cpython/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:

.. class:: AbstractContextManager

   An :term:`abstract base class` for classes that implement
   :meth:`object.__enter__` and :meth:`object.__exit__`. A default
   implementation for :meth:`object.__enter__` is provided which returns
   ``self`` while :meth:`object.__exit__` is an abstract method which by default
   returns ``None``. See also the definition of :ref:`typecontextmanager`.

   .. versionadded:: 3.6


.. class:: AbstractAsyncContextManager

   An :term:`abstract base class` for classes that implement
   :meth:`object.__aenter__` and :meth:`object.__aexit__`. A default
   implementation for :meth:`object.__aenter__` is provided which returns
   ``self`` while :meth:`object.__aexit__` is an abstract method which by default
   returns ``None``. See also the definition of
   :ref:`async-context-managers`.

   .. versionadded:: 3.7


.. 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.

   While many objects natively support use in with statements, sometimes a
   resource needs to be managed that isn't a context manager in its own right,
   and doesn't implement a ``close()`` method for use with ``contextlib.closing``

   An abstract example would be the following to ensure correct resource
   management::

      from contextlib import contextmanager

      @contextmanager
      def managed_resource(*args, **kwds):
          # Code to acquire resource, e.g.:
          resource = acquire_resource(*args, **kwds)
          try:
              yield resource
          finally:
              # Code to release resource, e.g.:
              release_resource(resource)

      >>> with managed_resource(timeout=3600) as resource:
      ...     # Resource is released at the end of this block,
      ...     # even if code in the block raises an exception

   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`.


.. decorator:: asynccontextmanager

   Similar to :func:`~contextlib.contextmanager`, but creates an
   :ref:`asynchronous context manager <async-context-managers>`.

   This function is a :term:`decorator` that can be used to define a factory
   function for :keyword:`async with` statement asynchronous context managers,
   without needing to create a class or separate :meth:`__aenter__` and
   :meth:`__aexit__` methods. It must be applied to an :term:`asynchronous
   generator` function.

   A simple example::

      from contextlib import asynccontextmanager

      @asynccontextmanager
      async def get_connection():
          conn = await acquire_db_connection()
          try:
              yield conn
          finally:
              await release_db_connection(conn)

      async def get_all_users():
          async with get_connection() as conn:
              return conn.query('SELECT ...')

   .. versionadded:: 3.7


.. 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.


.. _simplifying-support-for-single-optional-context-managers:

.. function:: nullcontext(enter_result=None)

   Return a context manager that returns *enter_result* from ``__enter__``, but
   otherwise does nothing. It is intended to be used as a stand-in for an
   optional context manager, for example::

      def myfunction(arg, ignore_exceptions=False):
          if ignore_exceptions:
              # Use suppress to ignore all exceptions.
              cm = contextlib.suppress(Exception)
          else:
              # Do not ignore any exceptions, cm has no effect.
              cm = contextlib.nullcontext()
          with cm:
              # Do something

   An example using *enter_result*::

      def process_file(file_or_path):
          if isinstance(file_or_path, str):
              # If string, open file
              cm = open(file_or_path)
          else:
              # Caller is responsible for closing file
              cm = nullcontext(file_or_path)

          with cm as file:
              # Perform processing on the file

   .. versionadded:: 3.7


.. 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:: 3.4


.. 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 an
   :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:: 3.4


.. function:: redirect_stderr(new_target)

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

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

   .. versionadded:: 3.5


.. 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.

.. class:: AsyncExitStack()

   An :ref:`asynchronous context manager <async-context-managers>`, similar
   to :class:`ExitStack`, that supports combining both synchronous and
   asynchronous context managers, as well as having coroutines for
   cleanup logic.

   The :meth:`close` method is not implemented, :meth:`aclose` must be used
   instead.

   .. method:: enter_async_context(cm)

      Similar to :meth:`enter_context` but expects an asynchronous context
      manager.

   .. method:: push_async_exit(exit)

      Similar to :meth:`push` but expects either an asynchronous context manager
      or a coroutine function.

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

      Similar to :meth:`callback` but expects a coroutine function.

   .. method:: aclose()

      Similar to :meth:`close` but properly handles awaitables.

   Continuing the example for :func:`asynccontextmanager`::

      async with AsyncExitStack() as stack:
          connections = [await stack.enter_async_context(get_connection())
              for i in range(5)]
          # All opened connections will automatically be released at the end of
          # the async with statement, even if attempts to open a connection
          # later in the list raise an exception.

   .. versionadded:: 3.7

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.


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, AbstractContextManager, ExitStack

   class ResourceManager(AbstractContextManager):

       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: %s', self.name)

        def __exit__(self, exc_type, exc, exc_tb):
            logging.info('Exiting: %s', self.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:`343` - The "with" statement
      The specification, background, and examples for the Python :keyword:`with`
      statement.

.. _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 are
:func:`suppress` and :func:`redirect_stdout`. Here's a very simple example of
reentrant use::

    >>> from contextlib import redirect_stdout
    >>> from io import StringIO
    >>> stream = StringIO()
    >>> write_to_stream = redirect_stdout(stream)
    >>> with write_to_stream:
    ...     print("This is written to the stream rather than stdout")
    ...     with write_to_stream:
    ...         print("This is also written to the stream")
    ...
    >>> print("This is written directly to stdout")
    This is written directly to stdout
    >>> print(stream.getvalue())
    This is written to the stream rather than stdout
    This is also written to the stream

Real world examples of reentrancy are more likely to involve multiple
functions calling each other and hence be far more complicated than this
example.

Note also that being reentrant is *not* the same thing as being thread safe.
:func:`redirect_stdout`, for example, is definitely not thread safe, as it
makes a global modification to the system state by binding :data:`sys.stdout`
to a different stream.


.. _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.

:class:`threading.Lock` is an example of a reusable, but not reentrant,
context manager (for a reentrant lock, it is necessary to use
:class:`threading.RLock` instead).

Another example of a reusable, but not reentrant, context manager is
:class:`ExitStack`, as it invokes *all* currently registered callbacks
when leaving any with statement, regardless of where those callbacks
were added::

    >>> from contextlib import ExitStack
    >>> stack = ExitStack()
    >>> with stack:
    ...     stack.callback(print, "Callback: from first context")
    ...     print("Leaving first context")
    ...
    Leaving first context
    Callback: from first context
    >>> with stack:
    ...     stack.callback(print, "Callback: from second context")
    ...     print("Leaving second context")
    ...
    Leaving second context
    Callback: from second context
    >>> with stack:
    ...     stack.callback(print, "Callback: from outer context")
    ...     with stack:
    ...         stack.callback(print, "Callback: from inner context")
    ...         print("Leaving inner context")
    ...     print("Leaving outer context")
    ...
    Leaving inner context
    Callback: from inner context
    Callback: from outer context
    Leaving outer context

As the output from the example shows, reusing a single stack object across
multiple with statements works correctly, but attempting to nest them
will cause the stack to be cleared at the end of the innermost with
statement, which is unlikely to be desirable behaviour.

Using separate :class:`ExitStack` instances instead of reusing a single
instance avoids that problem::

    >>> from contextlib import ExitStack
    >>> with ExitStack() as outer_stack:
    ...     outer_stack.callback(print, "Callback: from outer context")
    ...     with ExitStack() as inner_stack:
    ...         inner_stack.callback(print, "Callback: from inner context")
    ...         print("Leaving inner context")
    ...     print("Leaving outer context")
    ...
    Leaving inner context
    Callback: from inner context
    Leaving outer context
    Callback: from outer context