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Margit ANTAL

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Module 8: Testing FastAPI Backends

🚀 Overview

Why testing matters
Testing Basics
Types of testing
- Unit tests
- Integration tests
- End-to-end (E2E) tests

Why Testing Matters

Reliability of APIs
- Testing ensures that your APIs consistently produce the correct output for a given input.
Catching regressions (bugs) early
- Run automated tests when code changes
Confidence in deployments
- When your tests are passing --> the new version of your API is not breaking existing functionality.

Tset Types Overview

Test Type	Scope	Dependencies	Speed
Unit Tests	Single functions/methods	Mocked	Fast
Integration Tests	Multiple components together	Real dependencies	Moderate
End-to-End Tests	Full application stack	Real services	Slow

FastAPI Testing Tools

pytest: popular testing framework for Python
httpx: HTTP client for making requests in tests
FastAPI's TestClient: built on httpx, simplifies testing FastAPI apps

Dependencies Installation

FastAPI dependencies:
```
pip fastapi, uvicorn
```
Test dependencies:
```
pip install pytest httpx
```

Project's Structure

module08_testing_apis/
 ├─ main.py
 ├─ tests/
 │   └─ test_main.py

main.py

from fastapi import FastAPI

app = FastAPI()

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

Test Example

from fastapi.testclient import TestClient
from main import app

client = TestClient(app)

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

Explanation of the Test Code

Import TestClient from fastapi.testclient and the FastAPI app from main.py
Create a TestClient instance: simulates requests to the FastAPI app
Define a test function that makes a GET request to the root endpoint
Assert that the response status code is 200 and the JSON content matches expectations

Run pytest

From the project's root folder run:

python -m pytest

Result:

============================================= test session starts ==============================================
platform darwin -- Python 3.11.2, pytest-8.4.1, pluggy-1.6.0
rootdir: /Users/margitantal/PythonProjects/FASTAPI/module8_testing_apis
plugins: anyio-4.10.0
collected 1 item

tests/test_main.py .                                                                                     [100%]

============================================== 1 passed in 0.16s ===============================================

Organizing tests

Directory structure (tests/)
Grouping by feature/module
Naming conventions

Directory Structure

project/
│
├── app/
│   ├── main.py
│   ├── models.py
│   ├── routes/
│   │   ├── users.py
│   │   └── items.py
│   └── dependencies.py
│
└── tests/
    ├── __init__.py
    ├── conftest.py        # fixtures and setup
    ├── test_main.py       # basic smoke tests
    ├── unit/
    │   ├── test_models.py
    │   └── test_dependencies.py
    └── integration/
        ├── test_users.py
        └── test_items.py

Naming conventions

Naming is critical --> pytest discovers test files and functions automatically.
File naming: Prefix with test_ → e.g., test_users.py
Test function naming: Prefix with test_ → e.g.,

def test_create_user_success():
    ...
    
def test_create_user_duplicate_email():
    ...

Describe what is being tested and under what condition

Unit Testing

Scope: focused on single functions
Dependencies are mocked

What to Unit Test

fake_db = {
    1: {"id": 1, "name": "Alice"},
    2: {"id": 2, "name": "Bob"}
}

def get_user_from_db(user_id: int):
    return fake_db.get(user_id)

@app.get("/users/{user_id}")
def read_user(user_id: int, get_user=Depends(get_user_from_db)):
    user = get_user
    if not user:
        raise HTTPException(status_code=404, detail="User not found")
    return user

Unit Testing `read_user` (1)

import pytest
from fastapi.testclient import TestClient
from main import app, get_user_from_db

client = TestClient(app)

# --- Mock dependency ---
def mock_get_user(user_id: int):
    if user_id == 1:
        return {"id": 1, "name": "Mocked Alice"}
    return None

# --- Override dependency ---
app.dependency_overrides[get_user_from_db] = mock_get_user

Unit Testing `read_user` (2)

# --- Tests ---
def test_read_user_success():
    response = client.get("/users/1")
    assert response.status_code == 200
    assert response.json() == {"id": 1, "name": "Mocked Alice"}

def test_read_user_not_found():
    response = client.get("/users/999")
    assert response.status_code == 404
    assert response.json() == {"detail": "User not found"}

Integration Testing

Scope: tests functions together with their real dependencies (not isolated/mocked)
Database: uses a real database instance, but separate from the production database (e.g., test-specific or in-memory)

What to Integration Test (1)

from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.orm import declarative_base
from sqlalchemy.orm import sessionmaker, Session

