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

[Michael Foord]

Aahz wrote:
> On Sun, Mar 15, 2009, Michael Foord wrote:
>   
>> Aahz wrote:
>>     
>>> On Sun, Mar 15, 2009, Michael Foord 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).
>>>>
>>>> Isn't it better practise for exceptions to be used for exceptional   
>>>> circumstances rather than for control flow?
>>>>         
>>> It seems to me that we as a development community already made a decision
>>> when we switched to StopIteration as the primary mechanism for halting
>>> ``for`` loops.  (Not that it was really a new decision because parts of
>>> the Python community have always advocated using exceptions for control
>>> flow, but the ``for`` loop enshrines it.)  I doubt that using exceptions
>>> for control flow in ``with`` blocks will cause anywhere near so much a
>>> performance degradation.
>>>       
>>   
>> Well, StopIteration is still an implementation detail that only
>> occasionally bleeds through to actual programming. It says nothing
>> about whether using exceptions for non-exceptional circumstances
>> (control flow) is good practise. Personally I think it makes the
>> intent of code less easy to understand - in effect the exceptions
>> *are* being used as a goto.
>>     
>
> Let me know how you'd rewrite this more clearly without a control-flow
> exception:
>
>         try:
>             for field in curr_fields:
>                 for item in record[field]:
>                     item = item.lower()
>                     for filter in excludes:
>                         if match(item, filter):
>                             raise Excluded
>         except Excluded: 
>             continue 
>
> This is pretty much the canonical example showing why control-flow
> exceptions are a Good Thing.  They're a *structured* goto.
>   

You didn't include all the code - so impossible to match the exact 
semantics. Breaking out of multiple loops with a return is a cleaner way 
to handle it IMHO.


def find_excludes():
    for field in curr_fields:
        for item in record[field]:
            item = item.lower()
            for filter in excludes:
                if match(item, filter):
                    return
                   
while something:
    find_excludes()

Michael

-- 
http://www.ironpythoninaction.com/
http://www.voidspace.org.uk/blog