Pure Functions

Pure Functions

In this lesson — part of Functional Foundations — you'll learn pure functions in Python and why it matters in real work.

Why it matters

Functional style — pure functions and immutability — makes code easier to test and reason about.

Key ideas

• Pure functions
• Immutability
• No shared mutable state
• Predictable outputs

In practice

Here's how it looks in idiomatic Python:

def square(x):
    return x * x   # no side effects, output depends only on input

print(square(5))   # always 25
print(square(5))

Python note: A pure function has no side effects and returns the same result for the same arguments, making it trivial to test and cache.

Try it yourself

Exercise: In Python, rewrite a function that mutates a global so it's pure.

Recap

You now understand pure functions and can apply it in Python. Mark this lesson complete and continue to the next one.