diegosouzapw FastAPI Development
A comprehensive skill for building modern, high-performance Python APIs with FastAPI. Master async/await patterns, Pydantic data validation, dependency injection, authentication, database integration, and production-ready deployment strategies.
When to Use This Skill
Use this skill when:
- Building RESTful APIs with Python for web, mobile, or microservices
- Developing high-performance, asynchronous backend services
- Creating APIs with automatic interactive documentation (OpenAPI/Swagger)
- Implementing OAuth2, JWT authentication, or other security patterns
- Integrating with SQL or NoSQL databases in Python applications
- Building APIs that require strong data validation and type safety
- Developing microservices with automatic request/response validation
- Creating APIs with WebSocket support for real-time features
- Migrating from Flask, Django REST Framework, or other Python frameworks
- Building production-ready APIs with proper error handling and testing
Core Concepts
FastAPI Philosophy
FastAPI is built on three foundational principles:
- Fast to Code: Reduce development time with automatic validation and documentation
- Fast to Run: High performance comparable to NodeJS and Go (via Starlette and Pydantic)
- Fewer Bugs: Automatic validation reduces human errors by about 40%
- Standards-Based: Built on OpenAPI and JSON Schema standards
- Editor Support: Full autocomplete, type checking, and inline documentation
Key FastAPI Features
- Type Hints: Python 3.6+ type hints for validation and documentation
- Async Support: Native async/await for high-performance I/O operations
- Pydantic Models: Automatic request/response validation and serialization
- Dependency Injection: Elegant system for sharing logic across endpoints
- OpenAPI Docs: Automatic interactive API documentation
- Security: Built-in support for OAuth2, JWT, API keys, and more
- Testing: Easy to test with TestClient and async test support
Core Architecture Components
- FastAPI App: The main application instance
- Path Operations: Endpoint definitions with HTTP methods
- Pydantic Models: Data validation and serialization schemas
- Dependencies: Reusable logic for authentication, database, etc.
- Routers: Organize endpoints into modules
- Middleware: Process requests/responses globally
- Background Tasks: Execute code after returning responses
Getting Started
Installation
# Basic installation
pip install fastapi
# With ASGI server for production
pip install "fastapi[all]"
# Or install separately
pip install fastapi uvicorn[standard]
# Additional dependencies
pip install python-multipart # For form data
pip install python-jose[cryptography] # For JWT
pip install passlib[bcrypt] # For password hashing
pip install sqlalchemy # For SQL databases
pip install databases # For async database support
Minimal FastAPI Application
from fastapi import FastAPI
app = FastAPI()
@app.get("/")
async def root():
return {"message": "Hello World"}
# Run with: uvicorn main:app --reload
Pydantic Models for Data Validation
Basic Model Definition
from pydantic import BaseModel, Field, EmailStr, HttpUrl
from typing import Optional, List
from datetime import datetime
class User(BaseModel):
id: int
username: str = Field(..., min_length=3, max_length=50)
email: EmailStr
full_name: Optional[str] = None
is_active: bool = True
created_at: datetime = Field(default_factory=datetime.utcnow)
class UserCreate(BaseModel):
username: str = Field(..., min_length=3, max_length=50)
email: EmailStr
password: str = Field(..., min_length=8)
full_name: Optional[str] = None
class UserResponse(BaseModel):
id: int
username: str
email: EmailStr
full_name: Optional[str]
is_active: bool
class Config:
orm_mode = True # For SQLAlchemy models
Nested Models
class Image(BaseModel):
url: HttpUrl
name: str
class Item(BaseModel):
name: str
description: Optional[str] = None
price: float = Field(..., gt=0)
tax: Optional[float] = None
tags: List[str] = []
images: Optional[List[Image]] = None
# Request example:
# {
# "name": "Laptop",
# "price": 999.99,
# "tags": ["electronics", "computers"],
# "images": [
# {"url": "http://example.com/img1.jpg", "name": "Front view"}
# ]
# }
Model Validation and Examples
from pydantic import BaseModel, Field, validator
class Product(BaseModel):
name: str = Field(..., example="MacBook Pro")
price: float = Field(..., gt=0, example=1999.99)
discount: Optional[float] = Field(None, ge=0, le=100, example=10.0)
@validator('discount')
def discount_check(cls, v, values):
if v and 'price' in values:
discounted = values['price'] * (1 - v/100)
if discounted < 0:
raise ValueError('Discounted price cannot be negative')
return v
class Config:
schema_extra = {
"example": {
"name": "MacBook Pro 16",
"price": 2499.99,
"discount": 15.0
}
}
Path Operations and Routing
HTTP Methods and Path Parameters
from fastapi import FastAPI, Path, Query, Body
from typing import Optional
app = FastAPI()
# GET with path parameter
@app.get("/items/{item_id}")
async def read_item(
item_id: int = Path(..., title="The ID of the item", ge=1),
q: Optional[str] = Query(None, max_length=50)
):
return {"item_id": item_id, "q": q}
# POST with request body
@app.post("/items/")
async def create_item(item: Item):
return {"item": item, "message": "Item created"}
# PUT for updates
@app.put("/items/{item_id}")
async def update_item(
item_id: int,
item: Item = Body(...),
):
return {"item_id": item_id, "item": item}
# DELETE
@app.delete("/items/{item_id}")
async def delete_item(item_id: int):
return {"message": f"Item {item_id} deleted"}
# PATCH for partial updates
@app.patch("/items/{item_id}")
async def partial_update_item(
item_id: int,
item: dict = Body(...)
