contextlib: re-instate ExitStack from CPython 3.4

contextlib/contextlib.py

@@ -8,6 +8,8 @@ Not implemented:
 
 """
 
+import sys
+from collections import deque
 from ucontextlib import *
 
 
@@ -64,3 +66,134 @@ class suppress:
         #
         # 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