mirror of
https://github.com/python/cpython.git
synced 2025-11-04 15:41:43 +00:00
255 lines
8.1 KiB
Python
255 lines
8.1 KiB
Python
"""Utilities for with-statement contexts. See PEP 343."""
|
|
|
|
import sys
|
|
from collections import deque
|
|
from functools import wraps
|
|
|
|
__all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack"]
|
|
|
|
|
|
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
|
|
|
|
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")
|
|
|
|
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()
|
|
|
|
|
|
# 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):
|
|
# 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):
|
|
while 1:
|
|
exc_context = new_exc.__context__
|
|
if exc_context in (None, frame_exc):
|
|
break
|
|
new_exc = exc_context
|
|
new_exc.__context__ = old_exc
|
|
|
|
# Callbacks are invoked in LIFO order to match the behaviour of
|
|
# nested context managers
|
|
suppressed_exc = False
|
|
while self._exit_callbacks:
|
|
cb = self._exit_callbacks.pop()
|
|
try:
|
|
if cb(*exc_details):
|
|
suppressed_exc = True
|
|
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])
|
|
if not self._exit_callbacks:
|
|
raise
|
|
exc_details = new_exc_details
|
|
return suppressed_exc
|