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contextlib2 --- Updated utilities for context management
========================================================

.. module:: contextlib2
   :synopsis: Backports and future enhancements for the contextlib module

This module provides backports of features in the latest version of the
standard library's :mod:`contextlib` module to earlier Python versions. It
also serves as a real world proving ground for potential future enhancements
to that module.

Like :mod:`contextlib`, this module provides utilities for common tasks
involving the ``with`` statement.


Additions Relative to the Standard Library
------------------------------------------

This module is primarily a backport of the Python 3.5 version of
:mod:`contextlib` to earlier releases. However, it is also a proving ground
for new features not yet part of the standard library.

There are currently no such features in the module.

Refer to the :mod:`contextlib` documentation for details of which
versions of Python 3 introduce the various APIs provided in this module.


API Reference
=============

Functions and classes provided:

.. decorator:: contextmanager

   This function is a :term:`decorator` that can be used to define a factory
   function for :keyword:`with` statement context managers, without needing to
   create a class or separate :meth:`__enter__` and :meth:`__exit__` methods.

   A simple example (this is not recommended as a real way of generating HTML!)::

      from contextlib import contextmanager

      @contextmanager
      def tag(name):
          print("<%s>" % name)
          yield
          print("</%s>" % name)

      >>> with tag("h1"):
      ...    print("foo")
      ...
      <h1>
      foo
      </h1>

   The function being decorated must return a :term:`generator`-iterator when
   called. This iterator must yield exactly one value, which will be bound to
   the targets in the :keyword:`with` statement's :keyword:`as` clause, if any.

   At the point where the generator yields, the block nested in the :keyword:`with`
   statement is executed.  The generator is then resumed after the block is exited.
   If an unhandled exception occurs in the block, it is reraised inside the
   generator at the point where the yield occurred.  Thus, you can use a
   :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally` statement to trap
   the error (if any), or ensure that some cleanup takes place. If an exception is
   trapped merely in order to log it or to perform some action (rather than to
   suppress it entirely), the generator must reraise that exception. Otherwise the
   generator context manager will indicate to the :keyword:`with` statement that
   the exception has been handled, and execution will resume with the statement
   immediately following the :keyword:`with` statement.

   :func:`contextmanager` uses :class:`ContextDecorator` so the context managers
   it creates can be used as decorators as well as in :keyword:`with` statements.
   When used as a decorator, a new generator instance is implicitly created on
   each function call (this allows the otherwise "one-shot" context managers
   created by :func:`contextmanager` to meet the requirement that context
   managers support multiple invocations in order to be used as decorators).


.. function:: closing(thing)

   Return a context manager that closes *thing* upon completion of the block.  This
   is basically equivalent to::

      from contextlib import contextmanager

      @contextmanager
      def closing(thing):
          try:
              yield thing
          finally:
              thing.close()

   And lets you write code like this::

      from contextlib import closing
      from urllib.request import urlopen

      with closing(urlopen('http://www.python.org')) as page:
          for line in page:
              print(line)

   without needing to explicitly close ``page``.  Even if an error occurs,
   ``page.close()`` will be called when the :keyword:`with` block is exited.


.. function:: suppress(*exceptions)

   Return a context manager that suppresses any of the specified exceptions
   if they occur in the body of a with statement and then resumes execution
   with the first statement following the end of the with statement.

   As with any other mechanism that completely suppresses exceptions, this
   context manager should be used only to cover very specific errors where
   silently continuing with program execution is known to be the right
   thing to do.

   For example::

       from contextlib import suppress

       with suppress(FileNotFoundError):
           os.remove('somefile.tmp')

       with suppress(FileNotFoundError):
           os.remove('someotherfile.tmp')

   This code is equivalent to::

       try:
           os.remove('somefile.tmp')
       except FileNotFoundError:
           pass

       try:
           os.remove('someotherfile.tmp')
       except FileNotFoundError:
           pass

   This context manager is :ref:`reentrant <reentrant-cms>`.

