diff options
| -rw-r--r-- | Doc/library/contextlib.rst | 31 | ||||
| -rw-r--r-- | Lib/contextlib.py | 40 | ||||
| -rw-r--r-- | Lib/test/test_contextlib.py | 9 | ||||
| -rw-r--r-- | Misc/NEWS | 2 |
4 files changed, 81 insertions, 1 deletions
diff --git a/Doc/library/contextlib.rst b/Doc/library/contextlib.rst index 349b805..4b86755 100644 --- a/Doc/library/contextlib.rst +++ b/Doc/library/contextlib.rst @@ -115,6 +115,37 @@ Functions and classes provided: .. 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) + + .. versionadded:: 3.4 + .. class:: ContextDecorator() A base class that enables a context manager to also be used as a decorator. diff --git a/Lib/contextlib.py b/Lib/contextlib.py index aaab095..868fa6c 100644 --- a/Lib/contextlib.py +++ b/Lib/contextlib.py @@ -4,7 +4,8 @@ import sys from collections import deque from functools import wraps -__all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack", "ignored"] +__all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack", + "ignored", "redirect_stdout"] class ContextDecorator(object): @@ -140,6 +141,43 @@ class closing(object): def __exit__(self, *exc_info): self.thing.close() +class redirect_stdout: + """Context manager for temporarily redirecting stdout to another file + + # How to send help() to stderr + + with redirect_stdout(sys.stderr): + help(dir) + + # How to write help() to a file + + with open('help.txt', 'w') as f: + with redirect_stdout(f): + help(pow) + + # How to capture disassembly to a string + + import dis + import io + + f = io.StringIO() + with redirect_stdout(f): + dis.dis('x**2 - y**2') + s = f.getvalue() + + """ + + def __init__(self, new_target): + self.new_target = new_target + + def __enter__(self): + self.old_target = sys.stdout + sys.stdout = self.new_target + return self.new_target + + def __exit__(self, exctype, excinst, exctb): + sys.stdout = self.old_target + @contextmanager def ignored(*exceptions): """Context manager to ignore specified exceptions diff --git a/Lib/test/test_contextlib.py b/Lib/test/test_contextlib.py index 2892917..d8a0530 100644 --- a/Lib/test/test_contextlib.py +++ b/Lib/test/test_contextlib.py @@ -1,5 +1,6 @@ """Unit tests for contextlib.py, and other context managers.""" +import io import sys import tempfile import unittest @@ -653,6 +654,14 @@ class TestIgnored(unittest.TestCase): with ignored(LookupError): 'Hello'[50] +class TestRedirectStdout(unittest.TestCase): + + def test_redirect_to_string_io(self): + f = io.StringIO() + with redirect_stdout(f): + help(pow) + s = f.getvalue() + self.assertIn('pow', s) if __name__ == "__main__": unittest.main() @@ -32,6 +32,8 @@ Library - Issue #19158: a rare race in BoundedSemaphore could allow .release() too often. +- Issue #15805: Add contextlib.redirect_stdout(). + - Issue #18716: Deprecate the formatter module. - Issue #18037: 2to3 now escapes '\u' and '\U' in native strings. |