):
return {"item_id": item_id, "updated_fields": item}
Query Parameters with Validation
from fastapi import Query
from typing import List, Optional
@app.get("/search/")
async def search_items(
q: str = Query(..., min_length=3, max_length=50),
skip: int = Query(0, ge=0),
limit: int = Query(10, ge=1, le=100),
sort_by: Optional[str] = Query(None, regex="^(name|price|date)$"),
tags: List[str] = Query([], description="Filter by tags")
):
return {
"q": q,
"skip": skip,
"limit": limit,
"sort_by": sort_by,
"tags": tags
}
Response Models
from typing import List
@app.post("/users/", response_model=UserResponse)
async def create_user(user: UserCreate):
# Hash password, save to DB
db_user = {
"id": 1,
"username": user.username,
"email": user.email,
"full_name": user.full_name,
"is_active": True
}
return db_user
@app.get("/users/", response_model=List[UserResponse])
async def list_users(skip: int = 0, limit: int = 100):
users = [...] # Fetch from database
return users
# Exclude fields from response
class UserInDB(User):
hashed_password: str
@app.get("/users/{user_id}", response_model=UserResponse)
async def get_user(user_id: int):
user = get_user_from_db(user_id) # Returns UserInDB
return user # Password excluded automatically
Async/Await Patterns
When to Use async vs def
# Use async def when:
# - Making database queries with async driver
# - Calling external APIs with httpx/aiohttp
# - Using async I/O operations
# - Working with async libraries
@app.get("/async-example")
async def async_endpoint():
# Can use await inside
result = await async_database_query()
external_data = await async_http_call()
return {"result": result, "external": external_data}
# Use def when:
# - Working with synchronous libraries
# - Performing CPU-bound operations
# - No async operations needed
@app.get("/sync-example")
def sync_endpoint():
# Regular synchronous code
result = synchronous_database_query()
return {"result": result}
Async Database Operations
import asyncio
from databases import Database
DATABASE_URL = "postgresql://user:password@localhost/dbname"
database = Database(DATABASE_URL)
@app.on_event("startup")
async def startup():
await database.connect()
@app.on_event("shutdown")
async def shutdown():
await database.disconnect()
@app.get("/users/{user_id}")
async def get_user(user_id: int):
query = "SELECT * FROM users WHERE id = :user_id"
user = await database.fetch_one(query, {"user_id": user_id})
return user
@app.post("/users/")
async def create_user(user: UserCreate):
query = """
INSERT INTO users (username, email, hashed_password)
VALUES (:username, :email, :password)
RETURNING *
"""
hashed_password = hash_password(user.password)
new_user = await database.fetch_one(
query,
{
"username": user.username,
"email": user.email,
"password": hashed_password
}
)
return new_user
Concurrent Operations
import asyncio
import httpx
async def fetch_user(user_id: int):
async with httpx.AsyncClient() as client:
response = await client.get(f"https://api.example.com/users/{user_id}")
return response.json()
@app.get("/users/batch")
async def get_multiple_users(user_ids: List[int] = Query(...)):
# Fetch all users concurrently
users = await asyncio.gather(*[fetch_user(uid) for uid in user_ids])
return {"users": users}
Dependency Injection
Basic Dependencies
from fastapi import Depends
from typing import Optional
# Simple dependency function
async def common_parameters(
q: Optional[str] = None,
skip: int = 0,
limit: int = 100
):
return {"q": q, "skip": skip, "limit": limit}
@app.get("/items/")
async def read_items(commons: dict = Depends(common_parameters)):
return commons
@app.get("/users/")
async def read_users(commons: dict = Depends(common_parameters)):
return commons
Class-Based Dependencies
class Pagination:
def __init__(
self,
skip: int = Query(0, ge=0),
limit: int = Query(100, ge=1, le=100)
):
self.skip = skip
self.limit = limit
@app.get("/items/")
async def list_items(pagination: Pagination = Depends()):
return {
"skip": pagination.skip,
"limit": pagination.limit,
"items": [] # Fetch with pagination
}
Database Session Dependency
from sqlalchemy.orm import Session
from typing import Generator
def get_db() -> Generator[Session, None, None]:
db = SessionLocal()
try:
yield db
finally:
db.close()
@app.get("/users/{user_id}")
async def get_user(
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 user
Sub-Dependencies
from fastapi import Header, HTTPException
async def verify_token(x_token: str = Header(...)):
if x_token != "secret-token":
