summaryrefslogtreecommitdiffstats
path: root/Doc/library/contextlib.rst
blob: 82efd0cca15126f78806a4d18c5ba756e5371e9c (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
: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.


.. 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 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:`reusable but not reentrant <reusable-cms>`.

   .. versionadded:: 3.4


.. 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.

.. _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