python/Lib/contextlib.py at master · pylee/python · GitHub

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"""Utilities for with-statement contexts. See PEP 343."""

import sys

from functools import wraps

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

class GeneratorContextManager(object):

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

def __init__(self, gen):

self.gen = gen

def __enter__(self):

try:

return self.gen.next()

except StopIteration:

raise RuntimeError("generator didn't yield")

def __exit__(self, type, value, traceback):

if type is None:

try:

self.gen.next()

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, 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

except:

# only re-raise if it's *not* the exception that was

# passed to throw(), because __exit__() must not raise

# an exception unless __exit__() itself failed. But throw()

# has to raise the exception to signal propagation, so this

# fixes the impedance mismatch between the throw() protocol

# and the __exit__() protocol.

if sys.exc_info()[1] is not value:

raise

def contextmanager(func):

"""@contextmanager decorator.

Typical usage:

@contextmanager

def some_generator(<arguments>):

<setup>

try:

yield <value>

finally:

This makes this:

with some_generator(<arguments>) as <variable>:

<body>

equivalent to this:

<setup>

try:

<body>

finally:

"""

@wraps(func)

def helper(*args, **kwds):

return GeneratorContextManager(func(*args, **kwds))

return helper

@contextmanager

def nested(*managers):

"""Support multiple context managers in a single with-statement.

Code like this:

with nested(A, B, C) as (X, Y, Z):

<body>

is equivalent to this:

with A as X:

with B as Y:

with C as Z:

<body>

"""

exits = []

vars = []

exc = (None, None, None)

try:

for mgr in managers:

exit = mgr.__exit__

enter = mgr.__enter__

vars.append(enter())

exits.append(exit)

yield vars

except:

exc = sys.exc_info()

finally:

while exits:

exit = exits.pop()

try:

if exit(*exc):

exc = (None, None, None)

except:

exc = sys.exc_info()

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

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

# the right information. Another exception may

# have been raised and caught by an exit method

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

class closing(object):

"""Context to automatically close something at the end of a block.

Code like this:

with closing(<module>.open(<arguments>)) as f:

<block>

is equivalent to this:

f = <module>.open(<arguments>)

try:

<block>

finally:

f.close()

"""

def __init__(self, thing):

self.thing = thing

def __enter__(self):

return self.thing

def __exit__(self, *exc_info):

self.thing.close()

python/Lib/contextlib.py at master · pylee/python · GitHub

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