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140 lines
4 KiB
Python
140 lines
4 KiB
Python
"""Utilities for with-statement contexts. See PEP 343."""
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import sys
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from functools import wraps
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__all__ = ["contextmanager", "closing", "ContextDecorator"]
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class ContextDecorator(object):
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"A base class or mixin that enables context managers to work as decorators."
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def _recreate_cm(self):
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"""Return a recreated instance of self.
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Allows otherwise one-shot context managers like
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_GeneratorContextManager to support use as
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decorators via implicit recreation.
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Note: this is a private interface just for _GCM in 3.2 but will be
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renamed and documented for third party use in 3.3
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"""
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return self
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def __call__(self, func):
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@wraps(func)
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def inner(*args, **kwds):
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with self._recreate_cm():
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return func(*args, **kwds)
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return inner
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class _GeneratorContextManager(ContextDecorator):
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"""Helper for @contextmanager decorator."""
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def __init__(self, func, *args, **kwds):
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self.gen = func(*args, **kwds)
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self.func, self.args, self.kwds = func, args, kwds
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def _recreate_cm(self):
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# _GCM instances are one-shot context managers, so the
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# CM must be recreated each time a decorated function is
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# called
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return self.__class__(self.func, *self.args, **self.kwds)
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def __enter__(self):
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try:
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return next(self.gen)
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except StopIteration:
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raise RuntimeError("generator didn't yield")
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def __exit__(self, type, value, traceback):
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if type is None:
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try:
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next(self.gen)
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except StopIteration:
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return
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else:
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raise RuntimeError("generator didn't stop")
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else:
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if value is None:
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# Need to force instantiation so we can reliably
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# tell if we get the same exception back
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value = type()
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try:
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self.gen.throw(type, value, traceback)
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raise RuntimeError("generator didn't stop after throw()")
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except StopIteration as exc:
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# Suppress the exception *unless* it's the same exception that
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# was passed to throw(). This prevents a StopIteration
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# raised inside the "with" statement from being suppressed
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return exc is not value
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except:
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# only re-raise if it's *not* the exception that was
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# passed to throw(), because __exit__() must not raise
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# an exception unless __exit__() itself failed. But throw()
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# has to raise the exception to signal propagation, so this
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# fixes the impedance mismatch between the throw() protocol
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# and the __exit__() protocol.
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#
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if sys.exc_info()[1] is not value:
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raise
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def contextmanager(func):
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"""@contextmanager decorator.
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Typical usage:
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@contextmanager
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def some_generator(<arguments>):
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<setup>
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try:
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yield <value>
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finally:
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<cleanup>
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This makes this:
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with some_generator(<arguments>) as <variable>:
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<body>
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equivalent to this:
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<setup>
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try:
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<variable> = <value>
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<body>
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finally:
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<cleanup>
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"""
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@wraps(func)
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def helper(*args, **kwds):
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return _GeneratorContextManager(func, *args, **kwds)
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return helper
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class closing(object):
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"""Context to automatically close something at the end of a block.
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Code like this:
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with closing(<module>.open(<arguments>)) as f:
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<block>
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is equivalent to this:
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f = <module>.open(<arguments>)
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try:
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<block>
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finally:
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f.close()
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"""
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def __init__(self, thing):
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self.thing = thing
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def __enter__(self):
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return self.thing
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def __exit__(self, *exc_info):
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self.thing.close()
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