Python FASTAPI Testing

Introduction

This page is to capture how to go about testing with FAST API

MonkeyPatch

So you can patch code for testing with monkey patch

monkeypatch.setattr("module.path.name", replacement)

Patch a Function

So you can

monkeypatch.setattr("module.path.get_user", lambda id: fake_user)

Which is like

jest.mock("./getUser", () => () => fakeUser)

Patch a module‑level variable

Setting a variable

monkeypatch.setattr(settings, "API_KEY", "test-key")

Which is like

jest.mock("./settings", () => ({ API_KEY: "test-key" }))

Patch an imported reference

If a module does

from server.encryptors.factory import get_encryptor

Then you can patch it like this

monkeypatch.setattr("server.module.that.uses.it.get_encryptor", fake)

Comparison to Jest

Here is a table to help compare terminology

Concept	Jest	pytest
Test setup	beforeEach, beforeAll	Fixtures (function, module, session scope)
Test teardown	afterEach, afterAll	Fixture finalizers / yield fixtures
Dependency injection	Manual mocks, passing objects	Fixture injection by name
Auto‑applied setup	Global beforeEach in setup files	@pytest.fixture(autouse=True)
Mocking	jest.mock, jest.spyOn	monkeypatch.setattr, unittest.mock.patch
Test data factories	Helper functions	Fixtures returning objects

Fixtures

These are things you create to set up a test. The autouse means you do not need to pass the fixture to the tests.

#  Force argon2 for testing purposes
@pytest.fixture(autouse=True)
def force_argon2(monkeypatch: pytest.MonkeyPatch):
    monkeypatch.setattr(settings, "authentication_encryptor", EncryptorNames.ARGON2.value)
    monkeypatch.setattr(user_management.password_manager, "encryptor", get_encryptor(EncryptorNames.ARGON2))