Iterators & Generators

Iterators

An iterator is an object with __iter__() and __next__() methods. Lists, strings, and ranges are all iterable.

nums = [1, 2, 3]
it = iter(nums)
print(next(it))  # 1
print(next(it))  # 2

Generators with yield

A generator function uses yield instead of return. It produces values lazily, one at a time.

def countdown(n):
    while n > 0:
        yield n
        n -= 1

for x in countdown(3):
    print(x)

Try it yourself

Run the example below. Edit it and see what changes.

def evens(n):
    for i in range(n):
        if i % 2 == 0:
            yield i

for x in evens(6):
    print(x)
 

Common mistakes

• ⚠Calling next() past the end of an iterator raises StopIteration.
• ⚠Generators can only be iterated once — after that they're exhausted.