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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