How to test Python code with pytest
Python is a simple and powerful testing tool in Python. After installation, test files are automatically discovered according to naming rules. Write a function starting with test_ for assertion testing, use @pytest.fixture to create reusable test data, verify exceptions through pytest.raises, supports running specified tests and multiple command line options, and improves testing efficiency.
Testing Python code with pytest is simple, powerful, and widely used in the Python community. It helps you write clean, maintainable tests with minimal boilerplate. Here's how to get started and use it effectively.
Install and Set Up Pytest
First, install pytest using pip:
pip install pytestOnce installed, you can run tests from your project root. Pytest automatically discovers test files and functions following naming conventions.
By default, it looks for files named:
- test_*.py
- *_test.py
Inside these files, it runs functions starting with test_ .
Write Your First Test
Create a file called test_sample.py with a simple function and test:
def add(a, b):return ab
def test_add():
assert add(2, 3) == 5
assert add(-1, 1) == 0
Run the test in your terminal:
pytestYou'll see output showing how many tests passed or failed.
Use Fixtures for Reusable Setup
Pytest fixtures let you define reusable setup logic. For example, if you need a database connection or test data:
import pytest@pytest.fixture
def sample_data():
return [1, 2, 3, 4, 5]
def test_sum(sample_data):
assert sum(sample_data) == 15
The sample_data fixture is injected into any test that requests it by parameter name.
Test Exceptions and Edge Cases
To check that code raises expected exceptions, use pytest.raises :
def divide(a, b):if b == 0:
raise ValueError("Cannot divide by zero")
return a / b
def test_divide_by_zero():
with pytest.raises(ValueError, match="Cannot divide by zero"):
divide(10, 0)
Run Specific Tests and Use Options
You can run specific tests using patterns:
- pytest test_sample.py – run all tests in a file
- pytest test_sample.py::test_add – run one test function
- pytest -v – show verbose output
- pytest -x – stop after first failure
- pytest --tb=short – shorten traceback
Basically, pytest makes testing easy once you follow the naming rules and learn to use assertions and fixtures. It handles most of the plumbing so you can focus on writing meaningful tests.
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