raise HTTPException(status_code=400, detail="Invalid token")
return x_token
async def verify_key(x_key: str = Header(...)):
if x_key != "secret-key":
raise HTTPException(status_code=400, detail="Invalid key")
return x_key
async def verify_credentials(
token: str = Depends(verify_token),
key: str = Depends(verify_key)
):
return {"token": token, "key": key}
@app.get("/protected/")
async def protected_route(credentials: dict = Depends(verify_credentials)):
return {"message": "Access granted", "credentials": credentials}
Global Dependencies
async def log_requests():
print("Request received")
app = FastAPI(dependencies=[Depends(log_requests)])
# This dependency runs for ALL endpoints
Authentication and Security
OAuth2 Password Bearer with JWT
from fastapi import Depends, HTTPException, status
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from jose import JWTError, jwt
from passlib.context import CryptContext
from datetime import datetime, timedelta
from typing import Optional
SECRET_KEY = "your-secret-key-here"
ALGORITHM = "HS256"
ACCESS_TOKEN_EXPIRE_MINUTES = 30
pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")
class Token(BaseModel):
access_token: str
token_type: str
class TokenData(BaseModel):
username: Optional[str] = None
def verify_password(plain_password, hashed_password):
return pwd_context.verify(plain_password, hashed_password)
def get_password_hash(password):
return pwd_context.hash(password)
def create_access_token(data: dict, expires_delta: Optional[timedelta] = None):
to_encode = data.copy()
if expires_delta:
expire = datetime.utcnow() + expires_delta
else:
expire = datetime.utcnow() + timedelta(minutes=15)
to_encode.update({"exp": expire})
encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
return encoded_jwt
async def get_current_user(token: str = Depends(oauth2_scheme)):
credentials_exception = HTTPException(
status_code=status.HTTP_401_UNAUTHORIZED,
detail="Could not validate credentials",
headers={"WWW-Authenticate": "Bearer"},
)
try:
payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
username: str = payload.get("sub")
if username is None:
raise credentials_exception
token_data = TokenData(username=username)
except JWTError:
raise credentials_exception
user = get_user_from_db(username=token_data.username)
if user is None:
raise credentials_exception
return user
async def get_current_active_user(
current_user: User = Depends(get_current_user)
):
if not current_user.is_active:
raise HTTPException(status_code=400, detail="Inactive user")
return current_user
@app.post("/token", response_model=Token)
async def login(form_data: OAuth2PasswordRequestForm = Depends()):
user = authenticate_user(form_data.username, form_data.password)
if not user:
raise HTTPException(
status_code=status.HTTP_401_UNAUTHORIZED,
detail="Incorrect username or password",
headers={"WWW-Authenticate": "Bearer"},
)
access_token_expires = timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
access_token = create_access_token(
data={"sub": user.username}, expires_delta=access_token_expires
)
return {"access_token": access_token, "token_type": "bearer"}
@app.get("/users/me", response_model=User)
async def read_users_me(current_user: User = Depends(get_current_active_user)):
return current_user
API Key Authentication
from fastapi import Security
from fastapi.security import APIKeyHeader
API_KEY = "your-api-key"
api_key_header = APIKeyHeader(name="X-API-Key")
async def verify_api_key(api_key: str = Security(api_key_header)):
if api_key != API_KEY:
raise HTTPException(
status_code=status.HTTP_403_FORBIDDEN,
detail="Invalid API Key"
)
return api_key
@app.get("/secure-data")
async def get_secure_data(api_key: str = Depends(verify_api_key)):
return {"data": "This is secure data"}
OAuth2 with Scopes
from fastapi.security import OAuth2PasswordBearer, SecurityScopes
oauth2_scheme = OAuth2PasswordBearer(
tokenUrl="token",
scopes={
"items:read": "Read items",
"items:write": "Create and update items",
"users:read": "Read user information"
}
)
async def get_current_user_with_scopes(
security_scopes: SecurityScopes,
token: str = Depends(oauth2_scheme)
):
# Verify token and check scopes
if security_scopes.scopes:
authenticate_value = f'Bearer scope="{security_scopes.scope_str}"'
else:
authenticate_value = "Bearer"
credentials_exception = HTTPException(
status_code=status.HTTP_401_UNAUTHORIZED,
detail="Could not validate credentials",
headers={"WWW-Authenticate": authenticate_value},
)
