🚀 Lesson 2 — Building APIs in FastAPI (GET, POST, PUT, DELETE + Params)

This lesson is hands-on, practical, and industry-style, preparing you for real backend + AI applications.

🎯 What You Will Learn Today

✔ GET, POST, PUT, DELETE routes
✔ Path parameters
✔ Query parameters
✔ Request body using Pydantic
✔ Status codes & error handling
✔ Real industry example (Users API)

Let’s begin. 🔥

🧠 1. Basic GET Endpoint

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def home():
    return {"msg": "Welcome to Lesson 2 🚀"}

🛣 2. Path Parameters (Dynamic URLs)

Example:
/items/10 → return item with id 10

@app.get("/items/{item_id}")
def get_item(item_id: int):
    return {"item_id": item_id}

FastAPI automatically:

• converts item_id to int
• validates it
• sends 422 error if wrong type

🔎 3. Query Parameters

Example:
/search?keyword=phone&limit=5

@app.get("/search")
def search(keyword: str, limit: int = 10):
    return {"keyword": keyword, "limit": limit}

Here:

• keyword → required
• limit → optional (default = 10)

📦 4. Request Body (POST with JSON)

We use Pydantic Models to validate incoming JSON.

from pydantic import BaseModel

class User(BaseModel):
    name: str
    age: int
    email: str

POST endpoint:

@app.post("/users")
def create_user(user: User):
    return {"status": "User added", "data": user}

Send JSON:

{
  "name": "Rajeev",
  "age": 29,
  "email": "raj@example.com"
}

FastAPI validates:

• age must be int
• email must be str
• name required

🔄 5. PUT API (Update Resource)

@app.put("/users/{user_id}")
def update_user(user_id: int, user: User):
    return {"msg": "Updated", "user_id": user_id, "new_data": user}

PUT = replace whole object
PATCH = partial update (we’ll do later)

❌ 6. DELETE API

@app.delete("/users/{user_id}")
def delete_user(user_id: int):
    return {"msg": "User deleted", "user_id": user_id}

🚨 7. Error Handling + Status Codes

from fastapi import HTTPException

@app.get("/protected/{id}")
def read(id: int):
    if id <= 0:
        raise HTTPException(status_code=400, detail="ID must be positive!")
    return {"id": id}

🏭 8. Industry Example — Complete User CRUD API

This is exactly what companies expect.

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

app = FastAPI()

class User(BaseModel):
    name: str
    age: int
    email: str

db = {}  # temporary in-memory DB

@app.post("/users")
def create_user(user: User):
    user_id = len(db) + 1
    db[user_id] = user
    return {"id": user_id, "data": user}

@app.get("/users/{user_id}")
def get_user(user_id: int):
    if user_id not in db:
        raise HTTPException(404, "User not found")
    return db[user_id]

@app.put("/users/{user_id}")
def update_user(user_id: int, user: User):
    if user_id not in db:
        raise HTTPException(404, "User not found")
    db[user_id] = user
    return {"msg": "Updated", "data": user}

@app.delete("/users/{user_id}")
def delete_user(user_id: int):
    if user_id not in db:
        raise HTTPException(404, "User not found")
    del db[user_id]
    return {"msg": "Deleted"}

You now have:

✔ Add user
✔ Read user
✔ Update user
✔ Delete user
✔ Handle invalid user
✔ Validate input automatically

🧪 9. Try Everything in Swagger UI

Go to:

http://127.0.0.1:8000/docs

You can run APIs live without Postman.

📌 Lesson 2 Summary

You learned:

• GET, POST, PUT, DELETE
• Path & query parameters
• JSON body validation (Pydantic)
• Error handling
• Complete CRUD example

This is enough to build:

• backend for React/Next.js
• AI/ML model API
• microservices
• app backend

🚀 Ready for Lesson 3 — Pydantic Deep Dive?

Lesson 3 topics:

✔ Advanced validation
✔ Optional fields
✔ Default values
✔ Email validation
✔ Nested models
✔ Enums
✔ Config & constraints
✔ Industry examples

Should I continue with Lesson 3 now?

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