On Tue, Aug 9, 2016 at 3:30 PM, Nick Coghlan <ncogh...@gmail.com> wrote:
> On 9 August 2016 at 23:26, Nick Coghlan <ncogh...@gmail.com> wrote: > >> On 9 August 2016 at 06:18, Guido van Rossum <gu...@python.org> wrote: >> >>> I think Nick would be interested in understanding why this is the case. >>> What does the decorator do that could be so expensive? >>> >> >> Reviewing https://hg.python.org/cpython/file/default/Lib/contextlib.py >> #l57, Chris's analysis seems plausible to me >> > > Sorry Wolfgang - I missed that Chris was expanding on a comparison you > initially made! > > Either way, I agree that aspect does make up the bulk of the difference in > speed, so moving to C likely wouldn't help much. However, the speed > difference relative to the simpler warppers is far less defensible - I > think there are some opportunities for improvement there, especially around > moving introspection work out of _GeneratorContextManager.__init__ and > into the contextmanager decorator itself. > By moving the __doc__ introspection out of __init__ and by introducing __slots__ I got from -4.37x to -3.16x (__slot__ speedup was about +0.3x). Chris' SimplerContextManager solution is faster because it avoids the factory function but that is necessary for supporting the decoration of methods. Here's a PoC: diff --git a/Lib/contextlib.py b/Lib/contextlib.py index 7d94a57..45270dd 100644 --- a/Lib/contextlib.py +++ b/Lib/contextlib.py @@ -2,7 +2,7 @@ import abc import sys from collections import deque -from functools import wraps +from functools import wraps, update_wrapper __all__ = ["contextmanager", "closing", "AbstractContextManager", "ContextDecorator", "ExitStack", "redirect_stdout", @@ -57,25 +57,18 @@ class ContextDecorator(object): class _GeneratorContextManager(ContextDecorator, AbstractContextManager): """Helper for @contextmanager decorator.""" + __slots__ = ['gen', 'funcak'] + 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. + self.funcak = 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) + func, args, kwds = self.funcak + return self.__class__(func, args, kwds) def __enter__(self): try: @@ -157,6 +150,8 @@ def contextmanager(func): @wraps(func) def helper(*args, **kwds): return _GeneratorContextManager(func, args, kwds) + + update_wrapper(helper, func) return helper -- Giampaolo - http://grodola.blogspot.com
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