# Decode JWT and verify scopes...
return user
@app.get("/items/", dependencies=[Security(get_current_user_with_scopes, scopes=["items:read"])])
async def read_items():
return [{"item": "Item 1"}, {"item": "Item 2"}]
@app.post("/items/", dependencies=[Security(get_current_user_with_scopes, scopes=["items:write"])])
async def create_item(item: Item):
return {"item": item}
Database Integration
SQLAlchemy Setup
from sqlalchemy import create_engine, Column, Integer, String, Boolean
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
SQLALCHEMY_DATABASE_URL = "postgresql://user:password@localhost/dbname"
engine = create_engine(SQLALCHEMY_DATABASE_URL)
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base = declarative_base()
# Models
class UserModel(Base):
__tablename__ = "users"
id = Column(Integer, primary_key=True, index=True)
username = Column(String, unique=True, index=True)
email = Column(String, unique=True, index=True)
hashed_password = Column(String)
is_active = Column(Boolean, default=True)
Base.metadata.create_all(bind=engine)
CRUD Operations
from sqlalchemy.orm import Session
# Create
@app.post("/users/", response_model=UserResponse)
async def create_user(user: UserCreate, db: Session = Depends(get_db)):
db_user = UserModel(
username=user.username,
email=user.email,
hashed_password=get_password_hash(user.password)
)
db.add(db_user)
db.commit()
db.refresh(db_user)
return db_user
# Read
@app.get("/users/{user_id}", response_model=UserResponse)
async def read_user(user_id: int, db: Session = Depends(get_db)):
user = db.query(UserModel).filter(UserModel.id == user_id).first()
if not user:
raise HTTPException(status_code=404, detail="User not found")
return user
# Update
@app.put("/users/{user_id}", response_model=UserResponse)
async def update_user(
user_id: int,
user_update: UserCreate,
db: Session = Depends(get_db)
):
user = db.query(UserModel).filter(UserModel.id == user_id).first()
if not user:
raise HTTPException(status_code=404, detail="User not found")
user.username = user_update.username
user.email = user_update.email
if user_update.password:
user.hashed_password = get_password_hash(user_update.password)
db.commit()
db.refresh(user)
return user
# Delete
@app.delete("/users/{user_id}")
async def delete_user(user_id: int, db: Session = Depends(get_db)):
user = db.query(UserModel).filter(UserModel.id == user_id).first()
if not user:
raise HTTPException(status_code=404, detail="User not found")
db.delete(user)
db.commit()
return {"message": "User deleted successfully"}
Background Tasks
from fastapi import BackgroundTasks
def send_email(email: str, message: str):
# Simulate sending email
print(f"Sending email to {email}: {message}")
def process_file(filename: str):
# Simulate file processing
print(f"Processing file: {filename}")
@app.post("/send-notification/")
async def send_notification(
email: str,
background_tasks: BackgroundTasks
):
background_tasks.add_task(send_email, email, "Welcome!")
