pytest Basics¶

Introduction to pytest, a simpler and more powerful testing framework.

Simple pytest Tests¶

Write tests without unittest boilerplate.

```python

test_math.py - pytest style¶

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

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

def test_negative(): assert add(-1, 1) == 0

def test_string_concat(): assert add("hello", " world") == "hello world"

Run with: pytest test_math.py -v¶

```

test_math.py::test_addition PASSED test_math.py::test_negative PASSED test_math.py::test_string_concat PASSED

pytest Markers and Organization¶

Organize tests with markers and fixtures.

```python import pytest

@pytest.mark.slow def test_slow_operation(): import time time.sleep(0.1) assert True

@pytest.mark.parametrize("a,b,expected", [ (2, 3, 5), (0, 0, 0), (-1, 1, 0) ]) def test_add(a, b, expected): assert a + b == expected

@pytest.mark.skip(reason="Not implemented yet") def test_future_feature(): pass

Run with: pytest -v -m "not slow"¶

print("pytest organization example") ```

pytest organization example

Runnable Example: pytest_basics_tutorial.py¶

```python """ 08_pytest_basics.py - Introduction to pytest =========================================== Modern, powerful testing with pytest framework. """

pytest uses simple functions, not classes (though classes work too)¶

No need to import unittest or inherit from TestCase¶

=============================================================================¶

Definitions¶

=============================================================================¶

def add(a, b): """Add two numbers.""" return a + b

def test_add_positive_numbers(): """Test adding positive numbers.""" assert add(2, 3) == 5

def test_add_negative_numbers(): """Test adding negative numbers.""" assert add(-1, -1) == -2

def test_add_zero(): """Test adding zero.""" assert add(5, 0) == 5

pytest provides better assertion introspection than unittest¶

def test_list_operations(): """pytest shows detailed failure information.""" my_list = [1, 2, 3] assert 3 in my_list assert len(my_list) == 3

def test_dict_operations(): """Test dictionary operations.""" my_dict = {'name': 'Alice', 'age': 30} assert my_dict['name'] == 'Alice' assert 'age' in my_dict

pytest fixtures (simple version)¶

import pytest

=============================================================================¶

Main¶

=============================================================================¶

if name == "main": @pytest.fixture def sample_list(): """Fixture that provides a sample list.""" return [1, 2, 3, 4, 5]

def test_with_fixture(sample_list):
    """Test using a fixture."""
    assert len(sample_list) == 5
    assert sum(sample_list) == 15

# Testing exceptions with pytest
def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_divide_by_zero():
    """Test that division by zero raises ValueError."""
    with pytest.raises(ValueError):
        divide(10, 0)

def test_divide_by_zero_message():
    """Test exception message."""
    with pytest.raises(ValueError, match="Cannot divide"):
        divide(10, 0)

# Run with: pytest 08_pytest_basics.py -v

```

Exercises¶

Exercise 1. Write a function fizzbuzz(n) that returns "Fizz" if n is divisible by 3, "Buzz" if divisible by 5, "FizzBuzz" if divisible by both, and str(n) otherwise. Write pytest tests for all four cases.

Exercise 2. Predict the output of running pytest -v on this file:

```python def test_truthy(): assert [1, 2, 3]

def test_falsy(): assert [] ```

Exercise 3. Write a function parse_int(s) that converts a string to an integer or raises ValueError. Use pytest.raises to test the error case.

Exercise 4. Use @pytest.mark.parametrize to test a square(n) function with at least five different inputs, including negative numbers and zero.