[Python-ideas] Pass a function as the argument "step" of range()

[Steven D'Aprano]

On Thu, Jul 02, 2015 at 08:30:53AM +0200, Pierre Quentel wrote:
> In languages such as Javascript, the incrementation of a for loop counter
> can be done by an operation, for instance :
> 
> for(i=1; i<N; i*=2)
> 
> would iterate on the powers of 2 lesser than N.
> 
> To achieve the same thing in Python we currently can't use range() because
> it increments by an integer (the argument "step"). An option is to build a
> generator like :
> 
> def gen(N):
>     i = 1
>     while i<=N:
>         yield i
>         i *= 2

Given how simple generators are in Python, that's all you need for the 
most part. If you find yourself needing to do the above many times, with 
different expressions, you can write a factory:

def gen(start, end, func):
    def inner():
        i = start
        while i < end:
            yield i
            i = func(i)
    return inner()


for i in gen(1, 100, lambda x: 3**x - x**3):
    for j in gen(-1, 5, lambda x: x**2):
        for k in gen(1000, 20, lambda x: -(x**3)):
            pass



> My proposal is that besides an integer, range() would accept a function as
> the "step" argument, taking the current counter as its argument and
> returning the new counter value. Here is a basic pure-Python implementation

Why add this functionality to range? It has little to do with range, 
except that range happens to sometimes be used as the sequence in 
for-loops. range has nice simple and clean semantics, for something as 
uncommon as this request, I think a user-defined generator is fine.

range is not a tool for generating arbitrary sequences. It is a tool for 
generating sequences with equal-spaced values. Let's not complicate it.


-- 
Steven