return {"message": "Notification scheduled"}
@app.post("/upload/")
async def upload_file(
file: str,
background_tasks: BackgroundTasks
):
# Save file first
background_tasks.add_task(process_file, file)
return {"message": "File uploaded, processing in background"}
Error Handling
Custom Exception Handlers
from fastapi import Request, status
from fastapi.responses import JSONResponse
from fastapi.exceptions import RequestValidationError
class CustomException(Exception):
def __init__(self, name: str):
self.name = name
@app.exception_handler(CustomException)
async def custom_exception_handler(request: Request, exc: CustomException):
return JSONResponse(
status_code=418,
content={"message": f"Oops! {exc.name} did something wrong."},
)
@app.exception_handler(RequestValidationError)
async def validation_exception_handler(request: Request, exc: RequestValidationError):
return JSONResponse(
status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
content={"detail": exc.errors()},
)
@app.get("/items/{item_id}")
async def read_item(item_id: str):
if item_id == "error":
raise CustomException(name="Item")
return {"item_id": item_id}
Testing
Basic Tests with TestClient
from fastapi.testclient import TestClient
client = TestClient(app)
def test_read_main():
response = client.get("/")
assert response.status_code == 200
assert response.json() == {"message": "Hello World"}
def test_create_item():
response = client.post(
"/items/",
json={"name": "Test Item", "price": 10.5}
)
assert response.status_code == 200
assert response.json()["name"] == "Test Item"
def test_authentication():
response = client.post(
"/token",
data={"username": "testuser", "password": "testpass"}
)
assert response.status_code == 200
assert "access_token" in response.json()
Async Tests
import pytest
from httpx import AsyncClient
@pytest.mark.anyio
async def test_read_items():
async with AsyncClient(app=app, base_url="http://test") as ac:
response = await ac.get("/items/")
assert response.status_code == 200
assert isinstance(response.json(), list)
@pytest.mark.anyio
async def test_create_user():
async with AsyncClient(app=app, base_url="http://test") as ac:
response = await ac.post(
"/users/",
json={
"username": "newuser",
"email": "new@example.com",
"password": "securepass123"
}
)
assert response.status_code == 200
assert response.json()["username"] == "newuser"
Routers and Organization
APIRouter for Modular Code
# routers/users.py
from fastapi import APIRouter, Depends
router = APIRouter(
prefix="/users",
tags=["users"],
dependencies=[Depends(verify_token)],
responses={404: {"description": "Not found"}},
)
@router.get("/")
async def list_users():
return [{"username": "user1"}, {"username": "user2"}]
@router.get("/{user_id}")
async def get_user(user_id: int):
return {"user_id": user_id}
# main.py
from routers import users
app = FastAPI()
app.include_router(users.router)
Best Practices
1. Project Structure
my_fastapi_project/
├── app/
│ ├── __init__.py
│ ├── main.py
│ ├── config.py
│ ├── models/
│ │ ├── __init__.py
│ │ ├── user.py
│ │ └── item.py
│ ├── schemas/
│ │ ├── __init__.py
│ │ ├── user.py
│ │ └── item.py
│ ├── routers/
│ │ ├── __init__.py
│ │ ├── users.py
│ │ └── items.py
│ ├── dependencies/
│ │ ├── __init__.py
│ │ ├── auth.py
│ │ └── database.py
│ └── utils/
│ ├── __init__.py
│ └── security.py
├── tests/
│ ├── __init__.py
│ ├── test_users.py
│ └── test_items.py
├── requirements.txt
├── .env
└── README.md
2. Configuration Management
from pydantic import BaseSettings
class Settings(BaseSettings):
app_name: str = "My FastAPI App"
database_url: str
secret_key: str
algorithm: str = "HS256"
access_token_expire_minutes: int = 30
class Config:
env_file = ".env"
settings = Settings()
3. Documentation
app = FastAPI(
title="My API",
description="This is a very custom API",
version="1.0.0",
openapi_tags=[
{
"name": "users",
"description": "Operations with users.",
},
{
"name": "items",
"description": "Manage items.",
},
]
)
@app.post(
"/items/",
response_model=Item,
tags=["items"],
summary="Create an item",
description="Create an item with all the information",
response_description="The created item",
)
async def create_item(item: Item):
return item
Production Deployment
Docker Setup
# Dockerfile
FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY ./app /app
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]
Run with Gunicorn and Uvicorn Workers
gunicorn main:app --workers 4 --worker-class uvicorn.workers.UvicornWorker --bind 0.0.0.0:8000
Skill Version: 1.0.0 Last Updated: October 2025 Skill Category: Backend Development, API Development, Python Compatible With: FastAPI 0.100+, Python 3.7+, Pydantic 2.0+