   .. versionadded:: 0.5
      Part of the standard library in Python 3.4 and later


.. function:: redirect_stdout(new_target)

   Context manager for temporarily redirecting :data:`sys.stdout` to
   another file or file-like object.

   This tool adds flexibility to existing functions or classes whose output
   is hardwired to stdout.

   For example, the output of :func:`help` normally is sent to *sys.stdout*.
   You can capture that output in a string by redirecting the output to a
   :class:`io.StringIO` object::

        f = io.StringIO()
        with redirect_stdout(f):
            help(pow)
        s = f.getvalue()

   To send the output of :func:`help` to a file on disk, redirect the output
   to a regular file::

        with open('help.txt', 'w') as f:
            with redirect_stdout(f):
                help(pow)

   To send the output of :func:`help` to *sys.stderr*::

        with redirect_stdout(sys.stderr):
            help(pow)

   Note that the global side effect on :data:`sys.stdout` means that this
   context manager is not suitable for use in library code and most threaded
   applications. It also has no effect on the output of subprocesses.
   However, it is still a useful approach for many utility scripts.

   This context manager is :ref:`reentrant <reentrant-cms>`.

   .. versionadded:: 0.5
      Part of the standard library in Python 3.4 and later


.. function:: redirect_stderr(new_target)

   Similar to :func:`redirect_stdout`, but redirecting :data:`sys.stderr` to
   another file or file-like object.

   This context manager is :ref:`reentrant <reentrant-cms>`.

   .. versionadded:: 0.5
      Part of the standard library in Python 3.5 and later


.. class:: ContextDecorator()

   A base class that enables a context manager to also be used as a decorator.

   Context managers inheriting from ``ContextDecorator`` have to implement
   ``__enter__`` and ``__exit__`` as normal. ``__exit__`` retains its optional
   exception handling even when used as a decorator.

   ``ContextDecorator`` is used by :func:`contextmanager`, so you get this
   functionality automatically.

   Example of ``ContextDecorator``::

      from contextlib import ContextDecorator

      class mycontext(ContextDecorator):
          def __enter__(self):
              print('Starting')
              return self

          def __exit__(self, *exc):
              print('Finishing')
              return False

      >>> @mycontext()
      ... def function():
      ...     print('The bit in the middle')
      ...
      >>> function()
      Starting
      The bit in the middle
      Finishing

      >>> with mycontext():
      ...     print('The bit in the middle')
      ...
      Starting
      The bit in the middle
      Finishing

   This change is just syntactic sugar for any construct of the following form::

      def f():
          with cm():
              # Do stuff

   ``ContextDecorator`` lets you instead write::

      @cm()
      def f():
          # Do stuff

   It makes it clear that the ``cm`` applies to the whole function, rather than
   just a piece of it (and saving an indentation level is nice, too).

   Existing context managers that already have a base class can be extended by
   using ``ContextDecorator`` as a mixin class::

      from contextlib import ContextDecorator

      class mycontext(ContextBaseClass, ContextDecorator):
          def __enter__(self):
              return self

          def __exit__(self, *exc):
              return False

   .. note::
      As the decorated function must be able to be called multiple times, the
      underlying context manager must support use in multiple :keyword:`with`
      statements. If this is not the case, then the original construct with the
      explicit :keyword:`with` statement inside the function should be used.


.. class:: ExitStack()

   A context manager that is designed to make it easy to programmatically
   combine other context managers and cleanup functions, especially those
   that are optional or otherwise driven by input data.

   For example, a set of files may easily be handled in a single with
   statement as follows::

      with ExitStack() as stack:
          files = [stack.enter_context(open(fname)) for fname in filenames]
          # All opened files will automatically be closed at the end of
          # the with statement, even if attempts to open files later
          # in the list raise an exception

   Each instance maintains a stack of registered callbacks that are called in
   reverse order when the instance is closed (either explicitly or implicitly
   at the end of a :keyword:`with` statement). Note that callbacks are *not*
   invoked implicitly when the context stack instance is garbage collected.

