TCONFPY(3)TCONFPY(3)NAME tconfpy.py
Configuration File Support For Python Applications
SYNOPSIS
It is common to provide an external "configuration file" when writing
sophisticated applications. This gives the end-user the ability to
easily change program options by editing that file.
tconfpy is a Python module for parsing such configuration files.
tconfpy understands and parses a configuration "language" which has a
rich set of string-substitution, variable name, conditional, and vali‐
dation features.
By using tconfpy, you relieve your program of the major responsibility
of configuration file parsing and validation, while providing your
users a rich set of configuration features.
NOTE: Throughout this document we use the term "configuration file".
However, tconfpy can parse configurations both in files as well as in-
memory lists. Whenever you see the term "file", think "a file or a set
of configuration statements stored in a list".
ANOTHER NOTE: Throughout this document we refer to "symbols" and "vari‐
ables" interchangeably. Strictly speaking, this is not really right.
A "symbol" is an entry in a symbol table representing the state of some
"variable" that is visible to a person writing a configuration. But
it's safe to assume you're smart enough to understand this subtlety and
know what is meant by context ;)
JUST ONE LAST NOTE: If you run tconfpy directly, it will dump version
and copyright information, as well as the value of the current prede‐
fined System Variables:
python tconfpy.py
DOCUMENT ORGANIZATION
This document is divided into 4 major sections:
PROGRAMMING USING THE tconfpy API discusses how to call the configura‐
tion file parser, the options available when doing this, and what the
parser returns. This is the "Programmer's View" of the module and pro‐
vides in-depth descriptions of the API, data structures, and options
available to the programmer.
CONFIGURATION LANGUAGE REFERENCE describes the syntax and semantics of
the configuration language recognized by tconfpy. This is the "User's
View" of the package, but both programmers and people writing configu‐
ration files will find this helpful.
ADVANCED TOPICS FOR PROGRAMMERS describes some ways to combine the var‐
ious tconfpy features to do some fairly nifty things.
INSTALLATION explains how to install this package on various platforms.
This information can also be found in the READ-1ST.txt file distributed
with the package.
PROGRAMMING USING THE tconfpy APItconfpy is a Python module and thus available for use by any Python
program. This section discusses how to invoke the tconfpy parser, the
options available when doing so, and what the parser returns to the
calling program.
One small note is in order here. As a matter of coding style and
brevity, the code examples here assume the following Python import syn‐
tax:
from tconfpy import *
If you prefer the more pedestrian:
import tconfpy
you will have to prepend all references to a tconfpy object with
tconfpy.. So retval=ParseConfig(... becomes retval = tconfpy.ParseCon‐
fig(... and so on.
You will also find the test driver code provided in the tconfpy package
helpful as you read through the following sections. test-tc.py is a
utility to help you learn and exercise the tconfpy API. Perusing the
code therein is helpful as an example of the topics discussed below.
Core Objects
In order to use tconfpy effectively, you need to be familiar with the
objects used to communicate between your program and the parser. This
section provides a brief overview of these objects for reference use
later in the document.
The Symbol Table Object
A "symbol table" is a tconfpy object defined to hold the symbol
table proper, the results of the parse (upon return), and some
other data structures used internally by the parser.
The full Symbol Table object definition and initialization looks
like this:
class SymbolTable(object):
def __init__(self):
# These items are populated when the parser returns
self.Symbols = {}
self.DebugMsgs = []
self.ErrMsgs = []
self.WarnMsgs = []
self.LiteralLines = []
self.TotalLines = 0
self.Visited = []
# These items are for internal parser use only
# Never write an application to depend on their content
self.Templates = Template()
self.ALLOWNEWVAR = True
self.TEMPONLY = False
self.LITERALVARS = False
self.INLITERAL = False
self.DEBUG = False
self.CondStack = [["", True],]
This object is optionally passed to the API when beginning a
parse, if you wish to provide an initial symbol table. In this
case, only Symbols need be populated.
When the parse is complete, an object of this same type is
returned. Symbols, DebugMsgs, ErrMsgs, WarnMsgs, LiteralLines,
TotalLines, and Visited will be populated with the parse results
as appropriate. The remaining elements of the object are mean‐
ingless when the parser returns, and applications should never
make use of them.
The Variable Descriptor Object
The actual symbol table is kept in the SymbolTable.Symbols dic‐
tionary. There is one entry for each symbol (variable) encoun‐
tered in the parse. The symbol's name is used as the dictionary
key and an object of type VarDescriptor as its entry. This
"variable descriptor" is an object that describes the variable's
current and default values, whether it can be modified, and any
validation constraints:
class VarDescriptor(object):
# Default variable type is a writeable string with no constraints
def __init__(self):
self.Value = ""
self.Writeable = True
self.Type = TYPE_STRING
self.Default = ""
self.LegalVals = []
self.Min = None
self.Max = None
The Template Object
As described later in this document, it is possible to pre-
define a variable's type, default value, and validation con‐
traint(s) to be applied only if the variable is actually brought
into existence in the configuration file. This is a so-called
"template". Templates have their own object definition:
class Template(object):
def __init__(self):
self.Symbols = {}
In effect, this is a subset of the SymbolTable object. Tem‐
plate.Symbols is populated just like SymbolTable.Symbols using
symbol names and variable descriptors.
API Overview
The tconfpy API consists of a single call. Only the configuration file
to be processed is a required parameter, all the others are optional
and default as described below:
from tconfpy import *
retval = ParseConfig(cfgfile,
CallingProgram="tconfpy version-num",
InitialSymTable=SymbolTable(),
AllowNewVars=True,
Templates=Template(),
TemplatesOnly=False,
LiteralVars=True,
ReturnPredefs=True,
Debug=False
)
where:
cfgfile (Required Parameter - No Default)
Declares where the configuration is found. If this parameter is
a string, it is treated as the name of a file to parse. If this
parameter is a list, tconfpy presumes this to be an in-memory
configuration, and parses the list in sequential order, treating
each entry as a configuration line. This allows you to use
tconfpy for parsing either configuration files or in-memory con‐
figurations. If you pass anything other than a string or list
here, tconfpy will produce an error.
If you do pass the API an in-memory list, tconfpy treats each
entry as a line in a configuration "file". However, this means
that each element of the list must be a string. The parser
checks this first and only processes entries in the list that
are strings. Entries of any other type produce an error and are
ignored.
CallingProgram (Default: tconfpy + Version Number)
Change the prompt introducer string used for Debug, Error, and
Warning messages.
By default, each time tconfpy produces an Error, Warning, or
Debug message, it prepends it with the string tconfpy followed
by the version number. Since tconfpy is designed to be called
from applications programs, you may wish to substitute your own
program name or other information in these messages. You do so
by setting the CallingProgram keyword to the desired text.
InitialSymTable (Default: SymbolTable())
Used to pass a pre-initialized Symbol Table from the application
to the parser. Defaults to an empty symbol table.
AllowNewVars (Default: True)
Allow the user to create new variables in the configuration
file.
Templates (Default: Template())
This option is used to pass variable templates to the parser.
If present, tconfpy expects this option to pass an object of
type Template. See the section below entitled, Using Variable
Templates for all the details. By default, an empty template
table (no templates) is passed to the parser.
TemplatesOnly (Default: False)
If this option is set to True, tconfpy will not permit a new
variable to be created unless a variable template exists for it.
By default, tconfpy will use a variable template if one is
present for a new variable, but it does not require one. If a
new variable is created, and no Template exists for it, the
variable is just created as a string type with no restrictions
on content or length. When this option is set to True, then a
template must exist for each newly created variable.
LiteralVars (Default: False)
If set to True, this option enables variable substitutions
within .literal blocks of a configuration file. See the section
in the language reference below on .literal usage for details.
ReturnPredefs (Default: True)
tconfpy "prefefines" some variables internally. By default,
these are returned in the symbol table along with the variables
actually defined in the configuration file. If you want a
"pure" symbol table - that is, a table with only your variables
in it - set this option to False.
Debug (Default: False)
If set to True, tconfpy will provide detailed debugging informa‐
tion about each line processed when it returns.
retval
An object of type SymbolTable used to return parsing results.
The results of the parse will be returned in various data struc‐
tures:
SymbolTable.Symbols => All symbols and their values as a result of the parse
SymbolTable.DebugMsgs => Any Debug Messages if debug was requested
SymbolTable.ErrMsgs => Any Error Messages
SymbolTable.WarnMsgs => Any Warning Messages
SymbolTable.LiteralLines => Any Literal Text found in the configuration file
SymbolTable.TotalLines => Total number of lines processed
SymbolTable.Visited => List of configuration files processed
You can tell whether a parse was successful by examining
ErrMsgs. A successful parse will leave this data structure
empty (though there may well be Warning Messages present in
WarnMsgs.)
The Initial Symbol Table API Option
The simplest way to parse a configuration is just to call the parser
with the name of a file or an in-memory list containing the configura‐
tion statements:
retval = ParseConfig("MyConfigFile")
- OR -
myconfig = [configline1, configline2, ....]
retval = ParseConfig(myconfig)
Assuming your configuration is valid, ParseConfig() will return a sym‐
bol table populated with all the variables defined in the configuration
and their associated values. This symbol table will have only the sym‐
bols defined in that configuration (plus a few built-in and predefined
symbols needed internally by tconfpy).
However, the API provides a way for you to pass a "primed" symbol table
to the parser that contains predefined symbols/values of your own
choosing. Why on earth would you want to do this? There are a number
of reasons:
· You may wish to write a configuration file which somehow depends on
a predefined variable that only the calling program can know:
.if [APPVERSION] == 1.0
# Set configuration for original application version
.else
# Set configuration for newer releases
.endif
In this example, only the calling application can know its own ver‐
sion, so it sets the variable APPVERSION in a symbol table which is
passed to ParseConfig().
· You may wish to "protect" certain variable names be creating them
ahead of time and marking them as "Read Only" (VarDescriptor.Write‐
able=False ). This is useful when you want a variable to be avail‐
able for use within a configuration file, but you do not want users
to be able to change its value. In this case, the variable can be
referenced in a string substitution or conditional test, but cannot
be changed.
