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ucontextlib.py
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"""Utilities for with-statement contexts. See PEP 343.
Original source code: https://hg.python.org/cpython/file/3.4/Lib/contextlib.py
Not implemented:
- redirect_stdout;
- ExitStack.
- closing
- supress
"""
class ContextDecorator(object):
"A base class or mixin that enables context managers to work as decorators."
def _recreate_cm(self):
"""Return a recreated instance of self.
Allows an otherwise one-shot context manager like
_GeneratorContextManager to support use as
a decorator via implicit recreation.
This is a private interface just for _GeneratorContextManager.
See issue #11647 for details.
"""
return self
def __call__(self, func):
def inner(*args, **kwds):
with self._recreate_cm():
return func(*args, **kwds)
return inner
class _GeneratorContextManager(ContextDecorator):
"""Helper for @contextmanager decorator."""
def __init__(self, func, *args, **kwds):
self.gen = func(*args, **kwds)
self.func, self.args, self.kwds = func, args, kwds
def _recreate_cm(self):
# _GCM instances are one-shot context managers, so the
# CM must be recreated each time a decorated function is
# called
return self.__class__(self.func, *self.args, **self.kwds)
def __enter__(self):
try:
return next(self.gen)
except StopIteration:
raise RuntimeError("generator didn't yield") from None
def __exit__(self, type, value, traceback):
if type is None:
try:
next(self.gen)
except StopIteration:
return
else:
raise RuntimeError("generator didn't stop")
else:
if value is None:
# Need to force instantiation so we can reliably
# tell if we get the same exception back
value = type()
try:
self.gen.throw(type, value, traceback)
raise RuntimeError("generator didn't stop after throw()")
except StopIteration as exc:
# Suppress the exception *unless* it's the same exception that
# was passed to throw(). This prevents a StopIteration
# raised inside the "with" statement from being suppressed
return exc is not value
def contextmanager(func):
"""@contextmanager decorator.
Typical usage:
@contextmanager
def some_generator(<arguments>):
<setup>
try:
yield <value>
finally:
<cleanup>
This makes this:
with some_generator(<arguments>) as <variable>:
<body>
equivalent to this:
<setup>
try:
<variable> = <value>
<body>
finally:
<cleanup>
"""
def helper(*args, **kwds):
return _GeneratorContextManager(func, *args, **kwds)
return helper