Coverage for local/lib/python2.7/site-packages/sage/libs/gap/context_managers.py : 100%

""" 

Context Managers for LibGAP 

This module implements a context manager for global variables. This is 

useful since the behavior of GAP is sometimes controlled by global 

variables, which you might want to switch to a different value for a 

computation. Here is an example how you are suppose to use it from 

your code. First, let us set a dummy global variable for our example:: 

sage: libgap.set_global('FooBar', 123) 

Then, if you want to switch the value momentarily you can write:: 

sage: with libgap.global_context('FooBar', 'test'): 

....: print(libgap.get_global('FooBar')) 

test 

Afterward, the global variable reverts to the previous value:: 

sage: print(libgap.get_global('FooBar')) 

123 

The value is reset even if exceptions occur:: 

sage: with libgap.global_context('FooBar', 'test'): 

....: print(libgap.get_global('FooBar')) 

....: raise ValueError(libgap.get_global('FooBar')) 

Traceback (most recent call last): 

... 

ValueError: test 

sage: print(libgap.get_global('FooBar')) 

123 

""" 

############################################################################### 

# Copyright (C) 2012, Volker Braun <vbraun.name@gmail.com> 

# 

# Distributed under the terms of the GNU General Public License (GPL) 

# as published by the Free Software Foundation; either version 2 of 

# the License, or (at your option) any later version. 

# http://www.gnu.org/licenses/ 

############################################################################### 

from __future__ import print_function 

from sage.libs.gap.libgap import libgap 

class GlobalVariableContext(): 

def __init__(self, variable, value): 

""" 

Context manager for GAP global variables. 

It is recommended that you use the 

:meth:`sage.libs.gap.libgap.Gap.global_context` method and not 

construct objects of this class manually. 

INPUT: 

- ``variable`` -- string. The variable name. 

- ``value`` -- anything that defines a GAP object. 

EXAMPLES:: 

sage: libgap.set_global('FooBar', 1) 

sage: with libgap.global_context('FooBar', 2): 

....: print(libgap.get_global('FooBar')) 

2 

sage: libgap.get_global('FooBar') 

1 

""" 

self._variable = variable 

self._new_value = value 

def __enter__(self): 

""" 

Called when entering the with-block 

EXAMPLES:: 

sage: libgap.set_global('FooBar', 1) 

sage: with libgap.global_context('FooBar', 2): 

....: print(libgap.get_global('FooBar')) 

2 

sage: libgap.get_global('FooBar') 

1 

""" 

self._old_value = libgap.get_global(self._variable) 

libgap.set_global(self._variable, self._new_value) 

def __exit__(self, exc_type, exc_val, exc_tb): 

""" 

Called when exiting the with-block 

EXAMPLES:: 

sage: libgap.set_global('FooBar', 1) 

sage: with libgap.global_context('FooBar', 2): 

....: print(libgap.get_global('FooBar')) 

2 

sage: libgap.get_global('FooBar') 

1 

""" 

libgap.set_global(self._variable, self._old_value) 

return False