[Python-Dev] Rewrite @contextlib.contextmanager in C (original) (raw)

Wolfgang Maier wolfgang.maier at biologie.uni-freiburg.de
Mon Aug 8 17:14:59 EDT 2016

On 8/8/2016 22:38, Yury Selivanov wrote:

On 2016-08-08 4:18 PM, Guido van Rossum wrote: I think Nick would be interested in understanding why this is the case. What does the decorator do that could be so expensive? From the looks of it it doesn't do anything special. Although with @contextlib.contextmanager we have to instantiate a generator (the decorated one) and advance it in enter. So it's an extra object instantiation + extra code in enter and exit. Anyways, Nick knows much more about that code.

Right, I think a fairer comparison would be to:

class ctx2: def enter(self): self.it = iter(self) return next(self.it)

 def __exit__(self, *args):
     try:
         next(self.it)
     except StopIteration:
         pass

 def __iter__(self):
     yield

With this change alone the slowdown diminishes to ~ 1.7x for me. The rest is probably the extra overhead for being able to pass exceptions raised inside the with block back into the generator and such.

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