ChangeDirectory context manager (Python)

2009-01-24T12:41:53-08:00

Python recipe 576620 by Christophe Simonis (change, chdir, context_manager, directory). Revision 2.

A context manager that handle a temporary change of the working directory

Use PDFWriter with context manager support (Python)

2013-12-16T21:43:27-08:00

Python recipe 578790 by Vasudev Ram (contextmanager, context_manager, pdf, python, reportlab, with_statement, xtopdf).

PDFWriter - a core class of the xtopdf toolkit - can now be used with a Python context manager, a.k.a. the Python with statement.

Code example below. More details here:

http://jugad2.blogspot.in/2013/12/xtopdf-pdfwriter-now-has-context.html

Context manager for low-level redirection of stdout/stderr (Python)

2012-01-26T02:14:25-08:00

Python recipe 577564 by Greg Haskins (contextmanager, context_manager, f2py, fortran, python, redirect, stdout). Revision 3.

This context manager provides a convenient, Pythonic way to temporarily replace the file descriptors of stdout and stderr, redirecting to either os.devnull or files of your choosing. Swapping the C-level file descriptors is required when suppressing output from compiled extension modules, such as those built using F2PY. It functions equally well for pure-Python code. UPDATE: (see below).

Wrap any iterable context manager so it closes when consumed (Python)

2012-11-19T20:10:35-08:00

Python recipe 578342 by Andrew Barnert (contextmanager, context_manager, generator, iterable, iterator, with_statement).

There are a few types in Python—most notably, files—that are both iterators and context managers. For trivial cases, these features are easy to use together, but as soon as you need to use the iterator lazily or asynchronously, a with statement won't help. That's where this recipe comes in handy:

send_async(with_iter(open(path, 'r')))

This also allows you to "forward" closing for a wrapped iterator, so closing the outer iterator also closes the inner one:

sync_async(line.upper() for line in with_iter(open(path, 'r')))

Decorator and context manager from a single API (Python)

2010-06-27T15:15:01-07:00

Python recipe 577273 by Michael Foord (contextmanager, context_manager, decorator, decorators, with, with_statement). Revision 12.

Create objects that act as both context managers and as decorators, and behave the same in both cases.

Works with Python 2.4 - 2.7 and Python 3. The tests require unittest2 or Python 3.2 to run. (And because the tests use the with statement they won't work with Python 2.4.)

Example:

from contextdecorator import ContextDecorator

class mycontext(ContextDecorator):

    def __init__(self, *args):
        """Normal initialiser"""

    def before(self):
        """
        Called on entering the with block or starting the decorated function.

        If used in a with statement whatever this method returns will be the
        context manager.
        """

    def after(self, *exc):
        """
        Called on exit. Arguments and return value of this method have
        the same meaning as the __exit__ method of a normal context
        manager.
        """

Both before and after methods are optional (but providing neither is somewhat pointless). See the tests for more usage examples.

CleanupManager for with statements (Python)

2011-12-18T07:20:46-08:00

Python recipe 577981 by Nick Coghlan (contextmanager, context_manager). Revision 5.

Inspired by unittest.TestCase.addCleanup(), CleanupManager provides a means to programmatically add resources to be cleaned up when leaving a with statement. This makes it easy to use with optional resources, and those derived from sequences of inputs.

An more powerful version of this recipe with a few additional features is published under the name ContextStack as part of the contextlib2 module: http://contextlib2.readthedocs.org

This recipe is based on a suggestion originally posted by Nikolaus Rath at http://bugs.python.org/issue13585

Context manager for restoring a value (Python)

2010-01-09T07:14:06-08:00

Python recipe 576977 by George Sakkis (contextlib, context_manager). Revision 8.

Often one wants to rebind a name or modify a mutable object, perform a bunch of actions and finally restore the name/object to its original state. An example is redirecting stdout/stderr temporarily (http://www.diveintopython.org/scripts_and_streams/stdin_stdout_stderr.html). The restoring context manager shown below simplifies this pattern::

import sys

# prints in console
print "hello world!"

with restoring('sys.stdout'):
    with open('hello.txt', 'w') as sys.stdout:
        # prints in file
        print "hello world!"

# prints in console again
print "hello world!"

Context manager to prevent calling code from catching exceptions (Python)

2012-06-17T09:20:56-07:00

Python recipe 577863 by Oren Tirosh (context_manager, exception).

The following context manager causes any exceptions raised inside it to print a stack trace and exit immediately. The calling scope is not given a chance to catch the exception.

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