[Python-Dev] PEP 377 - allow __enter__() methods to skip the statement body

[Nick Coghlan]

Michael Foord wrote:
> Nick Coghlan wrote:
>>> Note that using exceptions for control flow can  be bad for other
>>> implementations of Python. For example exceptions on the .NET framework
>>> are very expensive. (Although there are workarounds such as not really
>>> raising the exception - but they're ugly).
>>>     
>>
>> Is it that exceptions are expensive, or setting up a try/except block is
>> expensive? The reason the SkipStatement idea is tenable at all (even in
>> CPython) is that try/except is fairly cheap when no exception is raised.
>>   
> 
> It is the raising of the exception that is expensive.

Then that isn't a huge drawback in this case - the SkipStatement
exception is only used in situations which would currently probably be
handled by raising an exception anyway (e.g. the change to
contextlib.contextmanager.__enter__() in the patch is to raise
SkipStatement where it currently raises RuntimeError).

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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