Introduction to FastAPI: Modern Python APIs Made Simple

FastAPI is one of the most exciting Python frameworks to emerge in recent years. It’s fast (one of the fastest Python frameworks available), it automatically generates API documentation, and it uses Python type hints for request validation. If you’ve outgrown Flask or want to build a production-ready API, FastAPI is worth learning.

Why FastAPI Over Flask?

Both are great, but FastAPI has distinct advantages:

• Automatic docs — FastAPI generates interactive Swagger UI at /docs with zero effort
• Type validation — define what data your endpoint expects; FastAPI validates it automatically
• Async support — built for async Python from the ground up
• Speed — comparable to Node.js and Go in benchmarks

Your First FastAPI App

Install FastAPI and an ASGI server:

pip install fastapi uvicorn

Create main.py:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Book(BaseModel):
    title: str
    author: str
    year: int

books = []

@app.get("/books")
def get_books():
    return books

@app.post("/books", status_code=201)
def create_book(book: Book):
    books.append(book.dict())
    return book

Run it:

uvicorn main:app --reload

Visit http://localhost:8000/docs — you’ll see a fully interactive API explorer generated automatically from your code. This alone saves hours of writing API documentation.

Request Validation with Pydantic

The Book class above is a Pydantic model. If someone sends invalid data — say, a string where year should be an integer — FastAPI returns a helpful error automatically:

{
  "detail": [
    {
      "loc": ["body", "year"],
      "msg": "value is not a valid integer",
      "type": "type_error.integer"
    }
  ]
}

No manual validation code needed. This is a huge productivity win.

Conclusion

FastAPI makes building Python APIs faster, safer, and more enjoyable. The automatic documentation alone makes it worth switching to for any serious project. Start by converting a simple Flask app to FastAPI — you’ll immediately see the benefits of type hints and auto-validation.

Read next: Building a REST API with Python and Flask

External resource: FastAPI Official Documentation