Testing with pytest

Catch regressions with small tests for normal and edge cases.

Beginner 13 min read Lesson 14 of 480 Challenge included

What you will be able to do

Write test functions pytest discovers
Assert expected errors
Cover empty and boundary inputs

Tests are executable documentation. They prove behavior still works after a change and make edge cases explicit.

test_math.pyPython

import pytest

def safe_divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_safe_divide():
    assert safe_divide(10, 2) == 5

def test_zero_division():
    with pytest.raises(ValueError):
        safe_divide(1, 0)

Files and functions start with test_ so pytest finds them.
Follow arrange, act, assert in each test.
Test the empty case, the boundary and the happy path.
One behaviour per test keeps failures easy to read.

Your turn

Test a word counter

Medium

Write two tests: one for an empty string, one for repeated words.

PYTHON · starter

def count_words(text):
    counts = {}
    for word in text.split():
        counts[word] = counts.get(word, 0) + 1
    return counts

# write two tests below

Check your understanding before moving on.

Knowledge checkQuestion 1

Which function name does pytest discover by default?

Select an answer to continue

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