Testing Flask Applications with Pytest

Testing Flask Applications with Pytest

Testing Flask Applications with Pytest. This articles serves as a guide to testing Flask applications with pytest.

This articles serves as a guide to testing Flask applications with pytest.

We'll first look at why testing is important for creating maintainable software and what you should focus on when testing. Then, we'll detail how to:

  • Create and run unit and functional tests with pytest
  • Utilize fixtures to initialize the state for test functions
  • Check the coverage of the tests using coverage.py

The source code (along with detailed installation instructions) for the Flask app being tested in this article can be found on GitLab at https://gitlab.com/patkennedy79/flask_user_management_example.


By the end of this article, you should be able to:

  1. Explain what to test in a Flask app
  2. Describe the differences between pytest and unittest
  3. Write unit and functional test functions with pytest
  4. Run tests with pytest
  5. Create fixtures for initializing the state for test functions
  6. Determine code coverage of your tests with coverage.py

Why Write Tests?

In general, testing helps ensure that your app will work as expected for your end users.

Software projects with high test coverage are never perfect, but it's a good initial indicator of the quality of the software. Additionally, testable code is generally a sign of a good software architecture, which is why advanced developers take testing into account throughout the entire development lifecycle.

Tests can be considered at three levels:

  • Unit
  • Functional (or integration)
  • End-to-end

Unit tests test the functionality of an individual unit of code isolated from its dependencies. They are the first line of defense against errors and inconsistencies in your codebase. They test from the inside out, from the programmer's point of view.

Functional tests test multiple components of a software product to make sure the components are working together properly. Typically, these tests are focused on functionality that the user will be utilizing. They test from the outside in, from the end user's point of view.

Both unit and Functional testing are fundamental parts of the Test-Driven Development (TDD) process.

Testing improves the maintainability of your code.

Maintainability refers to making bug fixes or enhancements to your code or to another developer needing to update your code at some point in the future.

Testing should be combined with a Continuous Integration (CI) process to ensure that your tests are constantly executing, ideally on each commit to your repository. A solid suite of tests can be critical to catching defects quickly and early in the development process before your end users come across them in production.

What to Test?

What should you test?

Again, unit tests should focus on testing small units of code in isolation.

For example, in a Flask app, you may use unit tests to test:

  1. Database models
  2. Utility functions that your view functions call

Functional tests, meanwhile, should focus on how the view functions operate.

For example:

  1. Nominal conditions (GET, POST, etc.) for a view function
  2. Invalid HTTP methods are handled properly for a view function
  3. Invalid data is passed to a view function

Focus on testing scenarios that the end user will interact with. The experience that the users of your product have is paramount!


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