· You may want to place limits on what values can be assigned to a
particular variable. When a variable is newly defined in a a con‐
figuration file, it just defaults to being a string variable with‐
out any limits on its length or content (unless you are using Vari‐
able Templates). But variables that are created by a program have
access to the variable's "descriptor". By setting various
attribues of the variable descriptor you can control variable type,
content, and range of values. In other words, you can have tconfpy
"validate" what values the user assigns to particular variables.
This substantially simplifies your application because no invalid
variable value will ever be returned from the parser.
How To Create An Initial Symbol Table
A tconfpy "Symbol Table" is really nothing more than a Python dictio‐
nary stored inside a container object. The key for each dictionary
entry is the variable's name and the value is a tconfpy-specific object
called a "variable descriptor". Creating new variables in the symbol
table involves nothing more than this:
from tconfpy import *
# Create an empty symbol table
MySymTable = SymbolTable()
# Create descriptor for new variable
MyVarDes = VarDescriptor()
# Code to fiddle with descriptor contents goes here
MyVarDes.Value = "MyVal"
# Now load the variable into the symbol table
MySymTable.Symbols["MyVariableName"] = MyVarDes
# Repeat this process for all variables, then call the parser
retval = ParseConfig("MyConfigFile", InitialSymTable=MySymTable)
The heart of this whole business the VarDescriptor object. It
"describes" the value and properties of a variable. These descriptor
objects have the following attributes and defaults:
VarDescriptor.Value = ""
VarDescriptor.Writeable = True
VarDescriptor.Type = TYPE_STRING
VarDescriptor.Default = ""
VarDescriptor.LegalVals = []
VarDescriptor.Min = None
VarDescriptor.Max = None
When tconfpy encounters a new variable in a configuration file, it just
instantiates one of these descriptor objects with these defaults for
that variable. That is, variables newly-defined in a configuration
file are entered into the symbol table as string types, with an initial
value of "" and with no restriction on content or length.
But, when you create variables under program control to "prime" an ini‐
tial symbol table, you can modify the content of any of these
attributes for each variable. These descriptor attributes are what
tconfpy uses to validate subsequent attempts to change the variable's
value in the configuration file. In other words, modifying a vari‐
able's descriptor tells tconfpy just what you'll accept as "legal" val‐
ues for that variable.
Each attribute has a specific role:
VarDescriptor.Value (Default: Empty String)
Holds the current value for the variable.
VarDescriptor.Writeable (Default: True)
Sets whether or not the user can change the variable's value.
Setting this attribute to False makes the variable Read Only.
VarDescriptor.Type (Default: TYPE_STRING)
One of TYPE_BOOL, TYPE_COMPLEX, TYPE_FLOAT, TYPE_INT, or
TYPE_STRING. This defines the type of the variable. Each time
tconfpy sees a value being assigned to a variable in the config‐
uration file, it checks to see if that variable already exists
in the symbol table. If it does, the parser checks the value
being assigned and makes sure it matches the type declared for
that variable. For example, suppose you did this when defining
the variable, foo:
VarDescriptor.Type = TYPE_INT
Now suppose the user puts this in the configuration file:
foo = bar
This will cause a type mismatch error because bar cannot be
coerced into an integer type - it is a string.
As a general matter, for existing variables, tconfpy attempts to
coerce the right-hand-side of an assignment to the type declared
for that variable. The least fussy operation here is when the
variable is defined as TYPE_STRING because pretty much every‐
thing can be coerced into a string. For example, here is how
foo = 3+8j is treated for different type declarations:
VarDescriptor.Type VarDescriptor.Value
------------------ -------------------
TYPE_BOOL Type Error
TYPE_COMPLEX 3+8j (A complex number)
TYPE_FLOAT Type Error
TYPE_INT Type Error
TYPE_STRING "3+8j" (A string)
This is why the default type for newly-defined variables in the
configuration file is TYPE_STRING: they can accept pretty much
any value.
NOTE: tconfpy always stores a variable's value in it native type
in the symbol table entry for that variable - i.e., You'll get
the variable's value back from the parser in the proper type.
However, when the variable is actually referenced within a con‐
figuration file, it is converted to a string for purposes of
configuration processing.
For instance, when doing conditional comparisons, the parser
coerces the variable's value into a string for purposes of the
comparsion. Say you have the floating point variable myfloat
whose value is 6.023 and the configuration file contains a
statement like:
.if myfloat == 6.023
When doing this conditional check, the parser will convert the
current value of myfloat into a string and compare it to the
string "6.023" on the Right Hand Side.
Similarly, variables are coerced as strings when they are refer‐
enced in substitutions:
# Assume 'myfloat' has been predefined to be a floating point variable
# Assume 'mybool' has been predefined to be a boolean variable
myfloat = 3.14
mybool = True
myvar = [myfloat] is [mybool]
# This sets 'myvar' to the string '3.14 is True'
This can be tricky when dealing with Boolean variables. As
described later in this document, you can do conditional tests
based on the state of a Boolean, but if you do this:
.if myboolean == whatever...
The parser converts myboolean to a string of either "True" or
"False", so watch out for this. As a general matter, you're
more likely to need the boolean tests that check the state of
the variable, but the above construct is handy if you want to
use regular string variables to control conditional bodies:
MYCONTROL = True
.if myboolean == MYCONTROL
Where, MYCONTROL is a regular old string variable - i.e., It has
not been defined to be a Boolean by either a Template or Initial
Symbol table passed to the parser.
VarDescriptor.Default (Default: Empty String)
This is a place to store the default value for a given variable.
When a variable is newly-defined in a configuration file,
tconfpy places the first value assigned to that variable into
this attribute. For variables already in the symbol table,
tconfpy does nothing to this attribute. This attribute is not
actually used by tconfpy for anything. It is provided as a con‐
venience so that the calling program can easily keep track of
each variable's default value. This makes it easy to do things
like "reset" every variable when restarting a program, for exam‐
ple.
VarDescriptor.LegalVals (Default: [])
Sometimes you want to limit a variable to a specific set of val‐
ues. That's what this attribute is for. LegalVals explictly
lists every legal value for the variable in question. If the
list is empty,then this validation check is skipped.
The exact semantics of LegalVals varies depending on the type of
the variable.
Variable Type What LegalVals Does
--------------------------------
Boolean Nothing - Ignored
Integer, Float, Complex List of numeric values the
user can assign to this variable
Examples: [1, 2, 34]
[3.14, 2.73, 6.023e23]
[3.8-4j, 5+8j]
String List of Python regular expressions.
User must assign a value to this
variable that matches at least
one of these regular expressions.
Example: [r'a+.*', r'^AnExactString$']
The general semantic here is "If Legal Vals is not an empty
list, the user must assign a value that matches one of the items
in LegalVals."
One special note applies to LegalVals for string variables.
tconfpy always assumes that this list contains Python regular
expressions. For validation, it grabs each entry in the list,
attempts to compile it as a regex, and checks to see if the
value the user wants to set matches. If you define an illegal
regular expression here, tconfpy will catch it and produce an
appropriate error.
You may also want to specify a set of legal strings that are
exact matches not open-ended regular expressions. For example,
suppose you have a variable, COLOR and you only want the user to
be able to only set it to one of, Red, White, or Blue. In that
case, use the Python regular expression metacharacters that
indicate "Start Of String" and "End Of String" do do this:
des = VarDescriptor()
des.LegalVals = [r'^Red$', r'^White$', r'^Blue$']
...
SymTable['COLOR'] = des
NOTE: If you want this test to be skipped, then set LegalVals to
an empty list, []. (This is the default when you first create
an instance of tconfpy.VarDescriptor.) Do not set it to a
Python None or anything else. tconfpy expects this attribute to
be a list in every case.
VarDescriptor.Min and VarDescriptor.Max (Default: None)
These set the minimum and maxium legal values for the variables,
but the semantics vary by variable type:
Variable Type What Min/Max Do
----------------------------
Boolean, Complex Nothing - Ignored
Integer, Float Set Minimum/Maxium allowed values.
String Set Minimum/Maximum string length
In all cases, if you want either of these tests skipped, set Min
or Max to the Python None.
All these various validations are logically "ANDed" together. i.e., A
new value for a variable must be allowed AND of the appropriate type
AND one of the legal values AND within the min/max range.
tconfpy makes no attempt to harmonize these validation conditions with
each other. If you specify a value in LegalVals that is, say, lower
than allowed by Min you will always get an error when the user sets the
variable to that value: It passed the LegalVals validation but failed
it for Min.
The Initial Symbol Table And Lexical Namespaces
The CONFIGURATION LANGUAGE REFERENCE section below discusses lexical
namespaces in some detail from the user's point-of-view. However, it
is useful for the programmer to understand how they are implemented.
tconfpy is written to use a predefined variable named NAMESPACE as the
place where the current namespace is kept. If you do not define this
variable in the initial symbol table passed to the parser, tconfpy will
create it automatically with an initial value of "".
From a programmer's perspective, there are are few important things to
know about namespaces and the NAMESPACE variable:
· You can manually set the initial namespace to something other than
"". You do this by creating the NAMESPACE variable in the initial
symbol table passed to the parser, and setting the Value attribute
of its descriptor to whatever you want as the initial namespace.
At startup tconfpy will check this initial value to make sure it
conforms to the rules for properly formed names - i.e., It it will
check for blank space, a leading $, the presence of square brack‐
ets, and so on. If the initial namespace value you provide is
illegal, tconfpy will produce an error and reset the initial names‐
pace to "".
· Because lexical namespaces are implemented by treating NAMESPACE as
just another variable, all the type and value validations available
for string variables can be applied to NAMESPACE. As discussed
above, this means you can limit the length and content of what the
user assigns to NAMESPACE. In effect, this means you can limit the
number and name of namespaces available for use by the user. There
is one slight difference here than for other variables. The root
namespace is always legal, regardless of what other limitations you
may impose via the LegalVals, Min, and Max attributes of the NAMES‐
PACE variable descriptor.
· When the call to ParseConfig() completes, the Value attribute of
the NAMESPACE variable descriptor will contain the namespace that
was in effect when the parse completed. i.e., It will contain the
last namespace used.