DATABASE_URL = "sqlite:///./app.db"  # real DB for dev, overridden in tests

engine = create_engine(DATABASE_URL, connect_args={"check_same_thread": False})
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base = declarative_base()

## --- Model ---
class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True, index=True)
    name = Column(String, index=True)

## Create tables
Base.metadata.create_all(bind=engine)

What to Integration Test (2)

def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

@app.get("/users/integration/{user_id}")
def read_user2(user_id: int, db: Session = Depends(get_db)):
    user = db.query(User).filter(User.id == user_id).first()
    if not user:
        raise HTTPException(status_code=404, detail="User not found")
    return {"id": user.id, "name": user.name}

Integration Testing `read_user2` (1)

import pytest
from fastapi.testclient import TestClient
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from main import app, Base, get_db, User

# --- Setup test database ---
SQLALCHEMY_DATABASE_URL = "sqlite:///./test.db"
engine = create_engine(SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False})
TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

# Create fresh schema
Base.metadata.drop_all(bind=engine)
Base.metadata.create_all(bind=engine)

Integration Testing `read_user2` (2)

def override_get_db():
    db = TestingSessionLocal()
    try:
        yield db
    finally:
        db.close()

app.dependency_overrides[get_db] = override_get_db
client = TestClient(app)

# --- Fixtures ---
@pytest.fixture
def setup_test_data():
    db = TestingSessionLocal()
    user = User(id=1, name="Integration Alice")
    db.add(user)
    db.commit()
    db.refresh(user)
    db.close()
    return user

Integration Testing `read_user2` (3)

def test_read_user2_success(setup_test_data):
    response = client.get(f"/users/integration/{setup_test_data.id}")
    assert response.status_code == 200
    assert response.json() == {"id": 1, "name": "Integration Alice"}

def test_read_user2_not_found():
    response = client.get("/users/integration/999")
    assert response.status_code == 404
    assert response.json() == {"detail": "User not found"}

🔍 Integration Testing - Key Points

Real DB: The test uses SQLite (test.db) with SQLAlchemy.
Fresh schema: We drop and recreate tables before tests to avoid stale state.
Fixtures: setup_test_data inserts test data into the DB.
Overrides: We override get_db so the app uses our test DB instead of production.
End result: This is a true integration test — FastAPI endpoint + SQLAlchemy ORM & database working together.

CI/CD and Test coverage

Continuous testing:
- GitHub Actions / GitLab CI
- Run tests on every push
Test coverage
```
python3 -m pytest -cov
```

End-to-End Testing

E2E Testing Setup
- Run full app + DB + external services
- Use docker-compose
Tools for E2E
- pytest with live server
- Postman/Newman collections
- Playwright (for frontend + backend)

Homework

Link to homework Section: Practical Exercises: Testing APIs

🎯 Remember

Write tests early
Use dependency overrides
Automate with CI/CD
Balance between unit, integration, and E2E

Provide feedback

Saved searches

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Module 8: Testing FastAPI Backends

🚀 Overview

Why Testing Matters

Tset Types Overview

FastAPI Testing Tools

Dependencies Installation

Project's Structure

Test Example

Explanation of the Test Code

Run pytest

Organizing tests

Directory Structure

Naming conventions

Unit Testing

What to Unit Test

Unit Testing `read_user` (1)

Unit Testing `read_user` (2)

Integration Testing

What to Integration Test (1)

What to Integration Test (2)

Integration Testing `read_user2` (1)

Integration Testing `read_user2` (2)

Integration Testing `read_user2` (3)

🔍 Integration Testing - Key Points

CI/CD and Test coverage

End-to-End Testing

Homework

🎯 Remember

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File metadata and controls

Module 8: Testing FastAPI Backends

🚀 Overview

Why Testing Matters

Tset Types Overview

FastAPI Testing Tools

Dependencies Installation

Project's Structure

Test Example

Explanation of the Test Code

Run pytest

Organizing tests

Directory Structure

Naming conventions

Unit Testing

What to Unit Test

Unit Testing read_user (1)

Unit Testing read_user (2)

Integration Testing

What to Integration Test (1)

What to Integration Test (2)

Integration Testing read_user2 (1)

Integration Testing read_user2 (2)

Integration Testing read_user2 (3)

🔍 Integration Testing - Key Points

CI/CD and Test coverage

End-to-End Testing

Homework

🎯 Remember

Unit Testing `read_user` (1)

Unit Testing `read_user` (2)

Integration Testing `read_user2` (1)

Integration Testing `read_user2` (2)

Integration Testing `read_user2` (3)