mirror of https://github.com/python/cpython
142 lines
3.0 KiB
Python
142 lines
3.0 KiB
Python
"""Utilities for with-statement contexts. See PEP 343."""
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import sys
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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 __context__(self):
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return self
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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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try:
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self.gen.throw(type, value, traceback)
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except StopIteration:
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pass
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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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def helper(*args, **kwds):
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return GeneratorContextManager(func(*args, **kwds))
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try:
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helper.__name__ = func.__name__
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helper.__doc__ = func.__doc__
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except:
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pass
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return helper
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@contextmanager
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def nested(*contexts):
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"""Support multiple context managers in a single with-statement.
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Code like this:
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with nested(A, B, C) as (X, Y, Z):
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<body>
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is equivalent to this:
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with A as X:
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with B as Y:
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with C as Z:
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<body>
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"""
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exits = []
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vars = []
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try:
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try:
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for context in contexts:
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mgr = context.__context__()
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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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else:
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exc = (None, None, None)
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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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exit(*exc)
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except:
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exc = sys.exc_info()
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else:
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exc = (None, None, None)
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if exc != (None, None, None):
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raise
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@contextmanager
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def closing(thing):
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"""Context manager 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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try:
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yield thing
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finally:
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thing.close()
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