How The tconfpy Parser Validates The Initial Symbol And Template Tables
When you pass an initial symbol and/or template table to the parser,
tconfpy does some basic validation that the table contents properly
conform to the VarDescriptor format and generates error messages if it
finds problems. However, the program does not check your specifica‐
tions to see if they make sense. For instance if you define an integer
with a minimum value of 100 and a maximum value of 50, tconfpy cheer‐
fully accepts these limits even though they are impossible. You'll
just be unable to do anything with that variable - any attempt to
change its value will cause an error to be recorded. Similarly, if you
put a value in LegalVals that is outside the range of Min to Max,
tconfpy will accept it quietly.
In the case of templates, tconfpy all makes sure that they are all
named "canonically". That is, a template name may not itself contain a
namespace. This effectively means that there can be no namespace sepa‐
rator characters (".") in the template name.
The AllowNewVars API Option
By default, tconfpy lets the user define any new variables they wish in
a configuration file, merely by placing a line in the file in this
form:
Varname = Value
However, you can disable this capability by calling the parser like
this:
retval = ParseConfig("myconfigfile", AllowNewVars=False)
This means that the configuration file can "reference" any predefined
variables, and even change their values (if they are Writeable), but it
cannot create new variables.
This feature is primarily intended for use when you pass an initial
symbol table to the parser and you do not want any other variables
defined by the user. Why? There are several possible uses for this
option:
· You know every configuration variable name the calling program will
use ahead of time. Disabling new variable names keeps the configu‐
ration file from getting cluttered with variables that the calling
program will ignore anyway, thereby keeping the file more readable.
· You want to insulate your user from silent errors caused by mis‐
spellings. Say your program looks for a configuration variable
called MyEmail but the user enters something like myemail =
foo@bar.com. MyEmail and myemail are entirely different variables
and only the former is recognized by your calling program. By
turning off new variable creation, the user's inadvertent mis‐
spelling of the desired variable name will be flagged as an error.
Note, however, that there is one big drawback to disabling new
variable creation. tconfpy processes the configuration file on a
line-by-line basis. No line "continuation" is supported. For
really long variable values and ease of maintenance, it is some‐
times helpful to create "intermediate" variables that hold tempo‐
rary values used to construct a variable actually needed by the
calling program. For example:
inter1 = Really, really, really, really, long argument #1
inter2 = Really, really, really, really, long argument #2
realvar = command [inter1] [inter2]
If you disable new variable creation you can't do this anymore
unless all the variables inter1, inter2, and realvar are predefined
in the initial symbol table passed to the parser.
Using Variable Templates
By default, any time a new variable is encountered in a configuration
file, it is created as a string type with no restrictions on its con‐
tent or length. As described above, you can predefine the variable in
the initial symbol table you pass to the parser. This allows you to
define that variable's type and to optionally place various restric‐
tions on the values it may take. In other words, you can "declare" the
variable ahead of time and tconfpy will do so-called "type and value
enforcement".
"Variable Templates" are a related kind of idea, with a bit of a twist.
They give you a way to "declare" variable type and content restrictions
for selected new variables discovered in the configuration file. In
other words, by using Variable Templates, you can make sure that a new
variable also has restrictions placed on its type and/or values.
The obvious question here is, "Why not just do this by predefining
every variable of interest in the initial symbol table passed to the
parser?" There are several answers to this:
· The tconfpy configuration language has very powerful "existential"
conditional tests. These test to see if a variable "exists". If
you predefine every variable you will ever need, then the kinds of
existential tests you can do will be somewhat limited (since every
variable does already exist).
With Variable Templates, you can define the type and value con‐
straints of a variable which will be applied, but only if you actu‐
ally bring that variable into existence. This allows constructs
like this to work:
.if [.PLATFORM] == posix
posix = True
.endif
.if [.PLATFORM] == nt
nt = True
.endif
.ifall posix
...
.endif
.ifall nt
...
.endif
.ifnone posix nt
...
.endif
In this example, notice that the variables posix and nt may- or may
not be actually created, depending on the value of .PLATFORM. The
logic later in the example depends upon this. If you were to pre‐
define these two variables (to specify type and/or value restric‐
tions), this type of logical flow would not be possible.
By providing Variable Templates for posix and nt, you can define
their type (likely Boolean in this case) ahead of time and this
will be applied if the variable does come into existence.
· The other reason for Variable Templates is more subtle, but gives
tconfpy tremendous versatility beyond just processing configuration
files. Variable Templates give you a way to use tconfpy to build
data validation tools.
Suppose you have a list of employee records exported in this gen‐
eral format (easy to do with most databases):
[Employee#]
LastName = ...
FirstName = ...
Address = ...
City = ...
... and so on
By using the empoyee's ID as a lexical namespace, we end up creat‐
ing new variables for each employee. Say the employee number is
1234. Then we would get, 1234.LastName, 1234.FirstName, and so on.
Now, here's the subtle part. Notice that the type and content
restrictions of these variables is likely to be the same for each
different employee.
By defining Variable Templates for each of the variables we intend
to use over and over again in different namespace contexts, we can
validate each of them to make sure their content, type, length, and
so forth are correct. This makes it possible to use tconfpy as the
underpinnings of a "data validation" or "cleansing" program.
· Another way to look at this is that Variable Templates give you a
way to define type/value restrictions on an entire "class" of vari‐
ables. Instead of having to explictly predefine variables for
every employee in our example above, you just define templates for
the variable set that is common to all employees. This is way sim‐
pler than predefining every possible variable combination ahead of
time.
The Templates And TemplatesOnly API Options
Variable Templates are supported with two API options: Templates And
TemplatesOnly. Templates is used to pass a symbol table (separate from
the main symbol table) containing the Variable Templates. By default,
this option is set to an object of type Template containing no tem‐
plates.
So what exactly is a "Variable Template"? It is the exact same thing
as a predefined variable you might pass in the initial symbol table.
In other words, it is a Python dictionary entry where the key is the
variable name and the entry is in VarDescriptor format. The big dif‐
ference here is that normal variables are stored in the symbol table in
a SymbolTable container object. But templated variables are stored in
a Template container object. (See Core Objects above for the details.)
Templated variables are thus defined (by the calling program) just like
you would any other variable - but stored in a different place:
from tconfpy import *
# Create an empty template table
MyTemplates = Template()
# Create descriptor for new variable
MyTemplateVarDes = VarDescriptor()
# Code to fiddle with descriptor contents goes here
MyTemplateVarDes.Type = TYPE_INT
... and so forth
# Now load the variable into the symbol table
MyTemplates.Symbols["MyTemplateName"] = MyTemplateVarDes
# Now pass the Templates to the parser
retval = ParseConfig("myfile", Templates=MyTemplates)
NOTE: You may optionally pass either an initial symbol table or a tem‐
plate table or both to the parser when you call it. That is, the ini‐
tial set of symbols is disjoint from any templates you've defined.
Semantically, the only difference between "regular" and templated vari‐
ables, is that a templated variable does not come into existence (i.e.
Become an entry) in the main symbol table until a variable by that name
is encountered in the configuration file. Then the variable is created
using the template as its entry in the main symbol table.
For example:
[1234]
LastName = Jones
FirstName = William
Address = 123 Main Street
City = Anywhere
State = WI
ZIP = 00000-0000
[1235]
LastName = Jones
FirstName = Susan
Address = 123 Main Street
City = Anywhere
State = WI
ZIP = 00000-0000
Suppose you define variable templates for LastName, FirstName, Address,
and so on. That is, you define variables by these names, and define
whatever type and content restrictions you want in each of their VarDe‐
scriptors. You then pass these to the parser via the Templates= option.
As tconfpy parses the file and encounters the new variables 1234.Last‐
Name ... 1235.ZIP, it uses the following
"rules" when creating new variables:
· See if there is a template variable whose name is the same as the
"base" name of the new variable. (The "base" name is just the
variable name without the prepended namespace.)
If there is a template with a matching name, see if the value the
user wants to assign to that variable passes all the type/valida‐
tion rules. If so, load the variable into the symbol table and set
its value as requested, using the VarDescriptor object from the
template. (This ensures that future attempts to change the vari‐
able's value will also be type/validation checked.)
If the assignment fails the validation tests, issue an appropriate
error and do not create the variable in the symbol table.
· If there is no template with a matching name, then just create a
new variable as usual - string type with no restrictions, unless
TemplatesOnly is set to True. Setting this option to True tells
the program that you want to allow the creation of only those vari‐
ables for which templates are defined. This is a way to restrict
just what new variables can be created in any namespace. Tem‐
platesOnly defaults to False which means you can create new vari‐
ables even when no template for them exists.
A few subtleties of templating variables are worth noting here:
· The same template is used over and over again to create a new vari‐
able of the same name in different namespaces. For example, sup‐
pose you've defined a template with the name, AccountNumber:
[ComputerSupplier]
AccountNumber = 1234-5
[Lawyer]
AccountNumber = 3456-3
This would create two separate variables in the symbol table, based
on the same template: ComputerSupplier.AccountNumber and
Lawyer.AccountNumber.
This works because tconfpy checks the so-called "canonical" name of
a variable when it is being created (the part of the name without
the namespace information) to see if a template exists for it. For
this reason, a template name must never contain a namespace. If
you attempt to create templates with names like Foo.Templatename,
the parser will reject it and produce an error.
· Notice that for variables actually in the symbol table, VarDescrip‐
tor.Value holds the current value for the variable. However, this
field is meaningless in a template. The template is only used when
creating a new variable to be added the normal symbol table. The
value field of the template's variable descriptor is never used for
anything - it is never read nor is it ever set.
· By definition, a templated variable does not actually exist (in the
symbol table) until you assign it some value in the configuration
file. This means that even if you mark the variable as Read Only
in its template, you are able to set it one time - to actually cre‐
ate it. Thereafter, the variable exists in the symbol table with
it's Writeable attribute set to False and future changes to the
variable are prevented
In summary, Variable Templates give you a way to place restrictions on
variable type and content in the event that the variable actually comes
into existence. They also give you a way to define such restrictions
for an entire class of variables without having to explicitly name each
such variable ahead of time. Finally, Variable Templates are an inter‐
esting way to use tconfpy as the basis for data validation programs.
