[Python-checkins] bpo-10049: Add a "no-op" (null) context manager to contextlib (GH-4464)
Nick Coghlan
webhook-mailer at python.org
Wed Nov 22 19:23:31 EST 2017
https://github.com/python/cpython/commit/0784a2e5b174d2dbf7b144d480559e650c5cf64c
commit: 0784a2e5b174d2dbf7b144d480559e650c5cf64c
branch: master
author: Jesse-Bakker <jessebakker00 at gmail.com>
committer: Nick Coghlan <ncoghlan at gmail.com>
date: 2017-11-23T10:23:28+10:00
summary:
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.
files:
A Misc/NEWS.d/next/Library/2017-11-22-17-21-01.bpo-10049.ttsBqb.rst
M Doc/library/contextlib.rst
M Lib/contextlib.py
M Lib/test/test_contextlib.py
diff --git a/Doc/library/contextlib.rst b/Doc/library/contextlib.rst
index 19793693b7b..48ca0da6b95 100644
--- a/Doc/library/contextlib.rst
+++ b/Doc/library/contextlib.rst
@@ -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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index 962cedab490..c1f8a84617f 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -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
diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py
index 64b6578ff94..1a5e6edad9b 100644
--- a/Lib/test/test_contextlib.py
+++ b/Lib/test/test_contextlib.py
@@ -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):
diff --git a/Misc/NEWS.d/next/Library/2017-11-22-17-21-01.bpo-10049.ttsBqb.rst b/Misc/NEWS.d/next/Library/2017-11-22-17-21-01.bpo-10049.ttsBqb.rst
new file mode 100644
index 00000000000..b6153c235d0
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-11-22-17-21-01.bpo-10049.ttsBqb.rst
@@ -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.
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