[Python-ideas] "While" suggestion

[Terry Reedy]

Boris Borcic wrote:
> Andrew Toulouse wrote:
>> If I'm understanding you correctly, this would address something I've 
>> wanted list comprehensions to do for a little while:
>>
>> [x for x in range(0,10) until greaterthanN(4,x)]
> 
> Note that you can do something like
> 
>  >>> 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].

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.