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a7e820a408
terminology in the alpha 1 documentation.
- "context manager" reverts to its alpha 1 definition
- the term "context specifier" goes away entirely
- contextlib.GeneratorContextManager is renamed GeneratorContext
There are still a number of changes relative to alpha 1:
- the expression in the with statement is explicitly called the
"context expression" in the language reference
- the terms 'with statement context', 'context object' or 'with
statement context' are used in several places instead of a bare
'context'. The aim of this is to avoid ambiguity in relation to the
runtime context set up when the block is executed, and the context
objects that already exist in various application domains (such as
decimal.Context)
- contextlib.contextmanager is renamed to contextfactory
This best reflects the nature of the function resulting from the
use of that decorator
- decimal.ContextManager is renamed to WithStatementContext
Simple dropping the 'Manager' part wasn't possible due to the
fact that decimal.Context already exists and means something
different. WithStatementContext is ugly but workable.
A technically unrelated change snuck into this commit:
contextlib.closing now avoids the overhead of creating a
generator, since it's trivial to implement that particular
context manager directly.
160 lines
4.1 KiB
Python
160 lines
4.1 KiB
Python
"""Utilities for with-statement contexts. See PEP 343."""
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import sys
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__all__ = ["contextfactory", "nested", "closing"]
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class GeneratorContext(object):
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"""Helper for @contextfactory 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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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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#
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if sys.exc_info()[1] is not value:
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raise
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def contextfactory(func):
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"""@contextfactory 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 GeneratorContext(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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helper.__dict__ = func.__dict__
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except:
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pass
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return helper
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@contextfactory
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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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exc = (None, None, None)
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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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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 __context__(self):
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return self
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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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