   This stack model is used so that context managers that acquire their
   resources in their ``__init__`` method (such as file objects) can be
   handled correctly.

   Since registered callbacks are invoked in the reverse order of
   registration, this ends up behaving as if multiple nested :keyword:`with`
   statements had been used with the registered set of callbacks. This even
   extends to exception handling - if an inner callback suppresses or replaces
   an exception, then outer callbacks will be passed arguments based on that
   updated state.

   This is a relatively low level API that takes care of the details of
   correctly unwinding the stack of exit callbacks. It provides a suitable
   foundation for higher level context managers that manipulate the exit
   stack in application specific ways.

   .. versionadded:: 0.4
      Part of the standard library in Python 3.3 and later

   .. method:: enter_context(cm)

      Enters a new context manager and adds its :meth:`__exit__` method to
      the callback stack. The return value is the result of the context
      manager's own :meth:`__enter__` method.

      These context managers may suppress exceptions just as they normally
      would if used directly as part of a :keyword:`with` statement.

   .. method:: push(exit)

      Adds a context manager's :meth:`__exit__` method to the callback stack.

      As ``__enter__`` is *not* invoked, this method can be used to cover
      part of an :meth:`__enter__` implementation with a context manager's own
      :meth:`__exit__` method.

      If passed an object that is not a context manager, this method assumes
      it is a callback with the same signature as a context manager's
      :meth:`__exit__` method and adds it directly to the callback stack.

      By returning true values, these callbacks can suppress exceptions the
      same way context manager :meth:`__exit__` methods can.

      The passed in object is returned from the function, allowing this
      method to be used as a function decorator.

   .. method:: callback(callback, *args, **kwds)

      Accepts an arbitrary callback function and arguments and adds it to
      the callback stack.

      Unlike the other methods, callbacks added this way cannot suppress
      exceptions (as they are never passed the exception details).

      The passed in callback is returned from the function, allowing this
      method to be used as a function decorator.

   .. method:: pop_all()

      Transfers the callback stack to a fresh :class:`ExitStack` instance
      and returns it. No callbacks are invoked by this operation - instead,
      they will now be invoked when the new stack is closed (either
      explicitly or implicitly at the end of a :keyword:`with` statement).

      For example, a group of files can be opened as an "all or nothing"
      operation as follows::

         with ExitStack() as stack:
             files = [stack.enter_context(open(fname)) for fname in filenames]
             # Hold onto the close method, but don't call it yet.
             close_files = stack.pop_all().close
             # If opening any file fails, all previously opened files will be
             # closed automatically. If all files are opened successfully,
             # they will remain open even after the with statement ends.
             # close_files() can then be invoked explicitly to close them all.

   .. method:: close()

      Immediately unwinds the callback stack, invoking callbacks in the
      reverse order of registration. For any context managers and exit
      callbacks registered, the arguments passed in will indicate that no
      exception occurred.


Examples and Recipes
====================

This section describes some examples and recipes for making effective use of
the tools provided by :mod:`contextlib2`. Some of them may also work with
:mod:`contextlib` in sufficiently recent versions of Python. When this is the
case, it is noted at the end of the example.


Cleaning up in an ``__enter__`` implementation
----------------------------------------------

As noted in the documentation of :meth:`ExitStack.push`, this
method can be useful in cleaning up an already allocated resource if later
steps in the :meth:`__enter__` implementation fail.

Here's an example of doing this for a context manager that accepts resource
acquisition and release functions, along with an optional validation function,
and maps them to the context management protocol::

   from contextlib2 import ExitStack

   class ResourceManager(object):

       def __init__(self, acquire_resource, release_resource, check_resource_ok=None):
           self.acquire_resource = acquire_resource
           self.release_resource = release_resource
           self.check_resource_ok = check_resource_ok

       def __enter__(self):
           resource = self.acquire_resource()
           if self.check_resource_ok is not None:
               with ExitStack() as stack:
                   stack.push(self)
                   if not self.check_resource_ok(resource):
                       msg = "Failed validation for {!r}"
                       raise RuntimeError(msg.format(resource))
                   # The validation check passed and didn't raise an exception
                   # Accordingly, we want to keep the resource, and pass it
                   # back to our caller
                   stack.pop_all()
           return resource

       def __exit__(self, *exc_details):
           # We don't need to duplicate any of our resource release logic
           self.release_resource()

This example will also work with :mod:`contextlib` in Python 3.3 or later.


