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"""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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from warnings import warn

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__all__ = ["contextmanager", "nested", "closing"]

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class GeneratorContextManager(object):

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"""Helper for @contextmanager decorator."""

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def __init__(self, gen):

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self.gen = gen

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def __enter__(self):

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try:

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return self.gen.next()

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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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self.gen.next()

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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, 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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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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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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<body>

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finally:

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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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@contextmanager

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def nested(*managers):

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"""Combine multiple context managers into a single nested context manager.

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This function has been deprecated in favour of the multiple manager form

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of the with statement.

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The one advantage of this function over the multiple manager form of the

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with statement is that argument unpacking allows it to be

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used with a variable number of context managers as follows:

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with nested(*managers):

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do_something()

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"""

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warn("With-statements now directly support multiple context managers",

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DeprecationWarning, 3)

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exits = []

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vars = []

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exc = (None, None, None)

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try:

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for mgr in managers:

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exit = mgr.__exit__

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enter = mgr.__enter__

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vars.append(enter())

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exits.append(exit)

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yield vars

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except:

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exc = sys.exc_info()

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finally:

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while exits:

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exit = exits.pop()

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try:

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if exit(*exc):

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exc = (None, None, None)

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except:

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exc = sys.exc_info()

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if exc != (None, None, None):

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# Don't rely on sys.exc_info() still containing

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# the right information. Another exception may

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# have been raised and caught by an exit method

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raise exc[0], exc[1], exc[2]

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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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