The LiteralVars API Option
tconfpy supports the inclusion of literal text anywhere in a configura‐
tion file via the .literal directive. This directive effectively tells
the tconfpy parser to pass every line it encounters "literally" until
it sees a corresponding .endlinteral directive. By default, tconfpy
does exactly this. However, tconfpy has very powerful variable substi‐
tution mechanisms. You may want to embed variable references in a
"literal" block and have them replaced by tconfpy.
Here is an example:
MyEmail = me@here.com # This defines variable MyEmail
.literal
printf("[MyEmail]"); /* A C Statement */
.endliteral
By default, ParseConfig() will leave everything within the .lit‐
eral/.endliteral block unchanged. In our example, the string:
printf("[MyEmail]"); /* A C Statement */
would be in the list of literals returned by ParseConfig().
However, we can ask tconfpy to do variable replacement within literal
blocks by setting LiteralVars=True in the ParseConfig() call:
retval = ParseConfig("myconfigfile", LiteralVars=True)
In this example, tconfpy would return:
printf("me@here.com"); /* A C Statement */
At first glance this seems only mildly useful, but it is actually very
handy. As described later in this document, tconfpy has a rich set of
conditional operators and string substitution facilities. You can use
these features along with literal block processing and variable substi‐
tution within those blocks. This effectively lets you use tconfpy as a
preprocessor for any other language or text.
The ReturnPredefs API Option
As described below, tconfpy internally "predefines" a number of vari‐
ables. These include variables that describe the current runtime envi‐
ronment as well as variables that substitute for language keywords.
These predefined variables are just stored in the symbol table like any
other variable. By default, they are returned with all the "real"
variables discovered in the configuration file. If you want only the
variables actually encountered in the configuration file itself, set
ReturnPredefs=False in the ParseConfig() API call. This will cause
tconfpy to strip out all the predefined variables before returning the
final symbol table.
Note that this option also removes the NAMESPACE variable since it is
understood to also be outside the configuration file (even though you
may have passed an initial version of NAMESPACE to the parser).
Note, also, that this option applies only to the variables predefined
by tconfpy itself. Any variables you predefine when passing an initial
symbol table will be returned as usual, regardless of the state of this
option.
The Debug API Option
tconfpy has a fairly rich set of debugging features built into its
parser. It can provide some detail about each line parsed as well as
overall information about the parse. Be default, debugging is turned
off. To enable debugging, merely set Debug=True in the API call:
retval = ParseConfig("myconfigfile", Debug=True)
How tconfpy Processes Errors
As a general matter, when tconfpy encounters an error in the configura‐
tion file currently being parsed, it does two things. First, it adds a
descriptive error message into the list of errors returned to the call‐
ing program (see the next section). Secondly, in many cases, noteably
during conditional processing, it sets the parser state so the block in
which the error occurred is logically False. This does not happen in
every case, however. If you are having problems with errors, enable
the Debugging features of the package and look at the debug output. It
provides detailed information about what caused the error and why.
tconfpy Return Values
When tconfpy is finished processing the configuration file, it returns
an object (of type SymbolTable) that contains the entire results of the
parse. This includes a symbol table, any relevant error or warning
messages, debug information (if you requested this via the API), and
any "literal" lines encountred in the configuration.
NOTE: Because the object is of type SymbolTable, it contains other data
structures used by the parser itself. These are purposely "cleaned
out" before the parser returns and should never be used by the calling
application for any reason. In other words, use only the data struc‐
tures documented below when the parser returns control to your calling
program.
The return object is an instance of the class twander.SymbolTable which
has been populated with the results of the parse. In the simplest
case, we can parse and extract results like this:
from tconfpy import *
retval = ParseConfig("myconfigfile", Debug=True)
retval now contains the results of the parse:
retval.Symbols
A Python dictionary which lists all the defined symbols and
their associated values. A "value" in this case is always an
object of type tconfpy.VarDescriptor (as described above).
retval.DebugMsgs
A Python list containing detailed debug information for each
line parsed as well as some brief summary information about the
parse. retval.DebugMsgs defaults to an empty list and is only
populated if you set Debug=True in the API call that initiated
the parse (as in the example above).
retval.ErrMsgs
A Python list containing error messages. If this list is empty,
you can infer that there were no parsing errors - i.e., The con‐
figuration file was OK.
retval.WarnMsgs
A Python list containing warning messages. These describe minor
problems not fatal to the parse process, but that you really
ought to clean up in the configuration file. It is possible to
create a configuration that produces no errors but does produce
warnings, for example. It is almost always the case that this
configuration is not being handled the way you intended. As a
general matter of good practice, apply "belt and suspenders"
rules in your applications, and demand that a configration be
free of both errors and warnings before proceding.
retval.LiteralLines
As described below, the tconfpy configuration language supports
a .literal directive. This directive allows the user to embed
literal text anywhere in the configuration file. This effec‐
tively makes tconfpy useful as a preprocessor for any other lan‐
guage or text. retval.LiteralLines is a Python list containing
all literal text discovered during the parse. The lines appear
there in the order they were discovered in the configuration
file.
retval.TotalLines
Contains a count of the number of the total number of lines pro‐
cessed during the parse.
retval.Visited
Contains a list of all the configuration files and/or in-memory
configurations processed.
CONFIGURATION LANGUAGE REFERENCEtconfpy recognizes a full-featured configuration language that includes
variable creation and value assignment, a full preprocessor with condi‐
tionals, type and value enforcement, and lexical namespaces. This sec‐
tion of the document describes that language and provides examples of
how each feature can be used.
tconfpy Configuration Language Syntax
tconfpy supports a fairly simple and direct configuration language syn‐
tax:
· Each line is treated independently. There is no line "continua‐
tion".
· The # can begin a comment anywhere on a line. This is done
blindly. If you need to embed this symbol somewhere within a vari‐
able value, use the [HASH] variable reference.
· Whitespace is (mostly) insignificant. Leading and trailing white‐
space is ignored, as is whitespace around comparison operators.
However, there are some places where whitespace matters:
Variable names may not contain whitespace
Directives must be followed by whitespace if they take
other arguments.
When assigning a value to a string variable, whitespace
within the value on the right-hand-side is preserved.
Leading- and trailing whitespace around the right-hand-
side of the assignment is ignored.
Whitespace within both the left- and right-hand-side
arguments of a conditional comparison
(.if ... == / != ...) is significant for purposes
of the comparison.
· Case is always significant except when assigning a value to Bool‐
eans (described in the section below entitled, Some Notes On Bool‐
ean Variables ).
· Regardless of a variable's type, all variable references return a
string representation of the variable's value! This is done so
that the variable's value can be used for comparison testing and
string substitution/concatenation. In other words, variables are
stored in their native type in the symbol table that is returned to
the calling program. However, they are treated as strings during
the parsing of the configuration file whenever they are used in a
comparison test or in a substitution.
· Text inside a literal block (see section below on the .literal
directive) is left untouched. Whitespace, the # symbol, and so on
are not intepreted in any way and are passed back to the calling
program as-is. The one exception to this rule is when variable
substitution inside literal blocks is enabled. This is discussed
in a later section of this document as well.
· Any line which does not conform to these rules and/or is not in the
proper format for one of the operations described below, is consid‐
ered an error.
Creating Variables And Assigning A Value
The heart of a configuration file is a "variable". Variables are
stored in a "Symbol Table" which is returned to the calling program
once the configuration file has been processed. The calling program
can predefine any variables it wishes before processing a configuration
file. You can normally also define your own new variables in the con‐
figuration file as desired (unless the programmer has inhibited new
variable creation).
Variables are assigned values like this:
MyVariable = Some string of text
If MyVariable is a new variable, tconfpy will create it on the spot.
If it already exists, tconfpy will first check and make sure that Some
string of text is a legal value for this variable. If not, it will
produce an error and refuse to change the current value of MyVariable.
Anytime you create a new variable, the first value assigned to it is
also considered its "default" value. This may (or may not) be meaning‐
ful to the application program.
Variables which are newly-defined in a configuration file are always
understood to be string variables - i.e., They hold "strings" of text.
However, it is possible for the applications programmer to predefine
variables with other types and place limitations on what values the
variable can take and/or how short or long a string variable may be.
(See the previous section, PROGRAMMING USING THE tconfpy API for all
the gory details.)
The programmer can also arrange for the configuration file to only have
access to variables predefined by the program ahead of time. In that
case, if you try to create a new variable, tconfpy will produce an
appropriate error and the new variable will not be created.
Variable Names
Variables can be named pretty much anything you like, with certain
restrictions:
· Variable names may not contain whitespace.
· Variable names may not begin with the $ character. The one excep‐
tion to this is when you are referencing the value of an environ‐
ment variable. References to environment variables begin with $:
# A reference to an environment variable is legal
x = [$TERM]
# Attempting to create a new variable starting with $ is illegal
$MYVAR = something
· Variable names cannot have the # character anywhere in them because
tconfpy sees that character as the beginning a comment.
· Variable names cannot begin with . character. tconfpy understands
a leading period in a variable name to be a "namespace escape".
This is discussed in a later section on lexical namespaces.
· Variable names cannot contain the [ or ] characters. These are
reserved symbols used to indicate a variable reference.
· You cannot have a variable whose name is the empty string. This is
illegal:
= String
· The variable named NAMESPACE is not available for your own use.
tconfpy understands this variable to hold the current lexical
namespace as described later in this document. If you set it to a
new value, it will change the namespace, so be sure this is what
you wanted to do.
Getting And Using The Value Of A Variable
You can get the value of any currently defined variable by referencing
it like this:
.... [MyVariable] ...
The brackets surrounding any name are what indicate that you want that
variable's value.
You can also get the value of any Environment Variable on your system
by naming the variable with a leading $:
... [$USER] ... # Gets the value of the USER environment variable
However you cannot set the value of an environment variable:
$USER = me # This is not permitted
This ability to both set and retrieve variable content makes it easy to
combine variables through "substitution":
MYNAME = Mr. Tconfpy
MYAGE = 101
Greeting = Hello [MYNAME], you look great for someone [MYAGE]!
