itertools.repeat VS itertools.cycle

Question

Is there any difference between itertools.repeat(n) and itertools.cycle(n)? As it seems, they produce the same output. Is one more efficient to use in a situation where I need an infinite loop of some element?

Answer 1

Simply, itertools.repeat will repeat the given argument, and itertools.cycle will cycle over the given argument. Don't run this code, but for example:

from itertools import repeat, cycle

for i in repeat('abcd'): print(i)
# abcd, abcd, abcd, abcd, ...

for i in cycle('abcd'): print(i)
# a, b, c, d, a, b, c, d, ...

Answer 2

These are equivalent but the first is clearer and a little faster:

it = repeat(x)
it = cycle([x])

However, cycle has the option of repeating entire sequences:

it = cycle(['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'])

And repeat has the option of setting a limit on the number of repetitions:

it = repeat(x, 5)         # Return five repetitions of x

Also, the intended use cases are different.

In particular, repeat was designed to provided a repeated argument to a mapped function:

it = imap(pow, repeat(2), range(10))

While cycle is intended for cyclic recurring behaviors. Here is a Python 3 example that returns 1/1 - 1/3 + 1/5 - 1/7 + 1/9 + ...:

it = accumulate(map(operator.truediv, cycle([1, -1]), count(1, 2)))

The latter example shows how all the parts fit together the create an "iterator algebra".

Hope you found this to be illuminating :-)