FastAPI: Basics

FastAPI is a modern Python framework for building APIs. Automatic documentation, type hints, high performance.

Installation

pip install fastapi uvicorn

First Application

# main.py
from fastapi import FastAPI

app = FastAPI()

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

@app.get("/items/{item_id}")
def get_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

uvicorn main:app --reload

Running and Documentation

• Application: http://127.0.0.1:8000
• Swagger UI: http://127.0.0.1:8000/docs
• ReDoc: http://127.0.0.1:8000/redoc

Path Parameters

@app.get("/users/{user_id}")
def get_user(user_id: int):  # automatic type validation
    return {"user_id": user_id}

@app.get("/files/{file_path:path}")  # path with /
def get_file(file_path: str):
    return {"path": file_path}

Query Parameters

@app.get("/tasks/")
def list_tasks(
    status: str | None = None,
    limit: int = 20,
    offset: int = 0,
):
    return {"status": status, "limit": limit, "offset": offset}

GET /tasks/?status=todo&limit=10

POST with Request Body

from pydantic import BaseModel

class TaskCreate(BaseModel):
    title: str
    status: str = "todo"
    priority: int = 1

@app.post("/tasks/", status_code=201)
def create_task(task: TaskCreate):
    return {"id": 1, **task.model_dump()}

HTTP Methods

@app.get("/tasks/{id}")       # retrieve
@app.post("/tasks/")          # create
@app.put("/tasks/{id}")       # replace
@app.patch("/tasks/{id}")     # partial update
@app.delete("/tasks/{id}")    # delete