[Cython] can we deprecate for-from loops?

[Robert Bradshaw]

On Mon, Oct 12, 2015 at 11:40 AM, Josh Ayers <josh at ayers.pw> wrote:
> When the step size is a variable, the range statement falls back to a
> Python for loop since the direction of the loop is unknown.  The "i from
> a <= i < b by c" syntax specifies the direction, so it is turned into a
> C for loop.  The difference is performance could be substantial.
>
> This produces a Python for loop:
>
> def python_loop(Py_ssize_t a, Py_ssize_t b, Py_ssize_t c):
>     cdef Py_ssize_t i
>     for i in range(a, b, c):
>         pass

I thought we had fixed that... apparently not. Doable (e.g. multiply
a, b, c, and then i by the sign of c, checking for 0 of course) but
messy.

> This produces a C for loop:
>
> def c_loop(Py_ssize_t a, Py_ssize_t b, Py_ssize_t c):
>     cdef Py_ssize_t i
>     for i from a <= i < b by c:
>         pass

Ah, there is a slight semantic difference: this will be an infinite
for...from loop if c is not positive (and a < b), but an empty
for...in loop for range.

However, there's another catch: for...from can be used with
non-integral values (doubles, pointers, ...) Unless we wanted to
introduce, say, something like cython.crange it may be more painful to
force users to express that as while loops.

I was hoping to get rid of it, but now I'm not sure it'd be that easy.

> On Mon, Oct 12, 2015, at 08:37 AM, Robert Bradshaw wrote:
>> On Sun, Oct 11, 2015 at 3:31 PM, Greg Ewing <greg.ewing at canterbury.ac.nz>
>> wrote:
>> > Stefan Behnel wrote:
>> >>
>> >> Hi!
>> >>
>> >> The syntax construct "for i from 0 <= i < 10" has been silently outdated
>> >> for years. Can we start issuing a warning that normal range() loops are
>> >> preferred?
>> >
>> >
>> > I'd be in favour of replacing it with just 'for 0 <= i < 10',
>> > but -1 on removing it altogether.
>> >
>> > I introduced it in Pyrex for a reason -- to clearly express
>> > iterations over ranges of integers with arbitrary combinations
>> > of open/closed endpoints,
>>
>> I agree that it expresses intent clearer than the range(...) construct.
>>
>> > for use in conjunction with C code.
>> > I believe that reason is still valid.
>>
>> I'm not sure why "in conjunction with C code" makes any difference to
>> the argument, now that the basic range loop produces the same code in
>> Cython. Put another way, does its clarity merit a PEP to introduce
>> this syntax into Python? If not, it's hard to justify in Cython given
>> that it is a fully redundant addition to the language.
>>
>> - Robert
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