Replacing any use of ``try-finally`` and flag variables
-------------------------------------------------------

A pattern you will sometimes see is a ``try-finally`` statement with a flag
variable to indicate whether or not the body of the ``finally`` clause should
be executed. In its simplest form (that can't already be handled just by
using an ``except`` clause instead), it looks something like this::

   cleanup_needed = True
   try:
       result = perform_operation()
       if result:
           cleanup_needed = False
   finally:
       if cleanup_needed:
           cleanup_resources()

As with any ``try`` statement based code, this can cause problems for
development and review, because the setup code and the cleanup code can end
up being separated by arbitrarily long sections of code.

:class:`ExitStack` makes it possible to instead register a callback for
execution at the end of a ``with`` statement, and then later decide to skip
executing that callback::

   from contextlib2 import ExitStack

   with ExitStack() as stack:
       stack.callback(cleanup_resources)
       result = perform_operation()
       if result:
           stack.pop_all()

This allows the intended cleanup up behaviour to be made explicit up front,
rather than requiring a separate flag variable.

If you find yourself using this pattern a lot, it can be simplified even
further by means of a small helper class::

   from contextlib2 import ExitStack

   class Callback(ExitStack):
       def __init__(self, callback, *args, **kwds):
           super(Callback, self).__init__()
           self.callback(callback, *args, **kwds)

       def cancel(self):
           self.pop_all()

   with Callback(cleanup_resources) as cb:
       result = perform_operation()
       if result:
           cb.cancel()

If the resource cleanup isn't already neatly bundled into a standalone
function, then it is still possible to use the decorator form of
:meth:`ExitStack.callback` to declare the resource cleanup in
advance::

   from contextlib2 import ExitStack

   with ExitStack() as stack:
       @stack.callback
       def cleanup_resources():
           ...
       result = perform_operation()
       if result:
           stack.pop_all()

Due to the way the decorator protocol works, a callback function
declared this way cannot take any parameters. Instead, any resources to
be released must be accessed as closure variables.

This example will also work with :mod:`contextlib` in Python 3.3 or later.


Using a context manager as a function decorator
-----------------------------------------------

:class:`ContextDecorator` makes it possible to use a context manager in
both an ordinary ``with`` statement and also as a function decorator. The
:meth:`ContextDecorator.refresh_cm` method even makes it possible to use
otherwise single use context managers (such as those created by
:func:`contextmanager`) that way.

For example, it is sometimes useful to wrap functions or groups of statements
with a logger that can track the time of entry and time of exit.  Rather than
writing both a function decorator and a context manager for the task,
:func:`contextmanager` provides both capabilities in a single
definition::

    from contextlib2 import contextmanager
    import logging

    logging.basicConfig(level=logging.INFO)

    @contextmanager
    def track_entry_and_exit(name):
        logging.info('Entering: {}'.format(name))
        yield
        logging.info('Exiting: {}'.format(name))

This can be used as both a context manager::

    with track_entry_and_exit('widget loader'):
        print('Some time consuming activity goes here')
        load_widget()

And also as a function decorator::

    @track_entry_and_exit('widget loader')
    def activity():
        print('Some time consuming activity goes here')
        load_widget()

Note that there is one additional limitation when using context managers
as function decorators: there's no way to access the return value of
:meth:`__enter__`. If that value is needed, then it is still necessary to use
an explicit ``with`` statement.

This example will also work with :mod:`contextlib` in Python 3.2.1 or later.


Context Management Concepts
===========================

.. _single-use-reusable-and-reentrant-cms:

Single use, reusable and reentrant context managers
---------------------------------------------------

Most context managers are written in a way that means they can only be
used effectively in a :keyword:`with` statement once. These single use
context managers must be created afresh each time they're used -
attempting to use them a second time will trigger an exception or
otherwise not work correctly.

