Looping from 1 to infinity in Python

Question

Using itertools.count:

import itertools
for i in itertools.count(start=1):
    if there_is_a_reason_to_break(i):
        break

In Python 2, range() and xrange() were limited to sys.maxsize. 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
    if there_is_a_reason_to_break(i):
        break

So it's probably best to use count().

def to_infinity():
    index = 0
    while True:
        yield index
        index += 1

for i in to_infinity():
    if i > 10:
        break

Simplest and best:

i = 0
while not there_is_reason_to_break(i):
    # some code here
    i += 1

It may be tempting to choose the closest analogy to the C code possible in Python:

from itertools import count

for i in count():
    if thereIsAReasonToBreak(i):
        break

But beware, modifying i will not affect the flow of the loop as it would in C. Therefore, using a while loop is actually a more appropriate choice for porting that C code to Python.

Reiterating thg435's comment:

from itertools import takewhile, count

def thereIsAReasonToContinue(i):
    return not thereIsAReasonToBreak(i)

for i in takewhile(thereIsAReasonToContinue, count()):
    pass # or something else

Or perhaps more concisely:

from itertools import takewhile, count

for i in takewhile(lambda x : not thereIsAReasonToBreak(x), count()):
    pass # or something else

takewhile imitates a "well-behaved" C for loop: you have a continuation condition, but you have a generator instead of an arbitrary expression. There are things you can do in a C for loop that are "badly behaved", such as modifying i in the loop body. It's possible to imitate those too using takewhile, if the generator is a closure over some local variable i that you then mess with. In a way, defining that closure makes it especially obvious that you're doing something potentially confusing with your control structure.

If you want to use a for loop, it's possible to combine built-in functions iter (see also this answer) and enumerate for an infinite for loop which has a counter. We're using iter to create an infinite iterator and enumerate provides the counting loop variable. The start value is zero by default, but you can set a different start value with the start argument.

for i, _ in enumerate(iter(bool, True), start=1):
    input(i)

Which prints:

1
2
3
4
5
...