01 Introduction To Fastapi

title: Introduction to FastAPI for REST APIs weight: 11 date: 2026-06-28 lastmod: 2026-06-28 description: Learn why FastAPI is ideal for building REST APIs with automatic OpenAPI docs, Pydantic validation, async support, and high performance comparable to Node.js and Go. tags: [api-development, fastapi]

FastAPI is a modern Python web framework for building REST APIs with automatic OpenAPI documentation, Pydantic-based request validation, native async support, and performance comparable to Node.js and Go.

```mermaid
flowchart TD
  A[FastAPI] --> B[Automatic OpenAPI]
  A --> C[Pydantic Validation]
  A --> D[Async Support]
  A --> E[High Performance]
  B --> F[Interactive Docs]
  C --> G[Type Safety]
  D --> H[Non-blocking I/O]
  style A fill:#e1f5fe
  style B fill:#c8e6c9
  style C fill:#c8e6c9
  style D fill:#fff9c4

FastAPI uses Python type hints to define request parameters, request bodies, and response models. It automatically generates OpenAPI 3.0 specs and interactive documentation. Built on Starlette and Pydantic, it is one of the fastest Python web frameworks available.

Think of FastAPI like a smart factory robot. You tell it what raw materials to expect (type hints), what product to make (response model), and it automatically documents the process (OpenAPI), inspects quality (validation), and works efficiently (async).

Example: Hello World with FastAPI

from fastapi import FastAPI

app = FastAPI(title="My API", version="1.0.0")

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

@app.get("/api/health")
def health_check():
    return {"status": "ok", "timestamp": "2026-06-28T10:00:00Z"}

# Run with: uvicorn main:app --reload

Expected output:

$ curl http://localhost:8000/api/health
{"status":"ok","timestamp":"2026-06-28T10:00:00Z"}

Example: Path and Query Parameters

from fastapi import FastAPI
from typing import Optional

app = FastAPI()

@app.get("/api/users/{user_id}")
def get_user(user_id: int, include_details: Optional[bool] = False):
    user = {"id": user_id, "name": "Alice", "email": "alice@example.com"}
    if include_details:
        user["role"] = "admin"
        user["created_at"] = "2026-01-15"
    return {"status": "success", "data": user}

# GET /api/users/42
# GET /api/users/42?include_details=true

Expected output:

{"status":"success","data":{"id":42,"name":"Alice","email":"alice@example.com","role":"admin","created_at":"2026-01-15"}}

Example: Automatic OpenAPI Documentation

from fastapi import FastAPI

app = FastAPI(
    title="Users API",
    description="API for managing users",
    version="1.0.0",
    docs_url="/docs",
    redoc_url="/redoc",
    openapi_url="/openapi.json"
)

@app.get("/api/users")
def list_users():
    """List all users with pagination."""
    return {"data": []}

Expected output:

Visit http://localhost:8000/docs for Swagger UI
Visit http://localhost:8000/redoc for ReDoc
Visit http://localhost:8000/openapi.json for OpenAPI spec

Common Mistakes

1. Not using type hints — FastAPI relies on Python type hints for validation and documentation. Omitting them loses automatic validation and docs.
2. Forgetting to run with uvicorn — FastAPI requires an ASGI server like uvicorn. Running with python directly does not work.
3. Using sync functions for blocking I/O — Sync functions block the event loop. Use async def for database queries and external API calls.
4. Not setting docs_url to None in production — Exposing Swagger UI in production may leak API structure. Disable or protect it.
5. Confusing path and query parameters — Path parameters are required. Query parameters are optional by default. Use the right type for each.

Practice Questions

1. What ASGI server is typically used with FastAPI?
2. How does FastAPI generate OpenAPI documentation automatically?
3. What is the difference between a path parameter and a query parameter?
4. Why should you use async def for database operations?
5. Challenge: Create a FastAPI app with endpoints for a todo list. Include GET /todos, POST /todos, GET /todos/{id}, and PUT /todos/{id}. Use type hints for all parameters and response models.

FAQ

Mini Project

Create a FastAPI application with three endpoints: a health check, a user listing with pagination, and a user creation endpoint. Define Pydantic models for request and response. Run with uvicorn and verify automatic OpenAPI docs at /docs.

What's Next

Now learn about installation and setup in Building REST APIs with FastAPI.