FastAPI is a modern, fast web framework for building APIs with Python 3.7+ that automatically generates OpenAPI and JSON Schema documentation. While FastAPI simplifies API development, manually creating and updating API documentation can still be a time-consuming task. In this blog post, we’ll explore how to leverage FastAPI’s automatic documentation generation capabilities, specifically focusing on Swagger and ReDoc, and how to streamline the process of documenting your APIs.

Table of Contents

Installation

First things first, make sure you have FastAPI installed. If not, you can install it using pip: pip install fastapi

Additionally, you’ll need an ASGI server to run your FastAPI application. For this example, we’ll use Uvicorn: pip install uvicorn

Auto API Documentation

FastAPI uses the OpenAPI standard to describe APIs, and it automatically generates API documentation that can be viewed in Swagger UI or ReDoc. The following example demonstrates how to create a simple FastAPI application and utilize automatic documentation generation.

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root():
    """Root endpoint returning a simple message."""
    return {"message": "Hello, FastAPI!"}

@app.get("/items/{item_id}")
def read_item(item_id: int, query_param: str = None):
    """Endpoint to retrieve item details."""
    return {"item_id": item_id, "query_param": query_param}
Code language: JavaScript (javascript)

In this example, the read_root and read_item functions represent two API endpoints. FastAPI automatically extracts information from the function signatures, including parameter types, default values, and docstrings.

Now, let’s run our FastAPI application using Uvicorn: uvicorn main:app --reload

Swagger Documentation

FastAPI uses Swagger UI to create an interactive documentation interface. To access the Swagger documentation, run your FastAPI application and navigate to http://127.0.0.1:8000/docs.

Swagger UI allows developers to explore and test API endpoints interactively. It displays detailed information about each endpoint, including parameters, request/response models, and example requests.

ReDoc Documentation

ReDoc is another documentation tool supported by FastAPI. It provides a clean, responsive, and customizable documentation interface. To access ReDoc documentation, navigate to http://127.0.0.1:8000/redoc.

ReDoc focuses on simplicity and readability, presenting the API documentation in a user-friendly format. It supports dark mode, multiple languages, and offers a seamless browsing experience for API consumers.

How It Works

FastAPI leverages Python’s type hints and function signatures to infer the structure of your API. The following elements contribute to the automatic generation of documentation:

  • Path Parameters and Query Parameters: FastAPI uses the declared function parameters to determine the parameters expected by your API endpoints. This includes path parameters, query parameters, and request bodies.
  • Request and Response Models: By using Python type hints, FastAPI infers the structure of request and response models. This not only aids in data validation but also contributes to the clarity of your API documentation.
  • Dependencies: FastAPI considers dependencies, such as authentication requirements, and includes them in the documentation. This ensures that users are aware of any prerequisites for accessing specific endpoints.

Streamlining Development

Auto-generating API documentation with FastAPI streamlines the development process in several ways:

  1. Real-time Updates: Any changes made to your API code are instantly reflected in the automatically generated documentation. This real-time synchronization ensures that your documentation is always up-to-date, reducing the chances of discrepancies between the code and the documentation.
  2. Reduced Maintenance Overhead: Traditional approaches to API documentation often involve manual updates, leading to increased maintenance overhead. FastAPI’s automatic documentation generation eliminates this burden, allowing developers to focus on coding without worrying about keeping the documentation in sync.
  3. Interactive Documentation: The generated documentation is interactive, providing a playground where users can test API endpoints directly from the documentation. This feature enhances the developer experience by allowing users to understand the API’s behavior and experiment with different requests and responses.

Conclusion

Automating API documentation with FastAPI, Swagger, and ReDoc streamlines the development process and enhances collaboration between developers and API consumers. Leveraging these tools not only saves time but also ensures that your API documentation stays up-to-date as your code evolves.

As you continue to build and enhance your FastAPI applications, embrace the power of automatic documentation generation to keep your API documentation accurate, comprehensive, and accessible.