Python API Development

Modern patterns for building production-ready APIs with FastAPI and Flask.

Decision Matrix: FastAPI vs Flask

Factor	FastAPI	Flask	Winner
Performance	High (async)	Moderate	FastAPI
Auto docs	Built-in (OpenAPI)	Manual	FastAPI
Validation	Pydantic (automatic)	Manual/extensions	FastAPI
Learning curve	Steeper	Gentler	Flask for beginners
Async support	Native	With extensions	FastAPI

General guidance:

•Use FastAPI when: New projects, async needed, want auto-docs, type safety important
•Use Flask when: Simple apps, team knows Flask, sync-only fine, want simplicity

FastAPI Patterns

Basic API Structure

python

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float

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

@app.post("/items/")
def create_item(item: Item):
    return item

Pydantic Models for Validation

python

from pydantic import BaseModel, Field

class User(BaseModel):
    username: str = Field(..., min_length=3, max_length=50)
    email: str
    age: int = Field(..., ge=0, le=150)

See pydantic-validation.md for:

•Custom validators
•Model inheritance
•Nested models

Dependency Injection

python

from fastapi import Depends

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

@app.get("/items/")
def read_items(db = Depends(get_db)):
    return db.get_items()

See dependency-injection-patterns.md for:

•Class-based dependencies
•Dependency override (testing)
•Sub-dependencies

Async Endpoints

python

@app.get("/data/")
async def get_data():
    async with httpx.AsyncClient() as client:
        response = await client.get("https://api.example.com/data")
        return response.json()

See async-api-patterns.md for:

•When async helps
•Database async patterns
•Concurrent request handling

Flask Patterns

Basic Flask API

python

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route("/users/<int:user_id>")
def get_user(user_id):
    user = db.get_user(user_id)
    return jsonify(user)

See flask-patterns.md for:

•Application factory pattern
•Flask extensions
•Request hooks

API Design Best Practices

RESTful Conventions

python

GET    /api/users          # List users
POST   /api/users          # Create user
GET    /api/users/123      # Get user
PUT    /api/users/123      # Update user
DELETE /api/users/123      # Delete user

Response Structure

python

# Success
{"data": {...}, "meta": {"timestamp": "..."}}

# Error
{"error": {"code": "...", "message": "...", "details": [...]}}

Authentication Patterns

See authentication-patterns.md for:

•JWT with FastAPI
•OAuth2 with FastAPI
•API keys
•Session-based (Flask)

Anti-Patterns to Avoid

Avoid	Use Instead
Verbs in URLs (`/getUser`)	Nouns + HTTP methods (`GET /users`)
No validation	Pydantic models (FastAPI)
Catching all exceptions	Specific error handlers

source: FastAPI docs, Flask docs, REST API best practices