CODESARRAY

Lesson 16, by Codesarray, , 4 min read

Python testing with pytest: write and run tests

How to test Python code with pytest: how it finds tests by name, how a test is a plain assert, what a failure shows, and how to run one test on many inputs with parametrize. One short test file with its real output.

By the end of this page you will know how to test Python code with pytest: how it finds your tests, how to write one with a plain assert, what a failing test shows you, and how to run one test on many inputs. It builds on Python functions, because a test is a function that calls your own.

How pytest finds tests

Pytest finds tests by name. It looks for files that start with test_, and inside them, functions that start with test. Run pytest in your project folder and it collects every one and runs it. A function with any other name, like add, is just ignored, so you can keep helpers and real code in the same file.

A test is a plain assert

A test is a function with a plain assert. If the expression is true, the test passes. If it is false, the test fails, and pytest shows you both sides of the comparison. If add(2, 3) came out as 5 but the test expected 6, pytest prints:

>       assert add(2, 3) == 6
E       assert 5 == 6
E        +  where 5 = add(2, 3)

There is no special assertion method to learn. You write the check the way you would say it.

Run one test on many inputs

pytest.mark.parametrize runs one test with many inputs. You give it the names of the arguments and a list of cases. Pytest runs the function once for each case and counts each as its own test. Add a case to the list and you get another test, without writing another function.

The code

import pytest


def add(a, b):
    return a + b


def test_add():
    assert add(2, 3) == 5


@pytest.mark.parametrize("a, b, total", [(0, 0, 0), (-1, 1, 0)])
def test_add_more(a, b, total):
    assert add(a, b) == total

Save it as test_add.py and run pytest:

test_add.py ...
3 passed in 0.10s

Pytest prints a few more lines around these, and the time will differ on your machine. Each dot is one passing test. There are three: the plain test, and the two parametrize cases.

The code line by line

  • Line 1 imports pytest, which parametrize needs.
  • Lines 4 and 5 define add, the function we are testing.
  • Lines 8 and 9 are the first test. It asserts that add(2, 3) is 5.
  • Line 12 lists two cases for the parametrize decorator: (0, 0, 0) and (-1, 1, 0).
  • Lines 13 and 14 are one test that runs once for each case, and asserts the total.

What you learned

  • Pytest finds files that start with test_ and functions that start with test.
  • A test is a function with a plain assert.
  • When an assert fails, pytest shows both sides.
  • @pytest.mark.parametrize runs one test on many inputs.
  • A dot in the output means a test passed.

Common questions

How do I install pytest? It is not part of the standard library. In a uv project run uv add --dev pytest, or use pip install pytest in an environment you made yourself. Then pytest runs from the project folder.

What if pytest says no tests ran? Check the names. A file called check_add.py is not collected, and neither is a function called check_add. Rename them to start with test_ and test.

How do I run just one test? Give the file and the function name: pytest test_add.py::test_add runs only that test. pytest -k more runs every test with more in its name, which here is the two parametrize cases.

How do I check that code raises an error? Use pytest.raises as a with block: with pytest.raises(ZeroDivisionError): 1 / 0. The test passes only if that error is raised inside the block.

How do I see each test by name? Run pytest -v. Each parametrize case gets its own line, such as test_add.py::test_add_more[-1-1-0] PASSED.

Next in the series: packaging with uv.

This article goes with the video Python Testing with pytest: Write and Run Tests. Watch it on YouTube.