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[Python-Dev] Rewrite @contextlib.contextmanager in C

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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