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Test a FastAPI app

This recipe demonstrates how to test FastAPI applications using Pytest.

Testing is crucial in API development to ensure reliability, maintainability, and confidence in your codebase.

Using Pytest with dependency injection (via fixtures) provides several benefits:

  • Allows you to isolate components, making tests more focused and easier to debug.
  • Enables you to mock external dependencies (like databases or third-party services) to test your API logic in isolation.
  • Helps maintain test consistency by reusing setup code across multiple tests.

This approach not only catches bugs early but also makes your code more modular and easier to refactor, as changes in one component can be tested without affecting others.

Additionally, well-structured tests serve as living documentation, helping new developers understand how different parts of your API are expected to work.

Code structure​

To test our FastAPI application, we will augment the existing code structure to include the following:

FilePurpose
pytest.iniPytest configuration
tests/conftest.pyRoot Pytest behaviours (e.g. global fixtures)
tests/test_app.pyTest suite for main application
tests/routes/v1/test_healthcheck.pyTest suite for healthcheck endpoint
tests/**/__init__.pyDummy file to register folders as modules under respective test folders
tree -L 4
.
├── pytest.ini
├── tests
│   ├── __init__.py
│   ├── conftest.py
│   ├── routes
│   │   ├── __init__.py
│   │   └── v1
│   │       ├── __init__.py
│   │       └── test_healthcheck.py
│   └── test_app.py

Pytest setup​

pytest.ini
[pytest]
testpaths = tests
python_files = test_*.py
python_functions = test_*
python_classes = Test*
addopts = -v
tests/conftest.py
import pytest
from fastapi.testclient import TestClient

from app import app

@pytest.fixture
def client():
    """Create a test client for the FastAPI application."""
    return TestClient(app)

Tests per FastAPI code​

tests/test_app.py
from fastapi import status

def test_root_endpoint(client):
    """Test the root endpoint returns the expected response."""
    response = client.get("/")
    assert response.status_code == status.HTTP_200_OK
    data = response.json()
    assert "app" in data
    assert "message" in data
    assert data["app"] == "Databricks FastAPI Example"
tests/routes/v1/test_healthcheck.py
from fastapi import status

def test_healthcheck(client):
    """Test the healthcheck endpoint."""
    response = client.get("/api/v1/healthcheck")
    assert response.status_code == status.HTTP_200_OK
    data = response.json()
    assert "status" in data
    assert data["status"] == "OK"
    assert "timestamp" in data

Running tests​

info

The tests we have included do not require the FastAPI application to be running, but we provide examples of dependency injection in the cookbook source code when not connected to a live environment.

# Run all tests
pytest

# Run specific tests
pytest tests/routes/v1/test_healthcheck.py

# Run with coverage report
pytest --cov=app tests/

Dependencies​

requirements.txt
fastapi
pytest
pytest-cov
uvicorn