Are infinite for loops possible in Python? [duplicate]

You can use the second argument of iter(), to call a function repeatedly until its return value matches that argument. This would loop forever as 1 will never be equal to 0 (which is the return value of int()):

for _ in iter(int, 1):
    pass

If you wanted an infinite loop using numbers that are incrementing you could use itertools.count:

from itertools import count

for i in count(0):
    ....

The quintessential example of an infinite loop in Python is:

while True:
    pass

To apply this to a for loop, use a generator (simplest form):

def infinity():
    while True:
        yield

This can be used as follows:

for _ in infinity():
    pass

Yes, use a generator that always yields another number: Here is an example

def zero_to_infinity():
    i = 0
    while True:
        yield i
        i += 1

for x in zero_to_infinity():
    print(x)

It is also possible to achieve this by mutating the list you're iterating on, for example:

l = [1]
for x in l:
    l.append(x + 1)
    print(x)

In Python 3, range() can go much higher, though not to infinity:

import sys

for i in range(sys.maxsize**10):  # you could go even higher if you really want but not infinity
    pass

Here's another solution using the itertools module:

import itertools

for _ in itertools.repeat([]):  # return an infinite iterator
    pass

It's also possible to combine built-in functions iter (see also this answer) and enumerate for an infinite for loop which has a counter:

for i, _ in enumerate(iter(bool, True)):
    input(i)

Which prints:

0
1
2
3
4
...

This uses iter to create an infinite iterator and enumerate provides the counting loop variable. You can even set a start value other than 0 with enumerate's start argument:

for i, _ in enumerate(iter(bool, True), start=42):
    input(i)

Which prints:

42
43
44
45
46
...

While there have been many answers with nice examples of how an infinite for loop can be done, none have answered why (it wasn't asked, though, but still...)

A for loop in Python is syntactic sugar for handling the iterator object of an iterable an its methods. For example, this is your typical for loop:

for element in iterable:
    foo(element)

And this is what's sorta happening behind the scenes:

iterator = iterable.__iter__()
try:
    while True:
        element = iterator.next()
        foo(element)
except StopIteration:
    pass

An iterator object has to have, as it can be seen, anextmethod that returns an element and advances once (if it can, or else it raises a StopIteration exception).

So every iterable object of which iterator'snextmethod does never raise said exception has an infinite for loop. For example:

class InfLoopIter(object):
    def __iter__(self):
        return self # an iterator object must always have this
    def next(self):
        return None

class InfLoop(object):
    def __iter__(self):
        return InfLoopIter()

for i in InfLoop():
    print "Hello World!" # infinite loop yay!

Best way in my opinion:

for i in range(int(1e18)):
    ...

The loop will run for thousands of years

The other solutions solutions have a few issues, such as:

• consuming a lot of memory which may cause memory overflow
• consuming a lot of processor power.
• creating deadlock.
• using 3rd party library

Here is an answer, which will overcome these problems.

from asyncio import run, sleep


async def generator():
    while True:
        await sleep(2)
        yield True


async def fun():
    async for _ in generator():
        print("Again")


if __name__ == '__main__':
    run(fun())

In case you want to do something that will take time, replace sleep with your desired function.

we can actually have a for infinite loop

list = []
for i in list:
   list.append(i)
   print("Your thing")