pytest basics

Learn to write tests with pytest.

pytest basics

pytest is the most popular Python test framework. It auto-discovers functions whose names start with test_ and runs them. Validation uses the assert statement.

A test function looks like def test_add():; inside it you write checks like assert add(2, 3) == 5. Run from the command line with pytest file.py.

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

def test_add():
    assert add(2, 3) == 5
    assert add(-1, 1) == 0

Passing tests show a green dot; failing ones show F. pytest output is short and readable.

Running and output

pytest runs test files from the command line. pytest test_module.py runs one file; bare pytest scans all test_*.py files.

Output summarizes each test on one line: passes show a dot, failures F, skips s. Total time and result count come at the end.

For verbose mode use pytest -v; each test name prints separately. For error detail pytest -v --tb=short is useful.

Exercise Write add and test_add

Write a function add(a, b) with body return a + b. Then define a test function test_add(); inside it run assert add(2, 3) == 5. At the end of the file, call test_add() and print("ok").

Show example solution

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

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

test_add()
print("ok")

Test yourself

Complete the test function name

def ____(self=None):
    assert add(2, 3) == 5

Show answer

def test_add(self=None):
    assert add(2, 3) == 5

Match the pytest output symbols

.

Passing test
F

Failing test
s

Skipped test
-v

Verbose mode
Which functions does pytest auto-discover?
- Those starting with test_
- Those starting with main
- All functions
- Only class methods
Show answer

Those starting with test_ — The test_ prefix is the standard for pytest's auto-discovery.

Next lesson: assert

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