How to Use pytest in Python: Simple Guide with Examples

To use pytest in Python, write test functions starting with test_ in files named test_*.py. Run tests by executing pytest in the terminal, and pytest will find and run all tests automatically.

Syntax

Pytest looks for test files named test_*.py or *_test.py. Inside these files, test functions must start with test_. Use assert statements to check expected results.

Run tests by typing pytest in your terminal inside the project folder.

python

def test_example():
    assert 1 + 1 == 2

Example

This example shows a simple test function that checks if adding two numbers works correctly. Running pytest will find this test and show if it passes.

python

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

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

Output

============================= test session starts ==============================
collected 1 item

test_sample.py .                                                          [100%]

============================== 1 passed in 0.01s ===============================

Common Pitfalls

Common mistakes include:

• Not naming test files or functions correctly (must start with test_).
• Using print instead of assert for checks.
• Running python file.py instead of pytest to run tests.

Always use assert to verify conditions and run tests with the pytest command.

python

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

# Wrong: function name does not start with test_
def check_add():
    assert add(2, 3) == 5

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

Quick Reference

• Test file names: start with test_ or end with _test.py
• Test function names: start with test_
• Assertions: use assert to check results
• Run tests: type pytest in terminal
• Install pytest: use pip install pytest if not installed

Key Takeaways

Name test files and functions starting with 'test_' for pytest to find them.

Use assert statements inside test functions to check expected results.

Run tests by typing 'pytest' in your terminal inside the project folder.

Avoid using print statements for testing; rely on assert for automatic checks.

Install pytest with 'pip install pytest' before running tests if needed.