What is FastAPI and why is it popular?

Question

Accepted Answer

**FastAPI** is a modern Python **web framework** for building APIs with automatic documentation.

**Key features:**
- **Fast** — based on Starlette and Pydantic.
- **Type hints** — automatic validation.
- **Auto docs** — Swagger UI and ReDoc.
- **Async** — native async/await support.
- **Standards-based** — OpenAPI and JSON Schema.

```python
from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

app = FastAPI()

class User(BaseModel):
    name: str
    email: str
    age: int | None = None

users_db: dict[int, User] = {}

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

@app.post("/users/")
async def create_user(user: User):
    users_db[len(users_db)] = user
    return {"message": "User created"}

# Run: uvicorn main:app --reload
# Docs: http://localhost:8000/docs
```

**vs Flask:**
- FastAPI: async, auto validation, type hints.
- Flask: simpler, larger ecosystem.

Python: What is FastAPI and why is it popular?