[Python-ideas] "While" suggestion

[Boris Borcic]

Terry Reedy wrote:
> 
>>
>>  >>> def yet(b) :
>>     if b : raise StopIteration
>>
>>      >>> list(x for x in range(0,10) if not yet(x>4))
>> [0, 1, 2, 3, 4]
> 
> I don't think it a good idea to abuse StopIteration in this way.  It is 
> intended only for use in iterators. The obvious list comp abbreviation 
> does not work in CPython 3.0.
> 
>  >>> [x for x in range(0,10) if not yet(x>4)]
> Traceback (most recent call last):
>   File "<pyshell#4>", line 1, in <module>
>     [x for x in range(0,10) if not yet(x>4)]
>   File "<pyshell#4>", line 1, in <listcomp>
>     [x for x in range(0,10) if not yet(x>4)]
>   File "<pyshell#1>", line 2, in yet
>     if b : raise StopIteration
> StopIteration
> 
> So it breaks the intended identity in Py3: list(genexp) == [genexp].

FYI, there is nothing specific to Py3, Python 2.5 and 2.4 behave the same.

> 
> This was discussed on the Py3 dev list and it was agreed the difference 
> in behavior was a bug but would be difficult to impossible to fix 
> without performance degradation and unnecessary to fix because the 
> problem only arises if someone uses StopIteration outside its documented 
> usage in iterators.

Whatever, consistency with Python 2+ is maintained.

Cheers, BB