Testing FastAPI

Testing FastAPI#

The document provides an in-depth overview of testing FastAPI applications, including handling asynchronous endpoints, dependency overrides, background tasks, and mocking external services. FastAPI’s design makes it highly suitable for efficient and reliable testing.

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Prerequisites#

Ensure you have the following libraries installed:

  • fastapi - for building the application.

  • pytest - a popular testing framework.

  • httpx - for making async HTTP requests.

  • pytest-asyncio - to support asynchronous tests in pytest.

Install these with:

pip install fastapi pytest httpx pytest-asyncio

The Project Structure#

Below is a simple project structure for testing a FastAPI application:

my_fastapi_app/
├── main.py                # Main application file
├── test_main.py           # Test file for application tests

Sample FastAPI Application#

Here’s a sample FastAPI application with basic routes and dependency injection in main.py:

# main.py
from fastapi import FastAPI, Depends, HTTPException
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str

fake_items_db = {"item123": {"name": "item123", "description": "A test item"}}

def get_item(item_id: str):
    item = fake_items_db.get(item_id)
    if item is None:
        raise HTTPException(status_code=404, detail="Item not found")
    return item

@app.get("/")
def read_root():
    return {"message": "Hello, World!"}

@app.get("/items/{item_id}")
def read_item(item_id: str, item: dict = Depends(get_item)):
    return item

@app.post("/items/")
def create_item(item: Item):
    fake_items_db[item.name] = item.dict()
    return item

Writing Basic Tests#

Using FastAPI’s TestClient and pytest, create a test_main.py file for tests. Here’s how to write basic tests for the endpoints defined in main.py:

# test_main.py
import pytest
from fastapi.testclient import TestClient
from main import app

client = TestClient(app)

def test_read_root():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "Hello, World!"}

def test_create_item():
    response = client.post("/items/", json={"name": "bar", "description": "A new item"})
    assert response.status_code == 200
    assert response.json() == {"name": "bar", "description": "A new item"}

def test_read_item():
    response = client.get("/items/item123")
    assert response.status_code == 200
    assert response.json() == {"name": "item123", "description": "A test item"}

def test_item_not_found():
    response = client.get("/items/notfound")
    assert response.status_code == 404
    assert response.json() == {"detail": "Item not found"}

Testing Asynchronous Endpoints#

For asynchronous endpoints, pytest-asyncio and httpx allow you to run async tests. This example modifies test_main.py with an async test:

import pytest
from httpx import AsyncClient
from main import app

@pytest.mark.asyncio
async def test_async_read_root():
    async with AsyncClient(app=app, base_url="http://test") as ac:
        response = await ac.get("/")
        assert response.status_code == 200
        assert response.json() == {"message": "Hello, World!"}

Testing with Dependency Overrides#

Testing with dependency overrides allows you to control dependencies in a test environment. Here, we override the get_item dependency to return a fixed value:

from main import app, get_item

def override_get_item(item_id: str):
    return {"name": "test_item", "description": "Mocked item"}

app.dependency_overrides[get_item] = override_get_item

def test_dependency_override():
    response = client.get("/items/item123")
    assert response.status_code == 200
    assert response.json() == {"name": "test_item", "description": "Mocked item"}

Testing Error Responses#

Check that error handling functions correctly by triggering specific errors and verifying response messages:

def test_item_not_found():
    response = client.get("/items/nonexistent")
    assert response.status_code == 404
    assert response.json() == {"detail": "Item not found"}

Mocking External Service Calls#

When endpoints rely on external APIs, mock these calls to avoid dependency on external systems.

Note: This following test will fail because There is No module named ‘main.external_service’, its just to show how we can mock external service, like ‘stripe api, smtp or etc.’

from unittest.mock import patch
from main import app

@patch("main.external_service.get_data")
def test_external_service_call(mock_get_data):
    mock_get_data.return_value = {"key": "mocked_data"}
    response = client.get("/items/external")
    assert response.status_code == 200
    assert response.json() == {"key": "mocked_data"}

How to Run Your Tests#

To execute your tests, use pytest from the command line within your project directory. This command will automatically discover and run all test files prefixed with test_:

pytest

For more detailed output, you can run:

pytest -v

This provides information on each test’s pass/fail status. To run specific tests or test files, specify the file path:

pytest test_main.py

After executing, pytest will display results in the terminal, detailing the success or failure of each test.

Sample Test Output#

Upon running tests with pytest, you should see output similar to:

=========================== test session starts ============================
collected 5 items

test_main.py .....                                                      [100%]

============================ 5 passed in 0.42s ============================

Conclusion#

Testing FastAPI applications involves various techniques, from simple endpoint testing to handling asynchronous code, dependencies, and mocking external APIs. By using pytest, httpx, and FastAPI’s testing tools, you can create comprehensive tests that ensure the reliability and maintainability of your application.