Close #13585: add contextlib.ExitStack to replace the ill-fated contextlib.nested API
diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index 2f8f00d..ead1155 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -1,9 +1,10 @@
"""Utilities for with-statement contexts. See PEP 343."""
import sys
+from collections import deque
from functools import wraps
-__all__ = ["contextmanager", "closing", "ContextDecorator"]
+__all__ = ["contextmanager", "closing", "ContextDecorator", "ExitStack"]
class ContextDecorator(object):
@@ -12,12 +13,12 @@
def _recreate_cm(self):
"""Return a recreated instance of self.
- Allows otherwise one-shot context managers like
+ Allows an otherwise one-shot context manager like
_GeneratorContextManager to support use as
- decorators via implicit recreation.
+ a decorator via implicit recreation.
- Note: this is a private interface just for _GCM in 3.2 but will be
- renamed and documented for third party use in 3.3
+ This is a private interface just for _GeneratorContextManager.
+ See issue #11647 for details.
"""
return self
@@ -138,3 +139,118 @@
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 throw 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):
+ if not self._exit_callbacks:
+ return
+ # This looks complicated, but it is really just
+ # setting up a chain of try-expect statements to ensure
+ # that outer callbacks still get invoked even if an
+ # inner one throws an exception
+ def _invoke_next_callback(exc_details):
+ # Callbacks are removed from the list in FIFO order
+ # but the recursion means they're invoked in LIFO order
+ cb = self._exit_callbacks.popleft()
+ if not self._exit_callbacks:
+ # Innermost callback is invoked directly
+ return cb(*exc_details)
+ # More callbacks left, so descend another level in the stack
+ try:
+ suppress_exc = _invoke_next_callback(exc_details)
+ except:
+ suppress_exc = cb(*sys.exc_info())
+ # Check if this cb suppressed the inner exception
+ if not suppress_exc:
+ raise
+ else:
+ # Check if inner cb suppressed the original exception
+ if suppress_exc:
+ exc_details = (None, None, None)
+ suppress_exc = cb(*exc_details) or suppress_exc
+ return suppress_exc
+ # Kick off the recursive chain
+ return _invoke_next_callback(exc_details)