209 – itertools.tee splits iterators

Iterators can only be traversed once, like is the case with generators:

squares = (x ** 2 for x in range(3))

for sq in squares:
    print(sq, end=", ")
# 0, 1, 4,

for sq in squares:
    print(sq, end=", ")
# <no output>

The second loop produces no output because the generator was exhausted during the first loop.

By using itertools.tee, you can work around this limitation, since itertools.tee allows you to create 2+ independent iterators out of a single iterator:

from itertools import tee

squares = (x ** 2 for x in range(3))
sq1, sq2 = tee(squares, 2)

for sq in sq1:
    print(sq, end=", ")
# 0, 1, 4,

for sq in sq2:
    print(sq, end=", ")
# 0, 1, 4,

This time, the second loop produces the same output as the first loop because both loops traverse two different iterators (sq1 and sq2), that were returned by itertools.tee.