This common limitation means that it is generally advisable to create
context managers directly in the header of the :keyword:`with` statement
where they are used (as shown in all of the usage examples above).

Files are an example of effectively single use context managers, since
the first :keyword:`with` statement will close the file, preventing any
further IO operations using that file object.

Context managers created using :func:`contextmanager` are also single use
context managers, and will complain about the underlying generator failing
to yield if an attempt is made to use them a second time::

    >>> from contextlib import contextmanager
    >>> @contextmanager
    ... def singleuse():
    ...     print("Before")
    ...     yield
    ...     print("After")
    ...
    >>> cm = singleuse()
    >>> with cm:
    ...     pass
    ...
    Before
    After
    >>> with cm:
    ...    pass
    ...
    Traceback (most recent call last):
        ...
    RuntimeError: generator didn't yield


.. _reentrant-cms:

Reentrant context managers
^^^^^^^^^^^^^^^^^^^^^^^^^^

More sophisticated context managers may be "reentrant". These context
managers can not only be used in multiple :keyword:`with` statements,
but may also be used *inside* a :keyword:`with` statement that is already
using the same context manager.

:class:`threading.RLock` is an example of a reentrant context manager, as is
:func:`suppress`. Here's a toy example of reentrant use (real world
examples of reentrancy are more likely to occur with objects like recursive
locks and are likely to be far more complicated than this example)::

    >>> from contextlib import suppress
    >>> ignore_raised_exception = suppress(ZeroDivisionError)
    >>> with ignore_raised_exception:
    ...     with ignore_raised_exception:
    ...         1/0
    ...     print("This line runs")
    ...     1/0
    ...     print("This is skipped")
    ...
    This line runs
    >>> # The second exception is also suppressed


.. _reusable-cms:

Reusable context managers
^^^^^^^^^^^^^^^^^^^^^^^^^

Distinct from both single use and reentrant context managers are "reusable"
context managers (or, to be completely explicit, "reusable, but not
reentrant" context managers, since reentrant context managers are also
reusable). These context managers support being used multiple times, but
will fail (or otherwise not work correctly) if the specific context manager
instance has already been used in a containing with statement.

An example of a reusable context manager is :func:`redirect_stdout`::

    >>> from contextlib import redirect_stdout
    >>> from io import StringIO
    >>> f = StringIO()
    >>> collect_output = redirect_stdout(f)
    >>> with collect_output:
    ...     print("Collected")
    ...
    >>> print("Not collected")
    Not collected
    >>> with collect_output:
    ...     print("Also collected")
    ...
    >>> print(f.getvalue())
    Collected
    Also collected

However, this context manager is not reentrant, so attempting to reuse it
within a containing with statement fails:

    >>> with collect_output:
    ...     # Nested reuse is not permitted
    ...     with collect_output:
    ...         pass
    ...
    Traceback (most recent call last):
      ...
    RuntimeError: Cannot reenter <...>


Obtaining the Module
====================

This module can be installed directly from the `Python Package Index`_ with
pip_::

    pip install contextlib2

Alternatively, you can download and unpack it manually from the `contextlib2
PyPI page`_.

There are no operating system or distribution specific versions of this
module - it is a pure Python module that should work on all platforms.

Supported Python versions are currently 2.7 and 3.2+.

.. _Python Package Index: http://pypi.python.org
.. _pip: http://www.pip-installer.org
.. _contextlib2 pypi page: http://pypi.python.org/pypi/contextlib2


Development and Support
-----------------------

contextlib2 is developed and maintained on GitHub_. Problems and suggested
improvements can be posted to the `issue tracker`_.

.. _GitHub: https://github.com/jazzband/contextlib2
.. _issue tracker: https://github.com/jazzband/contextlib2/issues


.. include:: ../NEWS.rst


Indices and tables
==================

* :ref:`genindex`
* :ref:`search`
