On Jan 30, 2009, at 12:56, Dag Sverre Seljebotn wrote:
> I continously come to think about more stuff here :-)
Cool :)
> I think we should leave the semantics of the for-from statement
> alone at
> the moment, since it was introduced to mimick the C construct (and for
> all I know code may depend on the C behaviour here).
Sounds reasonable.
As for the step problem ... you wrote:
> Then self.step.result() will return a function call, and your decop
> will invoke the function twice :-(
What you point out here is an interesting issue -- and I don't think
it relates only to my decop. In fact, as far as I can see, the
existing Cython behavior is quite odd here.
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. (If not, a local variable should
probably be used here too, even with the decref out of the picture, no?)
But there seems to be a problem with range() as well:
cdef int i, j
cdef int getstep():
return 2
for i in range(0,10,2): pass
for j in range(0,10,getstep()): pass
print i, j
# (10, 8)
This doesn't seem right? (I tried looking at the code generated for
the loop with getstep, but it was a bit too hairy for me to spot the
bug immediately :->)
On the other hand, here it seems that the right thing is done wrt. the
use of the step expression. As opposed to in the for-from loop,
getstep() is called only once...
Anyway... I'll have a look at adding the flag to the for-from-loop
that will use decrementation if it's constructed from a range-loop. It
seems that the local variable isn't an issue here (i.e. in for-from
loops constructed from range) after all (or am I mistaken)?
I'll hold off on adding a local variable to the plain for-from loop
for now (and as yet I have no idea what's up with the 10 vs 8 above),
as I don't know if the repeated evaluation of the step expression is
desired.
--
Magnus Lie Hetland
http://hetland.org
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