Python testing: prove behavior with small independent fixtures

The contract and its boundary

Consider a function that converts a user quantity into a nonnegative integer. Define the accepted domain before writing tests; parsing an arbitrary string is not the same as validating a quantity. Make invalid inputs raise a specific exception instead of returning a number that silently changes meaning. Tests should assert behavior, not implementation details such as local variable names.

import pytest

def quantity(value):
    if isinstance(value, bool) or not isinstance(value, int):
        raise TypeError('integer required')
    if value < 0:
        raise ValueError('negative quantity')
    return value

@pytest.mark.parametrize('value', [0, 1, 999])
def test_accepts_int(value):
    assert quantity(value) == value

def test_rejects_bool():
    with pytest.raises(TypeError):
        quantity(True)

def test_rejects_negative():
    with pytest.raises(ValueError):
        quantity(-1)

Why the bool case matters

Python's bool subclasses int, so a naive isinstance(True, int) check accepts True as 1. If your data contract requires actual numerical quantities rather than flags, explicitly reject it. A property-based test can explore many values, but targeted boundary cases document the intended behavior for future maintainers.

Separate unit tests from environment tests

The test above makes no network calls and modifies no persistent files. A test of filesystem import paths, real services or interpreter ABI belongs in another test layer with controllable setup. For tests involving temporary files, use pytest's tmp_path fixture to avoid changing the developer's current directory. Pin test dependencies and run the suite with the same interpreter as the application.

Proof of completion

Before changing the validation rule, add a failing test that captures the new edge case. After the fix, demonstrate red → green and show exact test output, Python version and pytest version. No execution is claimed for the reader's platform by the text of this lesson.