"""Utilities for with-statement contexts. See PEP 343."""
import sys
from collections import deque
from functools import wraps
__all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack",
"redirect_stdout", "suppress"]
class ContextDecorator(object):
"A base class or mixin that enables context managers to work as decorators."
def _recreate_cm(self):
"""Return a recreated instance of self.
Allows an otherwise one-shot context manager like
_GeneratorContextManager to support use as
a decorator via implicit recreation.
This is a private interface just for _GeneratorContextManager.
See issue #11647 for details.
"""
return self
def __call__(self, func):
@wraps(func)
def inner(*args, **kwds):
with self._recreate_cm():
return func(*args, **kwds)
return inner
class _GeneratorContextManager(ContextDecorator):
"""Helper for @contextmanager decorator."""
def __init__(self, func, args, kwds):
self.gen = func(*args, **kwds)
self.func, self.args, self.kwds = func, args, kwds
# Issue 19330: ensure context manager instances have good docstrings
doc = getattr(func, "__doc__", None)
if doc is None:
doc = type(self).__doc__
self.__doc__ = doc
# Unfortunately, this still doesn't provide good help output when
# inspecting the created context manager instances, since pydoc
# currently bypasses the instance docstring and shows the docstring
# for the class instead.
# See http://bugs.python.org/issue19404 for more details.
def _recreate_cm(self):
# _GCM instances are one-shot context managers, so the
# CM must be recreated each time a decorated function is
# called
return self.__class__(self.func, self.args, self.kwds)
def __enter__(self):
try:
return next(self.gen)
except StopIteration:
raise RuntimeError("generator didn't yield") from None
def __exit__(self, type, value, traceback):
if type is None:
try:
next(self.gen)
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 as 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:
<cleanup>
This makes this:
with some_generator(<arguments>) as <variable>:
<body>
equivalent to this:
<setup>
try:
<variable> = <value>
<body>
finally:
<cleanup>
"""
@wraps(func)
def helper(*args, **kwds):
return _GeneratorContextManager(func, args, kwds)
return helper
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()
class redirect_stdout:
"""Context manager for temporarily redirecting stdout to another file
# How to send help() to stderr
with redirect_stdout(sys.stderr):
help(dir)
# How to write help() to a file
with open('help.txt', 'w') as f:
with redirect_stdout(f):
help(pow)
"""
def __init__(self, new_target):
self._new_target = new_target
# We use a list of old targets to make this CM re-entrant
self._old_targets = []
def __enter__(self):
self._old_targets.append(sys.stdout)
sys.stdout = self._new_target
return self._new_target
def __exit__(self, exctype, excinst, exctb):
sys.stdout = self._old_targets.pop()
class suppress:
"""Context manager to suppress specified exceptions
After the exception is suppressed, execution proceeds with the next
statement following the with statement.
with suppress(FileNotFoundError):
os.remove(somefile)
# Execution still resumes here if the file was already removed
"""
def __init__(self, *exceptions):
self._exceptions = exceptions
def __enter__(self):
pass
def __exit__(self, exctype, excinst, exctb):
# Unlike isinstance and issubclass, CPython exception handling
# currently only looks at the concrete type hierarchy (ignoring
# the instance and subclass checking hooks). While Guido considers
# that a bug rather than a feature, it's a fairly hard one to fix
# due to various internal implementation details. suppress provides
# the simpler issubclass based semantics, rather than trying to
# exactly reproduce the limitations of the CPython interpreter.
#
# See http://bugs.python.org/issue12029 for more details
return exctype is not None and issubclass(exctype, self._exceptions)
# Inspired by discussions on http://bugs.python.org/issue13585
class ExitStack(object):
"""Context manager for dynamic management of a stack of exit callbacks
For example:
with ExitStack() as stack:
files = [stack.enter_context(open(fname)) for fname in filenames]
# All opened files will automatically be closed at the end of
# the with statement, even if attempts to open files later
# in the list raise an exception
"""
def __init__(self):
self._exit_callbacks = deque()
def pop_all(self):
"""Preserve the context stack by transferring it to a new instance"""
new_stack = type(self)()
new_stack._exit_callbacks = self._exit_callbacks
self._exit_callbacks = deque()
return new_stack
def _push_cm_exit(self, cm, cm_exit):
"""Helper to correctly register callbacks to __exit__ methods"""
def _exit_wrapper(*exc_details):
return cm_exit(cm, *exc_details)
_exit_wrapper.__self__ = cm
self.push(_exit_wrapper)
def push(self, exit):
"""Registers a callback with the standard __exit__ method signature
Can suppress exceptions the same way __exit__ methods can.
Also accepts any object with an __exit__ method (registering a call
to the method instead of the object itself)
"""
# We use an unbound method rather than a bound method to follow
# the standard lookup behaviour for special methods
_cb_type = type(exit)
try:
exit_method = _cb_type.__exit__
except AttributeError:
# Not a context manager, so assume its a callable
self._exit_callbacks.append(exit)
else:
self._push_cm_exit(exit, exit_method)
return exit # Allow use as a decorator
def callback(self, callback, *args, **kwds):
"""Registers an arbitrary callback and arguments.
Cannot suppress exceptions.
"""
def _exit_wrapper(exc_type, exc, tb):
callback(*args, **kwds)
# We changed the signature, so using @wraps is not appropriate, but
# setting __wrapped__ may still help with introspection
_exit_wrapper.__wrapped__ = callback
self.push(_exit_wrapper)
return callback # Allow use as a decorator
def enter_context(self, cm):
"""Enters the supplied context manager
If successful, also pushes its __exit__ method as a callback and
returns the result of the __enter__ method.
"""
# We look up the special methods on the type to match the with statement
_cm_type = type(cm)
_exit = _cm_type.__exit__
result = _cm_type.__enter__(cm)
self._push_cm_exit(cm, _exit)
return result
def close(self):
"""Immediately unwind the context stack"""
self.__exit__(None, None, None)
def __enter__(self):
return self
def __exit__(self, *exc_details):
received_exc = exc_details[0] is not None
# We manipulate the exception state so it behaves as though
# we were actually nesting multiple with statements
frame_exc = sys.exc_info()[1]
def _fix_exception_context(new_exc, old_exc):
# Context may not be correct, so find the end of the chain
while 1:
exc_context = new_exc.__context__
if exc_context is old_exc:
# Context is already set correctly (see issue 20317)
return
if exc_context is None or exc_context is frame_exc:
break
new_exc = exc_context
# Change the end of the chain to point to the exception
# we expect it to reference
new_exc.__context__ = old_exc
# Callbacks are invoked in LIFO order to match the behaviour of
# nested context managers
suppressed_exc = False
pending_raise = False
while self._exit_callbacks:
cb = self._exit_callbacks.pop()
try:
if cb(*exc_details):
suppressed_exc = True
pending_raise = False
exc_details = (None, None, None)
except:
new_exc_details = sys.exc_info()
# simulate the stack of exceptions by setting the context
_fix_exception_context(new_exc_details[1], exc_details[1])
pending_raise = True
exc_details = new_exc_details
if pending_raise:
try:
# bare "raise exc_details[1]" replaces our carefully
# set-up context
fixed_ctx = exc_details[1].__context__
raise exc_details[1]
except BaseException:
exc_details[1].__context__ = fixed_ctx
raise
return received_exc and suppressed_exc
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