FastAPI has exploded in popularity because:

✔ It’s fast (built on Starlette + Pydantic)
✔ Auto-docs with Swagger
✔ Type-safe
✔ Perfect for microservices & modern APIs

Let’s build a Users API with full CRUD.

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📦 Step 1 — Install FastAPI & Uvicorn

pip install fastapi uvicorn

Run server later with:

uvicorn main:app --reload

---

🏗 Step 2 — Create main.py

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def home():
    return {"message": "FastAPI is running 🚀"}

Run it — open:

http://127.0.0.1:8000/docs

Yes — you already get Swagger UI for free.

---

🧩 Step 3 — Create a User Model (Validation Included)

FastAPI uses Pydantic models for validation.

from pydantic import BaseModel, EmailStr

class User(BaseModel):
    id: int
    name: str
    email: EmailStr

We’ll use an in-memory list (like a fake DB first):

users = []

---

➕ Step 4 — Create User (POST)

@app.post("/users")
def create_user(user: User):
    users.append(user)
    return user

Now when you send JSON:

{
  "id": 1,
  "name": "John",
  "email": "john@email.com"
}

FastAPI will:

✔ Validate email
✔ Validate types
✔ Auto-document everything

---

📖 Step 5 — Get All Users (GET)

@app.get("/users")
def get_users():
    return users

---

🔍 Step 6 — Get User by ID

from fastapi import HTTPException

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

Clean, readable, predictable.

---

✏️ Step 7 — Update User (PUT)

@app.put("/users/{user_id}")
def update_user(user_id: int, updated: User):
    for i, user in enumerate(users):
        if user.id == user_id:
            users[i] = updated
            return updated
    raise HTTPException(status_code=404, detail="User not found")

---

❌ Step 8 — Delete User (DELETE)

@app.delete("/users/{user_id}")
def delete_user(user_id: int):
    for i, user in enumerate(users):
        if user.id == user_id:
            users.pop(i)
            return {"message": "User deleted"}
    raise HTTPException(status_code=404, detail="User not found")

You now have a complete CRUD API.

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🔥 Bonus — Partial Update (PATCH)

Sometimes you only want to update one field.

from typing import Optional

class UserUpdate(BaseModel):
    name: Optional[str]
    email: Optional[EmailStr]

@app.patch("/users/{user_id}")
def patch_user(user_id: int, data: UserUpdate):
    for user in users:
        if user.id == user_id:
            if data.name:
                user.name = data.name
            if data.email:
                user.email = data.email
            return user
    raise HTTPException(status_code=404, detail="User not found")

---

🧠 Common Problems & Fixes

❌ Pydantic Validation Errors

Means your JSON doesn’t match the model.
Check keys & types.

---

❌ “Address already in use”

Stop previous server:

ctrl + c

Or change port:

uvicorn main:app --reload --port 9000

---

❌ CORS Error (Frontend Calling API)

Install:

pip install fastapi[all]

Add:

from fastapi.middleware.cors import CORSMiddleware

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_methods=["*"],
    allow_headers=["*"],
)

---

🏗 Real-World Folder Structure (When App Grows)

app/
 ┣ models.py
 ┣ routes.py
 ┣ db.py
 ┣ main.py

Keeps things clean.

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📌 Best Practices

✔ Use environment variables
✔ Validate all inputs
✔ Use async where possible
✔ Prefer schemas over raw dicts
✔ Add logging

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🏁 Final Thoughts

FastAPI makes API development feel modern again:

👉 type-safe
👉 fast
👉 minimal boilerplate
👉 built-in docs

Once you grasp:

model → route → validation → response

…you can build production-grade APIs quickly and safely.