Testing with unittest

Python's unittest module is part of the standard library and provides a complete framework for writing and running automated tests. Good tests catch bugs before users do and give you confidence to refactor code.

The Structure of a Test

Every test file imports unittest, defines a class that inherits from unittest.TestCase, and writes methods whose names start with test_:

The class name conventionally starts with Test and describes what you're testing. Each test_* method tests one specific scenario.

Running Tests

Test discovery looks for files matching test*.py in the current directory and subdirectories.

Assertion Methods

unittest.TestCase provides many assertion methods. When an assertion fails, the test immediately stops and reports the failure:

assertRaises — Testing Exceptions

Use assertRaises to verify that code raises the expected exception:

setUp and tearDown

setUp() runs before each test method. tearDown() runs after each test method, even if the test fails. Use them to prepare and clean up shared state:

setUpClass and tearDownClass

Run once for the entire class, not before each test. Use for expensive setup like database connections:

Organizing Test Files

Conventional project structure:

Each module gets a corresponding test file. Run all tests from the project root:

Skipping Tests

A Complete Example

Here's a real-world test for a simple BankAccount class: