Introduction to FastAPI

FastAPI is a modern, fast (high-performance) web framework for building APIs with Python 3.7+ based on standard Python type hints. It is designed to be easy to use, highly performant, and to provide automatic data validation and serialization.

Key Concepts

1. Asynchronous Programming

FastAPI leverages Python's asynchronous capabilities, allowing it to handle a large number of concurrent connections efficiently. This is achieved using the async and await keywords, which are part of Python's asyncio library.

2. Type Hints

FastAPI uses Python's type hints to automatically validate and serialize data. This means that you can define the expected types for your function parameters and return values, and FastAPI will handle the rest. This not only makes your code more readable but also reduces the likelihood of runtime errors.

3. Dependency Injection

FastAPI supports dependency injection, which allows you to define dependencies that can be injected into your route handlers. This is useful for managing resources like database connections or authentication tokens in a clean and reusable way.

4. Automatic Documentation

FastAPI automatically generates interactive API documentation using Swagger UI and ReDoc. This documentation is based on the OpenAPI standard and is automatically updated as you modify your code. This makes it easier for developers to understand and interact with your API.

Examples

Example 1: Basic FastAPI Application

Here is a simple example of a FastAPI application that defines a single route:

from fastapi import FastAPI

app = FastAPI()

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

Example 2: Using Type Hints

In this example, we use type hints to define the expected input and output types for a route:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float

@app.post("/items/")
async def create_item(item: Item):
    return {"item": item}
    

Example 3: Dependency Injection

Here, we define a dependency that retrieves a user from a database:

from fastapi import FastAPI, Depends

app = FastAPI()

def get_user():
    return {"username": "john_doe"}

@app.get("/user/")
async def read_user(user = Depends(get_user)):
    return user
    

FastAPI is a powerful tool for building high-performance APIs with Python. By understanding its key concepts and features, you can create robust and maintainable web services.