8 posts tagged with "tests"

For beginners, testing FastAPI dependencies should start with one fundamental goal: isolation. You must ensure your route logic is tested without making real network calls, hitting a live database, or relying on complex configuration settings.

The key to achieving this is using FastAPI's built-in app.dependency_overrides dictionary. This allows you to replace any real dependency function with a simple, predictable mock function for the duration of a test.

side_effect and spec are powerful but often underutilized features of Python's unittest.mock library. Mastering them is key to writing tests that are both robust and expressive. This article will dive deep into these advanced concepts, showing you how to simulate complex scenarios and prevent common mocking pitfalls.

Patching a class's __init__ method is a common and powerful technique for unit testing code that relies on objects whose initialization performs unwanted side effects. These side effects can include making API calls, connecting to a database, or performing other time-consuming or stateful operations [1]. By mocking __init__, you can prevent these actions and verify that the class was instantiated correctly.

Mocking time and dates is a common and often essential practice when writing tests for time-dependent logic. If your code's behavior changes based on the current time (e.g., a function that checks for a subscription's expiration, a daily report generator, or a cache invalidation rule), relying on the system's clock will lead to unreliable and non-repeatable tests.

You need a way to "freeze" time or fast-forward it to a specific point so your tests run in a controlled, predictable environment. Python's unittest.mock.patch is the perfect tool for this, allowing you to replace datetime.now() with a mock that returns a fixed value.

Writing tests that are effective and maintainable is an art. While mocking is a powerful tool, it's not a silver bullet and can be misused. This final article of the mocking series will cover the best practices and common pitfalls of mocking in Python, ensuring your tests are robust, readable, and provide a high degree of confidence in your code.

Mocking entire classes and objects is a crucial skill for unit testing complex code. When a function instantiates a class or calls methods on a passed-in object, you need a way to control that object's behavior without executing its real logic. This article will cover advanced mocking techniques, focusing on how to mock classes and instance methods using both the standard unittest.mock library and the more convenient pytest-mock plugin. We will also clarify key distinctions between different types of test doubles.

External dependencies are a major source of pain in unit testing. They can be slow, unreliable, and difficult to control. Mocking these external services is the single most common and valuable use of the unittest.mock library [1]. This article will provide practical, hands-on examples for mocking three of the most frequent external dependencies: HTTP requests, database connections, and file system operations.

Mocking in Python is a fundamental practice for writing robust, reliable, and efficient tests. It allows developers to isolate a unit of code from its external dependencies, ensuring that a test is only concerned with the logic it is meant to verify [1, 2].