bpo-10049: Add a "no-op" (null) context manager to contextlib (GH-4464)
Adds a simpler and faster alternative to ExitStack for handling single optional context managers without having to change the lexical structure of your code.
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@ -137,6 +137,28 @@ Functions and classes provided:
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``page.close()`` will be called when the :keyword:`with` block is exited.
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.. _simplifying-support-for-single-optional-context-managers:
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.. function:: nullcontext(enter_result=None)
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Return a context manager that returns enter_result from ``__enter__``, but
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otherwise does nothing. It is intended to be used as a stand-in for an
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optional context manager, for example::
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def process_file(file_or_path):
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if isinstance(file_or_path, str):
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# If string, open file
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cm = open(file_or_path)
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else:
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# Caller is responsible for closing file
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cm = nullcontext(file_or_path)
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with cm as file:
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# Perform processing on the file
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.. versionadded:: 3.7
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.. function:: suppress(*exceptions)
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Return a context manager that suppresses any of the specified exceptions
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@ -433,24 +455,6 @@ statements to manage arbitrary resources that don't natively support the
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context management protocol.
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Simplifying support for single optional context managers
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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In the specific case of a single optional context manager, :class:`ExitStack`
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instances can be used as a "do nothing" context manager, allowing a context
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manager to easily be omitted without affecting the overall structure of
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the source code::
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def debug_trace(details):
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if __debug__:
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return TraceContext(details)
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# Don't do anything special with the context in release mode
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return ExitStack()
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with debug_trace():
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# Suite is traced in debug mode, but runs normally otherwise
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Catching exceptions from ``__enter__`` methods
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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@ -5,7 +5,7 @@ import _collections_abc
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from collections import deque
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from functools import wraps
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__all__ = ["asynccontextmanager", "contextmanager", "closing",
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__all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext",
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"AbstractContextManager", "ContextDecorator", "ExitStack",
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"redirect_stdout", "redirect_stderr", "suppress"]
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@ -469,3 +469,24 @@ class ExitStack(AbstractContextManager):
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exc_details[1].__context__ = fixed_ctx
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raise
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return received_exc and suppressed_exc
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class nullcontext(AbstractContextManager):
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"""Context manager that does no additional processing.
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Used as a stand-in for a normal context manager, when a particular
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block of code is only sometimes used with a normal context manager:
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cm = optional_cm if condition else nullcontext()
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with cm:
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# Perform operation, using optional_cm if condition is True
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"""
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def __init__(self, enter_result=None):
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self.enter_result = enter_result
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def __enter__(self):
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return self.enter_result
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def __exit__(self, *excinfo):
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pass
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@ -252,6 +252,16 @@ class ClosingTestCase(unittest.TestCase):
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1 / 0
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self.assertEqual(state, [1])
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class NullcontextTestCase(unittest.TestCase):
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def test_nullcontext(self):
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class C:
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pass
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c = C()
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with nullcontext(c) as c_in:
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self.assertIs(c_in, c)
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class FileContextTestCase(unittest.TestCase):
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def testWithOpen(self):
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@ -0,0 +1,3 @@
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Added *nullcontext* no-op context manager to contextlib. This provides a
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simpler and faster alternative to ExitStack() when handling optional context
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managers.
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