Mimic while-loop using only for-loops in python

Question

Having started to learn code with C, I had always assumed that for-loops and while-loops where essentialy always equivalent (as in one could always reproduce the behaviour of one using only the other). But in python while going from a for-loop to a while-loop is always trivial, I could not find a way to achieve the reverse.

Is there any way, in python, to reproduce the behaviour of a while-loop (infinite looping) using only for-loops ?

Here is a solution that doesn't work (because of the recursion limit) using a recursive generator:

def infinite_loopy():
    yield "All work and no play makes Jack a dull boy"
    for x in infinite_loopy():
        yield x

#here starts the supposedly infinite-loop
for x in infinite_loopy():
    print(x)

@Aurora0001 It's not a duplicate of that question as I would like **no usage of while-loops** — jadsq, Aug 24 '16 at 12:39
The accepted answer of that question uses no while loops, so it's exactly what you want, yes? If not, why not? — Kevin, Aug 24 '16 at 12:42
@Kevin My bad I only checked the question. But yeah that would count as a solution. — jadsq, Aug 24 '16 at 12:45
Yeah somehow I missed the proposed duplicate and drafted an answer that was exactly identical with the accepted answer in that other question... — Antti Haapala -- Слава Україні, Aug 24 '16 at 13:27
@Antti Haapala Does this realy count as a duplicate ? I feel like they are two different questions that append to have common answers. — jadsq, Aug 24 '16 at 13:33
@jadsq the **only** way you can have `for` loop iterate forever in Python is to have an iterable/iterator/generator of infinite length. — Antti Haapala -- Слава Україні, Aug 24 '16 at 13:34

Ami Tavory · Accepted Answer · 2016-08-24T13:24:04.503

5

You can do this by writing a non-yield iterator class:

class Infinite(object):
    def __iter__(self):
        return self

    def next(self): # For Python3, replace this with __next__
        return 1

# Loops forever
for i in Infinite():
    pass

(You can see it stalling on ideone if you have the patience - it's like watching paint dry).

edited Aug 24 '16 at 13:24

answered Aug 24 '16 at 12:42

Ami Tavory

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this solution is simple and pleasant to the eyes, +1 – Lost Aug 24 '16 at 13:06
@Lost Many thanks! – Ami Tavory Aug 24 '16 at 13:07
1

@AnttiHaapala Thanks for the correct comment. For Python3, `next` should be replaced with `__next__` (updated in the question). – Ami Tavory Aug 24 '16 at 13:24

score 5 · Answer 2 · answered Aug 24 '16 at 13:24

You can use the two-argument version of iter as follows:

for _ in iter(int, 1):
    print('All your loops are belong to us!')

The 2-argument form of iter calls the first argument as a function with no arguments. If the returned value equals (==) the second argument, StopIteration is raised, otherwise the return value is yielded. int() called without arguments returns 0 which is of course not equal to 1 thus yielding 0 forever, and we have an infinite loop.

RoadieRich · Answer 3 · 2016-08-24T13:12:47.937

4

You can use itertools.repeat, count or cycle:

import itertools

for _ in itertools.repeat(None):
    # infinite loop

for _ in itertools.count():
    # infinite loop

for _ in itertools.cycle([None]):
    # infinite loop

All of these can be represented by functions using while loops, but the itertools module is implemented in c (in cPython), and makes no use of while loops in the source code. Similarly for jython (java), and even PyPy (python). The same is true for count and cycle.

edited Aug 24 '16 at 13:12

answered Aug 24 '16 at 12:41

RoadieRich

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It's still possible that they're internally implemented using a `while`, no? I think that's how the OP responded to Aurora's comment just under the question. – Ami Tavory Aug 24 '16 at 12:46
The [doc](https://docs.python.org/2/library/itertools.html#itertools.repeat) for itertools.repeat indicate that it is roughly equivalent to a function containing a `while` – jadsq Aug 24 '16 at 12:48
@jadsq The [source for `repeat`](https://github.com/python/cpython/blob/master/Modules/itertoolsmodule.c#L4119) has no use of `while` whatsoever. – RoadieRich Aug 24 '16 at 12:56
@AmiTavory See edit – RoadieRich Aug 24 '16 at 13:06
@RoadieRich Oh, good answer, then (in a completely different direction). – Ami Tavory Aug 24 '16 at 13:07
@RoadieRich Alrigth then, its not extremely idomatic but its fair enough. – jadsq Aug 24 '16 at 13:10
@jadsq you don't get much more idiomatic than using the standard library for what it's designed for. – RoadieRich Aug 24 '16 at 13:13
@RoadieRich That's true. I just meant it as in "bare bones" python. – jadsq Aug 24 '16 at 13:28

score -1 · Answer 4 · edited May 23 '17 at 12:02

-1

You can use itertools.count() in for loop.

import itertools

def infinite_loopy():
    x = "All work and no play makes Jack a dull boy"
    for x in itertools.count():
        yield x

for i in infinite_loopy():
    print("All work and no play makes Jack a dull boy")

[Reference] Looping from 1 to infinity in Python

edited May 23 '17 at 12:02

Community

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answered Aug 24 '16 at 12:44

Daewon Lee

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Mimic while-loop using only for-loops in python

4 Answers4