Several observations are worth noting here:
· The substitution of variables takes place as soon as the parser
processes the line Greeting = .... That is, variable substitution
happens as it is encountered in the configuration file. The only
exception to this is if an attempt is made to refer to an unde‐
fined/non-existent variable. This generates an error.
· The variable Greeting now contains the string "Hello Mr. Tconfpy,
you look great for someone 101!" This is true even if variable
MYAGE has been defined by the calling program to be an integer
type. To repeat a previously-made point: All variable substitu‐
tion and comparison operations in a configuration file are done
with strings regardless of the actual type of the variables
involved.
· Variables must be defined before they can be referenced. tconfpy
does not support so-called "forward" references.
· Unless a variable as been marked as "Read Only" by the application
program, you can continue to change its value as you go. Simply
adding another line at the end of our example above will change the
value of Greeting to something new:
Greeting = Generic Greeting Message
In other words, the last assignment statement for a given variable
"wins". This may seem sort of pointless, but it actually has great
utility. You can use the .include directive to get, say, a "stan‐
dard" configuration provided by the system administrator for a par‐
ticular application. You can then selectively override the vari‐
ables you want to change in your own configuration file.
Indirect Variable Assignment
The dereferencing of a variable's value can take place on either the
right- or left-hand-side of an assignment statement. This means so-
called "indirect" variable assignments are permitted:
CurrentTask = HouseCleaning
[CurrentTask] = Dad
To understand what this does you need to realize that before tconfpy
does anything with a statement in a configuration file, it replaces
every variable reference with its associated value (or produces an
error for references to non-existent variables). So the second state‐
ment above is first converted to:
HouseCleaning = Dad
i.e., The value Dad is assigned to a (new) variable called HouseClean‐
ing. In other words, putting a variable reference on the left-hand-
side of an assignment like this allows you to access another variable
which is named "indirectly".
You have to be careful when doing this, though. Consider a similar,
but slightly different example:
CurrentTask = House Cleaning # This is fine
[CurrentTask] = Dad # Bad!
The reason this no longer works is that the indirect reference causes
the second line to parse to:
House Cleaning = Dad
This is illegal because whitespace is not permitted in variable names.
tconfpy will produce an error if it sees such a construct. As a gen‐
eral matter, any variable you construct through this indirection method
must still conform to all the rules of variable naming: It cannot con‐
tain whitespace, begin with $, contain #, [, or ] and so on.
Another example of how indirection can "bite" you is when the value of
the variable begins with a period. As you'll see in the following sec‐
tion on Lexical Namespaces, a variable name beginning with a period is
understood to be an "absolute" variable name reference (relative to the
root namespace). This can cause unexpected (though correct) behavior
when doing indirect variable access:
NAMESPACE = NS1
foo = .bar # Creates variable NS1.foo with value .bar
[foo] = baz # Means [NS1.foo] = baz
The second assignment statement in this example does not do what you
might initially think. Remember, tconfpy always does variable derefer‐
encing before anything else, so the second statement becomes:
.bar = baz
As you'll see in the section on Lexical Namespaces below, this actually
means, "Set the variable bar in the root namespace to the value baz."
In other words, if you do indirect variable assignment, and the content
of that variable begins with a period, you will be creating/setting a
variable in the root namespace. Be sure this is what you intended to
do.
Get into the habit of reading [something] as, "The current value of
something". See if you understand what the following does (if you
don't, try it out with test-tc.py):
foo = 1
bar = 2
[foo] = bar
[bar] = [foo]
You can get pretty creative with this since variable references can
occur pretty much anywhere in an assignment statement. The only place
they cannot appear is within another variable reference. That is, you
cannot "nest" references:
# The Following Is Fine
FOO = Goodness
BAR = Me
Oh[FOO][BAR] = Goodness Gracious Me!
# But This Kind Of Nesting Attempt Causes An Error
[FOO[BAR]] = Something Or Other
Introducing Lexical Namespaces
So far,the discussion of variables and references has conveniently
ignored the presence of another related tconfpy feature, "Lexical
Namespaces." Namespaces are a way to automatically group related vari‐
ables together. Suppose you wanted to describe the options on your car
in a configuration file. You might do this:
MyCar.Brand = Ferrari
MyCar.Model = 250 GTO
MyCar.Color = Red
# And so on ...
You'll notice that every variable start with the "thing" that each item
has in common - they are features of MyCar. We can simplify this con‐
siderably by introducing a lexical namespace:
[MyCar]
Brand = Ferrari
Model = 250 GTO
Color = Red
The first statement looks like a variable reference, but it is not. A
string inside square brackets by itself on a line introduces a names‐
pace. The first statement in this example sets the namespace to MyCar.
From that point forward until the namespace is changed again, every
variable assignment and reference is "relative" to the namespace. What
this really means is that tconfpy sticks the namspace plus a . in front
of every variable assigned or referenced. It does this automatically
and invisibly, so Brand is turned into MyCar.Brand and so on. You can
actually check this by loading the example above into a test configura‐
tion file and running the test-tc.py program on it. You will see the
"fully qualified" variable names that actually were loaded into the
symbol table, each beginning with MyCar. and ending with the variable
name you specified.
Realize that this is entirely a naming "trick". tconfpy has no clue
what the namespace means, it just combines the current namespace with
the variable name to create the actual variable name that will be
returned in the symbol table.
You're likely scratching your head wondering why on earth this feature
present in tconfpy. There are several good reasons for it:
· It reduces typing repetetive information throughout the configura‐
tion file. In turn, this reduces the likelyhood of a typographical
or spelling error.
· It helps visibly organize the configuration file. A namespace
makes it clear which variables are related to each other somehow.
This is no big deal in small configurations, but tconfpy was writ‐
ten with the idea of supporting configuration files that might con‐
tain thousands or even tens of thousands of entries.
· It simplifies the application programmer's job. Say I want to
write a program that extracts all the information about your car
from the configuration file, but I don't know ahead of time how
many things you will describe. All I really have to know is that
you are using MyCar as the namespace for this information. My pro‐
gram can then just scan the symbol table after the configuration
file has been parsed, looking for variables whose name begins with
MyCar.. So if you want to add other details about your auto like,
say, Age, Price, and so on, you can do so later and the program
does not have to be rewritten.
· It helps enforce correct configuration files. By default, you can
introduce new namespaces into the configuration file any time you
like. However, as described in the previous section on the tconfpy
API, the application programmer can limit you to a predefined set
of legal namespaces (via the LegalVals attribute of the NAMESPACE
variable descriptor). By doing this, the programmer is helping you
avoid incorrect configuration file entries by limiting just which
namespaces you can enter to reference or create variables.
Rules For Using Lexical Namespace
Creating and using lexical namespaces is fairly straightforward, but
there are a few restrictions and rules:
· The default initial namespace is the empty string, "". In this one
case, tconfpy does nothing to variables assigned or referenced.
That's why our early examples in the previous section worked. When
we assigned a value to a variable and then referenced that variable
value, we did so while in the so-called "root" namespace, "". When
the namespace is "", nothing is done to the variable names.
Bear in mind that the programmer can change this default namespace
to something other than "" before the configuration file is ever
processed. If they do this, they would be well advised to let
their users know this fact.
· There two ways to change to a new namespace:
[NewNameSpace] # May optionally have a comment
OR
NAMESPACE = NewNamespace # May optionally have a comment
If, at any point, you want to return to the root namespace, you can
use one of these two methods:
[]
OR
NAMESPACE =
So, why are there two ways to do the same thing? The first way is
the more common, and the more readable way to do it. It appears on
a line by itself and makes it clear that the namespace is being
changed. However, because variable references cannot be "nested",
you can only use strings of text here.
Suppose you want to change the namespace in a way that depends on
the value of another variable. For instance:
LOCATION = Timbuktu
NAMESPACE = [LOCATION]-East
In other words, the second form of a namespace change allows you to
employ the tconfpy string substitution and variable referencing
features. Bear in mind that tconfpy is case-sensitive so this will
not work as you expect:
Namespace = something
This just set the value of the variable Namespace to something and
has nothing whatsoever to do with lexical namespaces.
·
Whichever method you use to change it, the new namespace must fol‐
low all the same rules used for naming variables.
For example, both of the following will cause an error:
[$FOO]
OR
x = $FOO
NAMESPACE = [x]
· By default, all variable assignments and references are relative to
the currently active namespace:
[MyNameSpace]
foo = 123 # Creates a variable called MyNameSpace.foo
x = [bar] # Means: MyNameSpace.x = [MyNameSpace.bar]
· If you want to set or reference a variable in a namespace different
than the current namespace, you must use a so-called "absolute"
variable name. You do this by "escaping" the variable name. To
escape the name, begin it with a . and then use the full name
(including namespace) of that variable. (This is called the "fully
qualified variable name".) For example:
[NS1] # Switch to the namespace NS1
foo = 14 # Creates NS1.foo
[NS2] # Switch to the NS2 namespace
foo = [.NS1.foo] # Sets NS2.foo = 14
There is another clever way to do this without using the escape
character. tconfpy has no understanding whatsoever of what a lexi‐
cal namespace actually is. It does nothing more than "glue" the
current namespace to any variable names and references in your con‐
figuration file. Internally, all variables are named relative to
the root namespace. This means that you can use the fully quali‐
fied variable name without any escape character any time you are in
the root namespace:
[NS1] # Switch to the namespace NS1
foo = 14 # Creates NS1.foo
[] # Switch to the root namespace
foo = [NS1.foo] # Sets foo = 14 - no escape needed
· Lexical namspaces are implemented by having NAMESPACE just be noth‐
ing more than (yet) another variable in the symbol table. tconfpy
just understands that variable to be special - it treats it as the
repository of the current lexical namespace. This means you can
use the value of NAMESPACE in your own string substitutions:
MyVar = [NAMESPACE]-Isn't This Cool?
