How to extend/concatenate two iterators in Python

Question

I want concatenate two iterators in an efficient way.

Suppose we have two iterators (in Python3)

l1 = range(10)      # iterator over 0, 1, ..., 9
l2 = range(10, 20)  # iterator over 10, 11, ..., 19

If we convert them to lists, it is easy to concatenate like

y = list(l1) + list(l2)  # 0, 1, ,..., 19

However, this can be not efficient.

I would like to do something like

y_iter = l1 + l2  # this does not work

What is the good way to do this in Python3?

Answer 1

Use itertools.chain:

from itertools import chain
y_iter = chain(l1, l2)

It yields all the items from l1 and then all the items from l2. Effectively concatenating the sequence of yielded items. In the process it consumes both.

Answer 2

you can use the chain() function provided by the itertools

itertools.chain()