Generators, generator expressions, and loops

[Ian Kelly]

On Fri, Nov 16, 2018 at 7:57 AM Steve Keller <keller at no.invalid> wrote:
>
> I have looked at generators, generator expressions, and iterators and
> I try to get more familiar with these.
>
> 1. How would I loop over all (with no upper bound) integers or all
> powers of two, for example?
>
> In C it would be
>
>    for (int i = 0; ; i++) { ... }  or  for (int i = 1; ; i *= 2) { ... }
>
> In Python, I could define a generator
>
>     def powers():
>         i = 1
>         while True:
>             yield(i)
>             i *= 2
>
>     for i in powers():
>         ...
>
> More elegant are generator expressions but I cannot think of a way
> without giving an upper limit:
>
>     for i in (2 ** i for i in range(1000000)):
>         ...
>
> which looks ugly.  Also, the double for-loop (and also the two loops
> in the above exmaple, for + while in the generator) look unnatural,
> somehow, i.e. loop over all elements which are created by a loop.
>
> Is there a more beautyful way?

Some options:

from itertools import count

def powers():
    for i in count():
        yield 2 ** i


for i in (2 ** i for i in count()):
    ...


for i in map(lambda x: 2 ** x, count()):
    ...


from functools import partial
from operator import pow

for i in map(partial(pow, 2), count()):
    ...


Take your pick.