You can even use the current value of NAMESPACE when setting a new
namespace:
NAMESPACE = [NAMESPACE]-New
One final, but very important point is worth noting here. The
NAMESPACE variable itself is always understood to be relative to
the root namespace. No matter what the current namespace actually
is, [NAMESPACE] or NAMESPACE = ... always set a variable by that
name in the root namespace. Similarly, when we use a variable ref‐
erence to get the current namespace value (as we did in the example
above), NAMESPACE is understood to be relative to the root names‐
pace. That's why things like this work:
[MyNewSpace]
x = 100 # MyNewSpace.x = 100
y = [NAMESPACE]-1 # MyNewSpace.y = MyNewSpace-1
NAMESPACE = NewSpace # .NAMESPACE = NewSpace
Predefined Variables
tconfpy predefines a number of variables. The NAMESPACE variable we
discussed in the previous section is one of them, but there are a num‐
ber of others of which you should be aware. Note that all predefined
variables are relative to the root namespace. Except for the NAMESPACE
variable, they are all Read Only and cannot be modified in your config‐
uration file.
The first group of predefined variables are called "System Variables".
As the name implies, they provide information about the system on which
you're running. These are primarily useful when doing conditional
tests (described later in this document). For example, by doing condi‐
tional tests with System Variables you can have one configuration file
that works on both Unix and Windows operating systems. The System
Variables are:
Variable Name Contains
---------------------
.MACHINENAME - The name of the computer on which you are running.
May also include full domain name, depending on system.
.OSDETAILS - Detailed information about the operating system in use.
.OSNAME - The name of the operating system in use.
.OSRELEASE - The version of the operating system in use.
.OSTYPE - Generic name of the operating system in use.
.PLATFORM - Generic type of the operating system in use.
.PYTHONVERSION - The version of Python in use.
By combining these System Variables as well as the content of selected
Environment Variables, you can create complex conditional configura‐
tions that "adapt" to the system on which a Python application is run‐
ning. For example:
.if [.MACHINENAME] == foo.bar.com
BKU = tar
.else
BKU = [$BACKUPPROGRAM]
.endif
The other kind of predefined variables are called "Reserved Variables".
tconfpy understands a number of symbols as part of its own language.
For example, the string # tells tconfpy to begin a comment until end-
of-line. There may be times, however, when you need these strings for
your own use. In other words, you would like to use one of the strings
which comprise the tconfpy language for your own purposes and have
tconfpy ignore them. The Reserved Variables give you a way to do this.
The Reserved Variables are:
Variable Name Contains
---------------------
DELIML [
DELIMR ]
DOLLAR $
ELSE .else
ENDIF .endif
ENDLITERAL .endliteral
EQUAL =
EQUIV ==
HASH #
IF .if
IFALL .ifall
IFANY .ifall
IFNONE .ifnone
INCLUDE .include
LITERAL .literal
NOTEQUIV !=
PERIOD .
For instance, suppose you wanted to include the # symbol in the value
of one of your variables. This will not work, because tconfpy inter‐
prets it as the beginning of a comment, which is not what you want:
MyJersey = Is #23
So, we use one of the Reserved Variables to get what we want:
MyJersey = Is [HASH]23
One word of warning, though. At the end of the day, you still have to
create variable names or namespace names that are legal. You can't
"sneak" illegal characters into these names using Reserved Variables:
foo = [DOLLAR]MyNewNamespace # No problem
NAMESPACE = [foo] # No way - namespace cannot start with $
Type And Value Enforcement
By default, any variable (or namespace) you create in a configuration
file is understood to just hold a string of characters. There are no
limits to what that string may contain, how long it is, and so on.
However, tconfpy gives the programmer considerable power to enforce
variable types and values, if they so choose. (See the section above
entitled, PROGRAMMING USING THE tconfpy API for the details.) The pro‐
grammer can set all kinds of limitations about a variable's type, per‐
missible values, and (in the case of strings) how long or short it may
be. The programmer does this by defining these limitations for each
variable of interest prior to calling tconfpy to parse your configura‐
tion file. In that case, when tconfpy actually processes the configu‐
ration file, it "enforces" these restrictions any time you attempt to
change the value of one of these variables. If you try to assign a
value that fails one of these "validation" tests, tconfpy will produce
an error and leave the variable's value unchanged.
For instance, suppose the programmer has defined variable "Foo" to be a
floating point number, and that it must have a value between -10.5 and
100.1. In that case:
Foo = 6.023E23 # Error - Value is out of range
Foo = MyGoodness # Error - Value must be a FP number, not a string
Foo = -2.387 # Good - Value is both FP an in range
What Specific Validations Are Available?
The programmer has several different restrictions they can place on a
variable's value. You do not need to understand how they work, merely
what they are so that any error messages you see will make sense.
· The programmer may declare any variable to be Read Only. This
means you can still use references to that variable to extract its
value, but any attempt to change it value within the configuration
file will fail and produce an error.
· The programmer may specify the variable's type as string (the
default), integer, floating point, complex, or boolean.
· The programmer may specify the set of all legal values that can be
assigned to a variable. For instance, the programmer might specify
that the floating point variable Transcend can only be set to
either 3.14 or 2.73. Similarly, the programmer might specify that
the string variable COLOR can only ever be set to Red, Green, or
Blue. In fact, in the case of string variables, the programmer can
actually specify a set of patterns (regular expressions) the value
has to match. For instance, they can demand that you only set a
particular string variable to strings that begin with a and end
with bob.
· For integer and floating point variables, the programmer can spec‐
ify a legal value range for the variable. If you change the value
of such a variable, that value must be within the defined range or
you'll get an error.
· For string variables, the programmer can specify a minimum and max‐
ium length for the strings you assign to the variable in question.
· The programmer can limit you to only being able to use existing
variables. (.i.e. The Predefined variables and any variables the
programmer has defined ahead of time.) In that case, any attempt
to create a new variable in the configuration file will fail and
produce an error.
· The programmer can limit you to only being able to use namespaces
they have defined ahead of time. In that case, if you attempt to
enter a namespace not on the list the programmer created ahead of
time will fail and produce an error.
· The programmer can enable or prevent the substitution of variable
references in literal blocks (see below). If they disable this
option, something like [Foo] is left unchanged within the literal
block. i.e., It too, is treated "literally".
Notes On Variable Type/Value Enforcement
There are a few other things you should know about how tconfpy enforces
restrictions on variables:
· For purposes of processing the configuration file, variable refer‐
ences are always converted to strings regardless of the actual type
of the variable in question. (Variables are stored in the symbol
table in their actual type.)
For instance, suppose the programmer defines variable Foo to be
floating point. Then:
Foo = 1.23
Bar = Value is [Foo] # Creates a new *string* variable with the
# value: "Value is 1.23"
In other words, variable values are "coerced" into strings for the
purposes of substitution and conditional testing within a configu‐
ration file. This is primarily an issue with the conditional com‐
parisons below. For example, the following conditional is False
because the string representations of the two numbers are differ‐
ent. Assume f1 and f2 have been defined as floating point vari‐
ables by the calling program:
f1 = 1.0
f2 = 1.00
.if [f1] == [f2] # False because "1.0" is not the same string as "1.00"
...
· You cannot create anything but a string variable within a configu‐
ration file. This variable will have no restrictions placed on its
values. All validation features require the limitations to be
specified by the calling program ahead of time.
· Similarly, you cannot change any of the enforcement options from
within a configuration file. These features are only available
under program control, presumably by the application program that
is calling tconfpy.
· There is no way to know what the limitations are on a particular
variable from within the configuration file. Programmers who use
these features should document the variable restrictions they've
employed as a part of the documentation for the application in
question.
Some Further Notes On Boolean Variables
One last note here concerns Boolean variables. Booleans are actually
stored in the symbol table as the Python Boolean values, True or False.
However, tconfpy accepts user statements that set the value of the
Boolean in a number of formats:
Boolean True Boolean False
-------------------------
foo = 1 foo = 0
foo = True foo = False
foo = Yes foo = No
foo = On foo = Off
This is the one case where tconfpy is insensitive to case - tRUE, TRUE,
and true are all accepted, for example.
NOTE HOWEVER: If the user wants to do a conditional test on the value
of a Boolean they must observe case and test for either True or False:
boolvar = No
.if [boolvar] == False # This works fine
.if [boolvar] == FALSE # This does not work - Case is not being observed
.if [boolvar] == Off # Neither does this - Only True/False can be tested
As a practical matter, unless you actually need to do comparisons
involving "True" and "False" strings, it is best to use the Existential
Conditionals to test the state of a boolean variable. See the section
entitled, Existential Conditionals And Booleans below, for the details.
The .include Directive
At any point in a configuration file, you can "include" another config‐
uration file like this:
.include filename
In fact, you can use all the variable substitution and string concate‐
nation features we've already discussed to do this symbolically:
Base = MyConfig
Ver = 1.01
.include [Base]-[Ver].cfg
The whitespace after the .include directive is mandatory to separate it
from the file name. You can have as many .include statements in your
configuration file as you wish, and they may appear anywhere. The only
restriction is that they must appear on a line by themselves (with an
optional comment).
Why bother? There are several reasons:
· This makes it easy to break up large, complex configurations into
simpler (smaller) pieces. This generally makes things easier to
maintain.
· This makes is easy to "factor" common configuration information
into separate files which can then be used by different programs as
needed.
· The most common use for .include is to load a "standard" configura‐
tion for your program. Recall that the last assignment of a vari‐
able's value "wins". Suppose you want all the standard settings
for a program, but you just want to change one or two options.
Instead of requiring each user to have the whole set of standard
settings in their own configuration file, the system administrator
can make them available as a common configuration. You then
.include that file and override any options you like:
# Get the standard options
.include /usr/local/etc/MyAppStandardConfig.cfg
# Override the ones you like
ScreenColor = Blue
Currency = Euros
This makes maintenance of complex configuration files much simpler.
There is only one master copy of the configuration that needs to be
edited when system-wide changes are required.
One last thing needs to be noted here. tconfpy does not permit so-
called "circular" or "recursive" inclusions. If file a .includes file
b and file b .includes file a, you will have an infinite loop of inclu‐
sion, which, uh ..., is a Bad Thing. So, the parser checks each time
you attempt to open a new configuration file to see if it's already
been processed. If it has, an error is produced, and the .include line
that would have caused a circular reference is ignored. Thereafter,
the program will continue to process the remainder of the configuration
as usual.
Conditional Directives
One of the most powerful features of tconfpy is its "conditional pro‐
cessing" capabilities. The general idea is to test some condition and
include or exclude configuration information based on the outcome of
the test.
