In Python, the key to being able to suspend the execution of a function midway through is the use of coroutines, as we will see in this section. In order to understand coroutines, one needs to understand generators, and in order to understand those, one needs to have a grasp of iterators!

Most Python programmers are familiar with the concept of iterating some sort of collection (for example, strings, lists, tuples, file objects, and so on):

>>> for i in range(3):
...     print(i)
... 
0
1
2
>>> for line in open('exchange_rates_v1.py'):
...     print(line, end='')
... 
#!/usr/bin/env python3
import itertools
import time
import urllib.request
…

The reason why we can iterate all sorts of objects and not just lists or strings is the iteration protocol. The iteration protocol defines a standard interface for iteration: an object that implements __iter__ and __next__ (or __iter__ and next in Python 2.x) is an iterator and, as the name suggests, can be iterated over...