[Python-3000] Is this a bug with list comprehensions or not?

Terry Reedy tjreedy at udel.edu
Sat Jul 12 06:09:19 CEST 2008

Nick Coghlan wrote:
> Carl Johnson wrote:
>> Certainly, it's an inconsistency compared to generator expressions, 
>> but is it a bug? I know that Python 3000 is making it so that list 
>> comprehension variables don't leak anymore, so I take it that the goal 
>> is to have them be more similar to generator expressions than not. 
>> Then again, maybe there are good reasons for this behavior? My own 
>> feeling is that these two cases should behave the same way, but I 
>> could be wrong.
> Comprehensions and generator expressions in Py3k are more similar than 
> they are in 2.x, but they still aren't identical.
> [x for x in range(10) if f(x)] is shorthand for:
>   def _f(itr):
>       _[1] = []
>       for x in itr:
>           if f(x):
>               _[1].append(x)
>       return _[1]
>   expr_result = _f(range(10))
> While (x for x in range(10) if f(x)) is shorthand for:
>   def _g(itr):
>       for x in itr:
>           if f(x):
>               yield x
>   expr_result = _g(range(10))
>  From those expansions, I think it's pretty clear that the implicit 
> generator function in the second case is going to swallow the 
> StopIteration and treat it as the end of the iteration process, but no 
> such thing is going to happen with the implicit standard function in the 
> list comprehension case.

The idea I had, and which is what I believe Adam Olsen posted, is to 
augment the genexp to
    def _g(itr):
        for x in itr:
            if f(x):
                yield x
      except StopIteration: # possibly add 'as si'
        raise RuntimeError(<msg>) # <msg> might incorporate si msg

To me, it is more important that list(genexp) == corresponding_listcomp 
than that the definition of genexp be minimal.


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