[Cython] Fix for #196 (for loop bug)

[Dag Sverre Seljebotn]

Stefan Behnel wrote:
> Hi,
> 
> Dag Sverre Seljebotn wrote:
>> Magnus Lie Hetland wrote:
>>> For the for-from loop, this may be intended -- as you say, it mimics  
>>> the C behavior (although it seems to fly in the face of "Python  
>>> intuition"): Even with the existing Cython semantics, getstep() will  
>>> be called after *every iteration*.
>>>
>>> It seems to me that in
>>>
>>>    for 0 <= i < n by getstep(): pass
>>>
>>> one would expect (in Python) that the step expression be evaluated  
>>> only once. If that is not the case, fine. Then the for-from loop can  
>>> safely be left alone as it is.
>>>   
>> I think so -- the for-from loop is a C idiom from which one can expect C 
>> behaviour from my perspective, and if this has been the behaviour so far 
>> then backward compatability alone speaks against altering it. The range 
>> optimization has been around for much shorter and clearly should behave 
>> as Python range, so that story is different.
> 
> I vote for a) diverging behaviour between for-range and for-from for the
> loop-variable after loop termination, and b) the obvious evaluate-once
> semantics for the range() optimisation and the obvious (?)
> evaluate-on-each-step semantics for the for-from loop.

Lucky for you, that's what sits in the tree right now, thanks to Magnus :-)

Your thoughts on for-from seems like a good compromise, too. +1

Dag Sverre