@@ -137,6 +137,28 @@ Functions and classes provided:

``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 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

@@ -433,24 +455,6 @@ 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

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

@@ -5,7 +5,7 @@

from collections import deque

from functools import wraps

__all__ = ["asynccontextmanager", "contextmanager", "closing",

__all__ = ["asynccontextmanager", "contextmanager", "closing", "nullcontext",

"AbstractContextManager", "ContextDecorator", "ExitStack",

"redirect_stdout", "redirect_stderr", "suppress"]

@@ -469,3 +469,24 @@ def _fix_exception_context(new_exc, old_exc):

exc_details[1].__context__ = fixed_ctx

raise

return received_exc and suppressed_exc

class nullcontext(AbstractContextManager):

"""Context manager that does no additional processing.

Used as a stand-in for a normal context manager, when a particular

block of code is only sometimes used with a normal context manager:

cm = optional_cm if condition else nullcontext()

with cm:

# Perform operation, using optional_cm if condition is True

"""

def __init__(self, enter_result=None):

self.enter_result = enter_result

def __enter__(self):

return self.enter_result

def __exit__(self, *excinfo):

pass

@@ -252,6 +252,16 @@ def close(self):

1 / 0

self.assertEqual(state, [1])

class NullcontextTestCase(unittest.TestCase):

def test_nullcontext(self):

class C:

pass

c = C()

with nullcontext(c) as c_in:

self.assertIs(c_in, c)

class FileContextTestCase(unittest.TestCase):

def testWithOpen(self):

@@ -0,0 +1,3 @@

Added *nullcontext* no-op context manager to contextlib. This provides a

simpler and faster alternative to ExitStack() when handling optional context

managers.