Iterators, Generators and Next Steps

You will get values one at a time with iterators and generators, then plan your next practice.

10 min read 10 quiz questions

Get one value at a time

An iterator remembers its position in a collection. Call iter() to get an iterator and next() to request its next value.

Example: iter() and next()

colors = ['red', 'green', 'blue']
color_iterator = iter(colors)

print(next(color_iterator))
print(next(color_iterator))
print(next(color_iterator))

Each call to next() moves forward. A fourth call here would raise StopIteration because there are no values left.

Make a generator

Example: yield creates a generator

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

for number in countdown(3):
    print(number)
print('Go!')

A function containing yield is a generator. It pauses after each yielded value and continues when the next value is requested.

Example: a generator expression

squares = (number * number for number in range(1, 5))

for square in squares:
    print(square)

Parentheses make this a generator expression, not a list. Generators produce values when needed, so they can save memory when you work with many values.

Where to go next

Build small practice projects, such as a quiz, to-do list, or text adventure. Projects help you combine the skills you have learned.

Explore Python's standard library for built-in tools such as math, json, and pathlib. When a project needs extra packages, learn to use a virtual environment so its packages stay separate from other projects.

Key points

iter() creates an iterator from a collection.
next() gets the next iterator value.
yield turns a function into a generator.
Generator expressions use parentheses.
Generators can save memory by producing values on demand.

Worked example

Hand out daily pocket money one day at a time with a generator — values appear lazily, none stored up front.

Example

def pocket_money(amount, days):
    for _ in range(days):
        yield amount

wallet = pocket_money(20, 3)
print(next(wallet), next(wallet))

How it works, step by step

yield pauses the function and hands out one value.
next() resumes until the next yield.
Nothing beyond the requested days is ever computed.

References & Further Reading

Related entries from PyLabs's own reference library.

iter() (Return an iterator for a collection or other iterable.)
next() (Get the next item from an iterator.)
enumerate() (Pair each item with a counter.)

Your notes

Practice exercise

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Write a generator function named count_up that yields 1, 2, and 3. Loop over it and print each value on its own line.

# Write your code below

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Iterators, Generators and Next Steps quiz

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