What's the point? You can build large, complex configurations that
test things like environment variables, one of the Predefined Vari‐
ables, or even a variable you've set previously in the configuration
file. In other words, resulting configuration is then produced in a
way that is appropriate for that particular system, on that particular
day, for that particular user, ...
By using conditional directives, you can create a single configuration
file that works for every user regardless of operating system, loca‐
tion, and so on.
There are two kinds of conditional directives. "Existential Condition‐
als" test to see if a configuration or environment variable exists.
Existential Conditionals pay no attention to the value of the variables
in question, merely whether or not those variables have been defined.
"Comparison Conditionals" actually compare two strings. Typically, one
or more variable references appear in the compared strings. In this
case, the value of the variable is important.
The general structure of any conditional looks like this:
ConditionalDirective Argument(s)
This is included if the conditional was True
.else # Optional
This is included if the conditional was False
.endif # Required
Except for the whitespace after the conditional directive itself,
whitespace is not significant. You may indent as you wish.
Conditionals may also be "nested". You can have a conditional within
another conditional or .else block:
ConditionalDirective Argument(s)
stuff
ConditionalDirective Argument(s)
more stuff
.endif
interesting stuff
.else
yet more stuff
ConditionalDirective Argument(s)
other stuff
.endif
ending stuff
.endif
There are no explicit limits to how deeply you can nest a configura‐
tion. However, you must have an .endif that terminates each condi‐
tional test. Bear in mind that tconfpy pays no attention to your
indentation. It associates an .endif with the last conditional it
encountered. That's why it's a really good idea to use some consistent
indentation style so you can understand the logical structure of the
conditions. It's also a good idea to put comments throughout such con‐
ditional blocks so it's clear what is going on.
There are a few general rules to keep in mind when dealing with condi‐
tionals:
· There must be whitespace between the conditional directive and its
arguments (which may- or may not have whitespace in them).
· As with any other kind of tconfpy statement, you may place comments
anywhere at the end of a conditional directive line or within the
conditional blocks.
· Each conditional directive must have a corresponding .endif. If
you have more conditionals than .endifs or vice-versa, tconfpy will
produce an error message to that effect. It can get complicated to
keep track of this, especially with deeply nested conditionals. It
is therefore recommended that you always begin and end conditional
blocks within the same file. i.e., Don't start a conditional in
one file and then .include another file that has the terminating
.endif in it.
· The .else clause is optional. However, it can only appear after
some preceding conditional directive.
· As in other parts of the tconfpy language, variable names and ref‐
erences in conditional directives are always relative to the cur‐
rently active namespace unless they are escaped with a leading
period. Similarly, in this context, Environment Variables, Prede‐
fined Variables, and the NAMESPACE Variable are always relative to
the root namespace, no matter what namespace is currently active.
Existential Conditional Directives
There are three Existential Conditionals: .ifall, .ifany, and .ifnone.
Each has the same syntax:
ExistentialDirective varname ...
included if test was True
.else # optional
included if test was False
.fi
In other words, existential conditionals require one or more variable
names. In each case, the actual content of that variable is ignored.
The test merely checks to see if a variable by that name exists. Noth‐
ing else may appear on an existential conditional line, except, per‐
haps, a comment.
The three forms of existential conditional tests implement three dif‐
ferent kinds of logic:
.ifall var1 var2 ...
This is a logical "AND" operation. ALL of the variables, var1, var2 ...
must exist for this test to be True.
.ifany var1 var2 ...
This is a logical "OR" operation. It is True of ANY of the variables,
var1, var2 ... exist.
.ifnone var1 var2 ...
This is a logical "NOR" operation. It is True only if NONE of the variables,
var1, var2 ... exist.
Here is an example:
FOO = 1
BAR = 2
z = 0
.ifall FOO BAR
x = 1
.endif
.ifany FOO foo fOo
y = 2
.endif
.ifnone BAR bar Bar SOmething
z=3
.endif
When tconfpy finishes processing this, x=1, y=2, and z=0.
You can also use references to environment variables in an existential
conditional test:
.ifany $MYPROGOPTIONS
options = [$MYPROGOPTIONS]
.else
options = -b20 -c23 -z -r
.endif
Finally, you can use variable references here to get the name of a
variable to test by "indirection" (as we saw in the previous section on
accessing/setting variables indirectly). This should be used sparingly
since it can be kind of obscure to understand, but it is possible to do
this:
foo = MyVarName
.ifany [FOO]
...
.endif
This will test to see if either the variable MyVarName exists.
You can also do indirection through an environment variable. Use this
construct with restraint - it can introduce serious obscurity into your
configuration file. Still, it has a place. Say the TERM environment
variable is set to vt100:
.ifany [$TERM]
...
.endif
This will test to see if a variable called vt100 exists in the symbol
table. This is a handy way to see if you have a local variable defined
appropriate for the currently defined terminal, for instance.
Existential Conditionals And Booleans
As we've just seen, .ifall/any/none check to see if the variables named
on the Right Hand Side "exist". This is true for all kinds of vari‐
ables, regardless of their type. For instance:
.ifall Boolean1 Boolean2
...
.endif
Foo = Boolean3
.ifall [Foo]
...
.endif
The first conditional will be True only if both variables, Boolean1 and
Boolean2 exist. Similarly, the second conditional will be True only if
Boolean3 exists.
However, when using indirection on a boolean variable (i.e. A variable
that has been pre-defined to be a boolean type by the calling program),
the state of the boolean is returned. For example:
.ifall [Boolean1] [Boolean2]
...
.endif
In this case, the [Boolean..] indirection is understood to mean,
"Return the logical state of the boolean variable in question". This
allows the existential conditionals to act like logical operators when
their targets are boolean. In the example above, the test will only be
True if both booleans are logically True.
You can even mix and match:
.ifall Boolean1 [Boolean2]
...
.endif
This conditional will be True only if Boolean1 exists and Boolean2 is
True.
It's worth mentioning just why the semantics of indirect boolean refer‐
ences are different than for other variable types. If booleans were
treated the same as other variables, then [Boolean] would return a
string representation of the boolean variable's state - i.e., It would
return "True" or "False". In effect, we would be doing this:
.ifall/any/none True False True True False ...
This more-or-less makes no sense, since we're checking to see if vari‐
ables named "True" and "False" exist. What does make a lot of sense is
to use the state of a boolean variable to drive the conditional logic.
There is one other reason this feature is provided. Earlier versions
of tconfpy did not have this feature - booleans were treated like any
other variable. This meant that doing logical tests on the state of
the boolean required a kind of tortuous construct using the Comparsion
Conditionals (described in the next section):
# Example of AND logic using Comparison Conditional
.if [Bool1][Bool2][Bool3] == TrueTrueTrue
...
.endif
This is ugly, hard to read, and hard to maintain. By contrast, the
method just described allows booleans to be used in their intended man‐
ner - to make logical choices. .ifall becomes a logical "AND" func‐
tion. .ifany becomes a logical "OR" function, and .ifnone becomes a
logical "NOR" function when these tests are applied to indirect boolean
references.
Both methods of testing boolean state remain supported, so you can use
the style most appropriate for your application.
Comparison Conditional Directives
There are two Comparison Conditionals:
.if string1 == string2 # True if string1 and string2 are identical
.if string1 != string2 # True if string1 and string2 are different
As a general matter, you can put literal strings on both sides of such
a test, but the real value of these tests comes when you use variable
references within the tested strings. In this case, the value of the
variable does matter. It is the variable's value that is replaced in
the string to test for equality or inequality:
MyName = Tconfpy
.if [MyName] == Tconfpy
MyAge = 100.1
.else
MyAge = Unknown
.fi
These are particularly useful when used in combination with the tconfpy
Predefinded Variable or environment variables. You can build configu‐
rations that "sense" what system is currently running and "adapt"
accordingly:
AppFiles = MyAppFiles
.if [.OSNAME] == FreeBSD
files = [$HOME]/[AppFiles]
.endif
.if [.OSNAME] == Windows
files = [$USERPROFILE]\[AppFiles]
.endif
.ifnone [files]
ErrorMessage = I don't know what kind of system I am running!
.endif
The .literal Directive
By default, tconfpy only permits statements it "recognizes" in the con‐
figuration file. Anything else is flagged as an unrecognized statement
or "syntax error". However, it is possible to "embed" arbitrary text
in a configuration file and have tconfpy pass it back to the calling
program without comment by using the .literal directive. It works like
this:
.literal
This is literal text that will be passed back.
.endliteral
This tells tconfpy to ignore everything between .literal and .endlit‐
eral and just pass it back to the calling program (in retval.Literals -
see previous section on the tconfpy API). Literal text is returned in
the order it is found in the configuration file.
What good is this? It is a nifty way to embed plain text or even pro‐
grams written in other languages within a configuration file and pass
them back to the calling program. This is especially handy when used
in combination with tconfpy conditional features:
.if [.PLATFORM] == posix
.literal
We're Running On A Unix-Like System
.endliteral
.else
.literal
We're Not Running On A Unix-Like System
.endliteral
.endif
In other words, we can use tconfpy as a "preprocessor" for other text
or computer languages. Obviously, the program has to be written to
understand whatever is returned as literal text.
By default, tconfpy leaves text within the literal block completely
untouched. It simply returns it as it finds it in the literal block.
However, the programmer can invoke tconfpy with an option (Literal‐
Vars=True) that allows variable substitution within literal blocks.
This allows you to combine the results of your configuration into the
literal text that is returned to the calling program. Here is how it
works:
.ifall $USER
Greeting = Hello [$USER]. Welcome to [.MACHINENAME]!
.else
Greeting = Hello Random User. Welcome To Random Machine!
.endif
# Now embed the greeting in a C program
.literal
#include <stdio.h>
main()
{
printf("[Greeting]");
}
.endliteral
If the calling program sets LiteralVars=True, the literal block will
return a C program that prints the greeting defined at the top of this
example. If they use the default LiteralVars=False, the C program
would print [Greeting].
In other words, it is possible to have your literal blocks make refer‐
ence to other configuration variables (and Predefined or Environment
Variables). This makes it convenient to combine both configuration
information for the program, and other, arbitrary textual information
that the program may need, all in a single configuration file.
