Generators and yield

What you'll learn

Write functions with yield
Understand why generators save memory
Use generators with next(), for and sum()

Producing values one after another

A normal function gives back one value with return and is then finished. A generator uses yield: it produces a value, pauses – and carries on exactly there next time.

def countdown(start):
    while start > 0:
        yield start
        start -= 1
    yield "Liftoff!"

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

Press Steps: you can see Python jumping back and forth between the loop and the generator.

Step by step with next()

With next() you fetch the next value yourself:

def colours():
    yield "red"
    yield "amber"
    yield "green"

traffic_light = colours()
print(next(traffic_light))
print(next(traffic_light))
print(next(traffic_light))

One more next(traffic_light) would raise a StopIteration error – the generator is empty.

Why generators?

A list with a million numbers needs a lot of memory. A generator only calculates each number when it’s needed. With round brackets there are even generator expressions:

total = sum(x * x for x in range(1_000_000))
print(total)

No list is built – and still the right result comes out.

Remember: If you see yield in a function, it’s a generator. You can use generators in for loops, with list(), sum() or next().

Quick check

Before you start the task: what does this program print? Type your guess – then Python works it out.

def squares(n):
    for i in range(n):
        yield i * i


g = squares(4)
print(next(g), next(g))
print(list(g))

Your task

Your turn

Write a generator fibonacci(n) that produces the first n Fibonacci numbers: 0, 1, 1, 2, 3, 5, 8, … (every number is the sum of the two before it). Use yield – no list.

def fibonacci(n):
    # a and b are the last two numbers
    a, b = 0, 1
    # produce n numbers with yield


print(list(fibonacci(10)))

Something to think about
First yield a, then calculate on with a, b = b, a + b.
Which command helps?
A generator uses yield instead of return.

Gap code – fill in the ___

                        def fibonacci(n):
    # a and b are the last two numbers
    a, b = 0, 1
    for _ in ___:
        ___
        a, b = ___


print(list(fibonacci(10)))

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

def fibonacci(n):
    # a and b are the last two numbers
    a, b = 0, 1
    for _ in range(n):
        yield a
        a, b = b, a + b


print(list(fibonacci(10)))