Introduction to pytest

Introduction to pytest#

Testing is the practice of running code with known inputs and checking that the outputs match expectations. A test that passes gives confidence that a function works; a test that fails pinpoints exactly what broke.

Installing pytest#

pip install pytest

Running Tests#

Place test functions in files named test_*.py or *_test.py. Run all tests in the current directory:

pytest

Run a specific file:

pytest test_mymodule.py

pytest prints a dot for each passing test and F for each failure, followed by a detailed report of what went wrong.

Writing a First Test#

Suppose we have a function in mymath.py:

def factorial(n: int) -> int:
    if n == 0:
        return 1
    return n * factorial(n - 1)

A test file test_mymath.py:

from mymath import factorial

def test_factorial_zero() -> None:
    assert factorial(0) == 1

def test_factorial_positive() -> None:
    assert factorial(5) == 120

def test_factorial_one() -> None:
    assert factorial(1) == 1

Running pytest produces:

...
3 passed in 0.01s

If a test fails — say we accidentally write factorial(5) == 119 — pytest shows exactly which assertion failed and what the actual values were:

FAILED test_mymath.py::test_factorial_positive
AssertionError: assert 120 == 119

How pytest Finds Tests#

pytest discovers tests automatically:

  • Files matching test_*.py or *_test.py

  • Functions inside those files starting with test_

  • Methods starting with test_ inside classes starting with Test

You do not need a main function or any special imports beyond the module under test.