Notice too that the # character can be freely included within a literal
block. You don't have to use a Reserved Variable reference like [HASH]
here because everything (including whitespace) inside a literal block
is left untouched.
If you fail to provide a terminating .endliteral, the program will
treat everthing as literal until it reaches the end of the configura‐
tion file. This will generate an appropriate warning, but will work as
you might expect. Everything from the .literal directive forward will
be treated literally. As a matter of good style, you should always
insert an explicit .endliteral, even if it is at the end of file.
Placing an .endliteral in the configuration file without a preceding
.literal will also generate a warning message, and the statement will
be ignored.
GOTCHAS
tconfpy is a "little language". It is purpose-built to do one and only
one thing well: process configuration options. Even so, it is complex
enough that there are a few things that can "bite" you when writing
these configuration files:
· Probably the most common problem is attempting to do this:
foo = bar
.if foo == bar
...
.endif
But this will not work. tconfpy is very strict about requiring you
to explicitly distinguish between variable names and variable ref‐
erences.
The example above checks to see if the string foo equals the string
bar - which, of course, it never does.
What you probably want is to compare the value of variable foo with
some string:
foo = bar
.if [foo] == bar
...
.endif
Now you're comparing the value of the variable foo with the string
bar.
This was done for a very good reason. Because you have to explic‐
itly note whether you want the name or value of a variable (instead
of having tconfpy infer it from context), you can mix both literal
text and variable values on either side of a comparison or assign‐
ment:
foo = bar
foo[foo]foo = bar # Means: foobarfoo = bar
.if foo[foo] == foobar # Means: .if foobar == foobar
· Namespaces are a handy way to keep configuration options organized,
especially in large or complex configurations. However, you need
to keep track of the current namespace when doing things:
foo = bar
....
[NS-NEW]
.if [foo] == something # Checks value of NS-NEW.foo - will cause error
# since no such variable exists
· Remember that "last assignment wins" when setting variable values:
myvar = 100
... a long configuration file
myvar = 200
At the end of all this, myvar will be set to 200. This can be
especially annoying if you .include a configuration file after
you've set a value and the included file resets it. As a matter of
style, it's best to do all the .includes at the top of the master
configuration file so you won't get bitten by this one.
· Remember that case matters. Foo, foo, and foO are all different
variable names.
· Remember that all variable references are string replacements no
matter what the type of the variable actually is. tconfpy type and
value enforcement is used to return the proper value and type to
the calling program. But within the actual processing of a config‐
uration file, variable references (i.e., the values of variables)
are always treated as strings.
·
It is possible to load your own, user-defined, type in the variable
descriptor object when you pre-define a symbol table to be passed
to the parser. The problem is that this is more-or-less useless.
The parser attempts to coerce data assignments in the configuration
into the specified type. But, using only the assignment statements
available in this language, you cannot define values in a meaning‐
ful way for user-defined types. So, assignment of user-defined
variable types will always fail with a type error. Again, tconfpy
is designed as a small configuration processing language, not as a
general purpose programming language. In short, user-defined types
are not supported in the variable descriptor processing and will
always cause a type error to occur.
ADVANCED TOPICS FOR PROGRAMMERS
Here are some ideas on how you might combine tconfpy features to
enhance your own applications.
Guaranteeing A Correct Base Configuration
While it is always nice to give users lots of "knobs" to turn, the
problem is that the more options you give them, the more they can mis‐
configure a program. This is especially a problem when you are doing
technical support. You'd really like to get them to a "standard" con‐
figuration and then work from there to help solve their problem. If
your write your program with this in mind, tconfpy gives you several
ways to easily do this:
· Provide a "standard" system-, or even, enterprise-wide configura‐
tion file for your application. This file presumably has all the
program options set to "sane" values. All the user has to do is
create a configuration file with one line in it:
.include /wherever/the/standard/config/file/is
· Predefine every option variable the program will support. Populate
the initial symbol table passed to ParseConfig() with these defini‐
tions. By properly setting the Type, LegalVals, and Min/Max for
each of these variables ahead of time, you can prevent the user
from ever entering option values that make no sense or are danger‐
ous.
· Make sure ever program option has a reasonable Default value in its
variable descriptor. Recall that this attribute is provided for
the programmer's convenience. (When a variable descriptor is first
instantiated, it defaults to a string type and sets the default
attribute to an empty string. However, you can change both type
and default value under program control.) If you predefine a vari‐
able in the initial symbol table passed to the parser, tconfpy will
leave this attribute alone. However, variables that are created
for the first time in the configuration file will have this
attribute set to the first value assigned to the variable. Now
provide a "reset" feature in your application. All it has to do is
scan through the symbol table and set each option to its default
value.
Enforcing Mandatory Configurations
The tconfpy type and value validation features give you a handy way to
enforce what the legal values for a particular option may be. However,
you may want to go further than this. For instance, you may only want
to give certain classes of users the ability to change certain options.
This is easily done. First, predefine all the options of interest in
the symbol table prior to calling the tconfpy parser. Next, have your
program decide which options the current user is permitted to change.
Finally, mark all the options they may not change as "Read Only", by
setting the "Writeable" attribute for those options to False. Now call
the parser.
This general approach allows you to write programs that support a wide
range of options which are enabled/disabled on a per-user, per-machine,
per-domain, per-ip, per-company... basis.
Iterative Parsing
There may be situations where one "pass" through a configuration file
may not be enough. For example, your program may need to read an ini‐
tial configuration to decide how to further process the remainder of a
configuration file. Although it sounds complicated, it is actually
pretty easy to do. The idea is to have the program set some variable
that selects which part of the configuration file to process, and then
call the parser. When the parser returns the symbol table, the program
examines the results, makes whatever adjustments to the symbol table it
needs to, and passes it back to the parser for another "go". You can
keep doing this as often as needed. For instance:
# Program calls the parser with PASS set to 1
.if [PASS] == 1
# Do 1st Pass Stuff
.endif
# Program examines the results of the first pass, does
# what is has to, and sets PASS to 2
.if [PASS] == 2
# Do 2nd Pass Stuff
.endif
# And so on
In fact, you can even make this iterative parsing "goal driven". The
program can keep calling the parser, modifing the results, and calling
the parser again until some "goal" is met. The goal could be that a
particular variable gets defined (like CONFIGDONE). The goal might be
that a variable is set to a particular value (like, SYSTEMS=3).
It might even be tempting to keep parsing iteratively until tconfpy no
longer returns any errors. This is not recommended, though. A well-
formed configuration file should have no errors on any pass. Iterating
until tconfpy no longer detects errors makes it hard to debug complex
configuration files. It is tough to distinguish actual configuration
errors from errors would be resolved in a future parsing pass.
INSTALLATION
There are three ways to install tconfpy depending on your preferences
and type of system. In each of these installation methods you must be
logged in with root authority on Unix-like systems or as the Adminis‐
trator on Win32 systems.
Preparation - Getting And Extracting The Package
For the first two installation methods, you must first download the
latest release from:
http://www.tundraware.com/Software/tconfpy/
Then unpack the contents by issuing the following command:
tar -xzvf py-tconfpy-X.XXX.tar.gz (where X.XXX is the version number)
Win32 users who do not have tar installed on their system can find a
Windows version of the program at:
http://unxutils.sourceforge.net/
Install Method #1 - All Systems (Semi-Automated)
Enter the directory created in the unpacking step above. Then issue
the following command:
python setup.py install
This will install the tconfpy module and compile it.
You will manually have to copy the 'test-tc.py' program to a directory
somewhere in your executable path. Similarly, copy the documentation
files to locations appropriate for your system.
Install Method #2 - All Systems (Manual)
Enter the directory created in the unpacking step above. Then, manu‐
ally copy the tconfpy.py file to a directory somewhere in your PYTHON‐
PATH. The recommended location for Unix-like systems is:
.../pythonX.Y/site-packages
For Win32 systems, the recommended location is:
...\PythonX.Y\lib\site-packages
Where X.Y is the Python release number.
You can precompile the tconfpy module by starting Python interactively
and then issuing the command:
import tconfpy
Manually copy the 'test-tc.py' program to a directory somewhere in your
executable path. Copy the documentation files to locations appropriate
for your system.
Install Method #3 - FreeBSD Only (Fully-Automated)
Make sure you are logged in as root, then:
cd /usr/ports/devel/py-tconfpy
make install
This is a fully-automated install that puts both code and documentation
where it belongs. After this command has completed you'll find the
license agreement and all the documentation (in the various formats)
in:
/usr/local/share/doc/py-tconfpy
The 'man' pages will have been properly installed so either of these
commands will work:
man tconfpy
man test-tc
Bundling tconfpy With Your Own Programs
If you write a program that depends on tconfpy you'll need to ensure
that the end-users have it installed on their systems. There are two
ways to do this:
· Tell them to download and install the package as described above.
This is not recommended since you cannot rely on the technical
ability of end users to do this correctly.
· Just include 'tconfpy.py' in your program distribution directory.
This ensures that the module is available to your program regard‐
less of what the end-user system has installed.
THE tconfpy MAILING LIST
TundraWare Inc. maintains a mailing list to help you with your tconfpy
questions and bug reports. To join the list, send email to major‐
domo@tundraware.com with a single line of text in the body (not the
Subject line) of the message:
subscribe tconfpy-users your-email-address-goes-here
You will be notified when your subscription has been approved. You
will also receive detailed information about how to use the list,
access archives of previous messages, unsubscribe, and so on.
OTHERtconfpy requires Python 2.3 or later.
BUGS AND MISFEATURES
None known as of this release.
COPYRIGHT AND LICENSINGtconfpy is Copyright (c) 2003-2005 TundraWare Inc. For terms of use,
see the tconfpy-license.txt file in the program distribution. If you
install tconfpy on a FreeBSD system using the 'ports' mechanism, you
will also find this file in /usr/local/share/doc/py-tconfpy.
AUTHOR
Tim Daneliuk
tconfpy@tundraware.com
DOCUMENT REVISION INFORMATION
$Id: tconfpy.3,v 1.159 2005/01/20 09:32:57 tundra Exp $
TundraWare Inc. TCONFPY(3)
