ctxvar

A module to access variables defined in calling frames.

Copyright 2012 by Kay-Uwe (Kiwi) Lorenz

Published under New BSD License, see LICENSE.txt for details.

Inspired by Perl6’s $*foo variables, which are “dynamically overridable global variables” (Synopsis 2 on Twigils), I have created this module.

Basically this enables you to access local variables from calling frames:

>>> def a(): x = 1 ; return b()
>>> def b(): return ctx('x')
>>> a()
1

If you request to get a variable from calling contexts, there will be searched locals of each calling frame until requested variable is found, or there will be raised a NameError exception.

If you rely on this feature and you set a variable x, which should be accessed in a called function, you quickly hit the problem, that your x might be used in another called function locally and you do not get the expected value in your called function, but the one of the other x:

>>> def a(): x = 1 ; return b()
>>> def b(): x = 2 ; return c()
>>> def c(): # expects x from a 
...     return ctx('x')
>>> a()
2

So it is good to brand context variables with some characters, which cannot be used in usual python syntax for variables:

>>> def a(): ctx('*x', 1) ; return b()
>>> def b(): x = 2 ; return c()
>>> def c(): # expects *x from a
...    return ctx('*x')
>>> a()
1

If you try to access a variable, which cannot be found, there will be raised a NameError:

>>> ctx('***does not exist***')
Traceback (most recent call last):
...
NameError: name '***does not exist***' not defined in calling frames

You can use ctxget() for specifying a default in such a case:

>>> ctxget('***does not exist***', 'default')
'default'

There are two convenience functions for normal import:

>>> import ctxvar
>>> ctxvar.set('*foo', 'bar')
>>> ctxvar.get('*foo', 'default')
'bar'

If you access a context var more than once within a function, it may be helpful to copy it to local frame, while getting the value:

>>> def a(): ctx('*x', 1) ; return b()
>>> def b(): x = 2 ; return c()
>>> def c(): # expects *x from a
...    first = ctx('*x', Here)
...    return ctx('*x') # this will access the local copy
>>> a()
1

>>> def a(): FOO = "Hello" ; return b()
>>> def b(): BAR = "World" ; return c()
>>> def c(): return ctxstr("${FOO} ${BAR}")
>>> a()
'Hello World'

This will go back the frames only once.

ctxvar.ctx(name, value=Undef, caching=None, back=1)

Set or get a context variable.

Parameters:
name
name of variable
value

Any value the variable shall be set to. Default is Undef, a local class, which indicates that rather a value shall be retrieved than stored.

You may also set the value to Here, this will still retrieve the value but will create a copy of the variable in current frame.

caching
Set caching on, if there shall be created a copy in each frame between current one and the one holding the variable. Similar to value = Here.
back
Number of frames initially go back. This usually needs not to be set and is rather for internal calls, see set()
Returns:
value of variable
Raises:
NameError - if top level frame is reached and variable could not be found.
ctxvar.ctxget(name, default=None)

get a context variable. If not exists return default value

ctxvar.ctxstr(s, default='')

expand each ${...} s to its ctxvar. if default is None, replace by match

class ctxvar.Here

A Symbolic class for indicating, that you want to copy a context variable into local scope. See ctx().

class ctxvar.Undef

A Symbolic class different from None for use as default value in a function, if None could be passed as value. See ctx().

ctxvar.set(name, value)

set context variable name = value

ctxvar.get(name, default=None)

get a context variable. If not exists return default value

Installation

From Sources

Get Mercurial repository from https://bitbucket.org/klorenz/ctxvar:

$ hg clone https://bitbucket.org/klorenz/ctxvar

And run python installation:

$ python setup.py install

License

New BSD License

Copyright (c) 2012, Kay-Uwe (Kiwi) Lorenz

All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  • Neither the name of “Kay-Uwe (Kiwi) Lorenz” nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS “AS IS” AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL “Kay-Uwe (Kiwi) Lorenz” BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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