Doc/library/contextlib.rst

:mod:`contextlib` --- Utilities for :keyword:`with`\ -statement contexts
========================================================================

.. module:: contextlib
   :synopsis: Utilities for with-statement contexts.

**Source code:** :source:`Lib/contextlib.py`

--------------

This module provides utilities for common tasks involving the :keyword:`with`
statement. For more information see also :ref:`typecontextmanager` and
:ref:`context-managers`.


Utilities
---------

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).

   .. versionchanged:: 3.2
      Use of :class:`ContextDecorator`.


.. 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.


.. 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.

   .. versionadded:: 3.2


.. 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:: 3.3

   .. 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:`contextlib`.


Supporting a variable number of context managers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The primary use case for :class:`ExitStack` is the one given in the class
documentation: supporting a variable number of context managers and other
cleanup operations in a single :keyword:`with` statement. The variability
may come from the number of context managers needed being driven by user
input (such as opening a user specified collection of files), or from
some of the context managers being optional::

    with ExitStack() as stack:
        for resource in resources:
            stack.enter_context(resource)
        if need_special resource:
            special = acquire_special_resource()
            stack.callback(release_special_resource, special)
        # Perform operations that use the acquired resources

As shown, :class:`ExitStack` also makes it quite easy to use :keyword:`with`
statements to manage arbitrary resources that don't natively support the
context management protocol.


Simplifying support for single optional context managers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In the specific case of a single optional context manager, :class:`ExitStack`
instances can be used as a "do nothing" context manager, allowing a context
manager to easily be omitted without affecting the overall structure of
the source code::

   def debug_trace(details):
       if __debug__:
           return TraceContext(details)
       # Don't do anything special with the context in release mode
       return ExitStack()

   with debug_trace():
       # Suite is traced in debug mode, but runs normally otherwise


Catching exceptions from ``__enter__`` methods
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It is occasionally desirable to catch exceptions from an ``__enter__``
method implementation, *without* inadvertently catching exceptions from
the :keyword:`with` statement body or the context manager's ``__exit__``
method. By using :class:`ExitStack` the steps in the context management
protocol can be separated slightly in order to allow this::

   stack = ExitStack()
   try:
       x = stack.enter_context(cm)
   except Exception:
       # handle __enter__ exception
   else:
       with stack:
           # Handle normal case

Actually needing to do this is likely to indicate that the underlying API
should be providing a direct resource management interface for use with
:keyword:`try`/:keyword:`except`/:keyword:`finally` statements, but not
all APIs are well designed in that regard. When a context manager is the
only resource management API provided, then :class:`ExitStack` can make it
easier to handle various situations that can't be handled directly in a
:keyword:`with` statement.


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 contextlib import contextmanager, ExitStack

   class ResourceManager:

       def __init__(self, acquire_resource, release_resource, check_resource_ok=None):
           self.acquire_resource = acquire_resource
           self.release_resource = release_resource
           if check_resource_ok is None:
               def check_resource_ok(resource):
                   return True
           self.check_resource_ok = check_resource_ok

       @contextmanager
       def _cleanup_on_error(self):
           with ExitStack() as stack:
               stack.push(self)
               yield
               # 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()

       def __enter__(self):
           resource = self.acquire_resource()
           with self._cleanup_on_error():
               if not self.check_resource_ok(resource):
                   msg = "Failed validation for {!r}"
                   raise RuntimeError(msg.format(resource))
           return resource

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


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 contextlib 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 a particular application uses this pattern a lot, it can be simplified
even further by means of a small helper class::

   from contextlib 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 contextlib 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


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.

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,
inheriting from :class:`ContextDecorator` provides both capabilities in a
single definition::

    from contextlib import ContextDecorator
    import logging

    logging.basicConfig(level=logging.INFO)

    class track_entry_and_exit(ContextDecorator):
        def __init__(self, name):
            self.name = name

        def __enter__(self):
            logging.info('Entering: {}'.format(name))

        def __exit__(self, exc_type, exc, exc_tb):
            logging.info('Exiting: {}'.format(name))

Instances of this class 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.

.. seealso::

   :pep:`0343` - The "with" statement
      The specification, background, and examples for the Python :keyword:`with`
      statement.