How to generate an OpenAPI/Swagger spec with FastAPI

Many developers start their API development with FastAPI, and with good reason. FastAPI has rapidly gained traction in the Python community for its excellent performance, intuitive design, and flexibility. It enables developers to craft API solutions that not only run fast but also meet their users’ unique needs.

FastAPI is great for building your core API, but you’ll want to layer on SDKs and docs to provide your users with easy integration. For that, you’ll want an OpenAPI file.

The good news is that FastAPI provides you with an OpenAPI file out of the box. The less good news is that you’ll need some tweaking to get the OpenAPI spec to a level where it becomes usable with other tooling.

This article will show you how to improve the default OpenAPI spec generation to make the most of the generated schema.

Generating an OpenAPI Spec With FastAPI

Understanding how FastAPI generates OpenAPI schemas can help you make more informed decisions when you customize your FastAPI setup.

The process is fairly straightforward: FastAPI builds the OpenAPI schema based on the routes and models you’ve defined in your application. For every route in your FastAPI application, FastAPI adds an operation to the OpenAPI schema. For every model used in these routes, FastAPI adds a schema definition. The request and response bodies, parameters, and headers all draw from these schema definitions.

While this process works well out of the box, FastAPI also offers several customization options that can change the generated OpenAPI schema. We’ll cover some of these options in the following sections.

Our FastAPI Example App: APItizing Burgers

Let’s get this out of the way: the name came in a daydream shortly before lunchtime.

To guide us through this journey, we’ll use a simple example FastAPI application: the “APItizing Burgers” burger shop API. This API includes two models, Burger and Order, and provides basic CRUD operations for managing burgers and orders at our hypothetical burger shop. Additionally, we have a webhook defined for order status events.

We’ll look at how we optimized this FastAPI application and refined our models and routes so that the generated OpenAPI specification is intuitive and easy to use. We will also explore how we can use this schema to generate SDKs using Speakeasy. The source code for our example API is available in the apitizing-burgers (opens in a new tab) repository.

The repository consists of two directories: app and sdk.

The app directory contains only our FastAPI server definition: app/main.py. This is where we’ll look at what we customized.

The sdk directory and the two specifications, openapi.yaml and openapi.json, are generated by running gen.sh in the root of the project.

Join us as we dive into FastAPI customization and discover how these tweaks can streamline your SDK generation process.

WARN

When using Pydantic to define models, a known issue is that the serialization of datetime objects is not timezone-aware. This will cause a mismatch with the OpenAPI format date-time, which requires RFC 3339 date-time strings with timezones included. Consider using AwareDatetime (opens in a new tab) fields in Pydantic models to enable the appropriate validation (opens in a new tab) and ensure your SDK behavior matches the response definition from your server.

Basic FastAPI Setup

Let’s get started with the basics – some things you probably do already.

These straightforward examples are trivial but will help you better understand the three steps in the automation pipeline: How FastAPI setup influences OpenAPI specifications, which, in turn, influences SDK code.

Add a List of Servers to Your FastAPI App

This may seem obvious, but while first working with FastAPI in development, the generated docs, development server, and API operations all work out of the box without the need to manually specify your server address.

However, when generating SDKs, your OpenAPI spec needs to list servers.

In our app/main.py, we added our local server as shown.

Add a Title, Summary, Description, and Version to Your FastAPI App

In our app/main.py, if we have the following.

Route-Level Customizations: Enhancing Usability

With the basics out of the way, let’s look at a few more substantial recommendations.

When developers use your generated SDK, they may wish to see what all the possible responses for an API call could be.

With FastAPI, you can add additional responses to each route by specifying a response type.

In our app/main.py, we added this abbreviated code.

Group FastAPI Operations With OpenAPI Tags and Tag Metadata

As your API develops and grows bigger, you’re likely to split it into separate files. FastAPI provides conveniences (opens in a new tab) to help reduce boilerplate and repetition when splitting an API into multiple modules.

While this separation may reduce cognitive overhead while you’re working in particular sections of the API code, it doesn’t mean similar groups are automatically created in your documentation and SDK code.

We recommend you add tags to all operations in FastAPI, whether you’re building a big application or only have a handful of operations, so that operations can be grouped by tag in generated SDK code and documentation.

The most straightforward way to add tags is to edit each operation and add a list of tags. This example highlights the tags list.

Each tagged path in our OpenAPI spec also gets a list of tags.

Customize the OpenAPI

When FastAPI outputs an OpenAPI spec, it generates a unique OpenAPI operationId for each path. By default, this unique ID is generated by the FastAPI generate_unique_id function.

This can often lead to cumbersome and unintuitive names. To improve usability, we have two methods of customizing these generated strings.

The preferred method is to use a custom function when you generate unique IDs for paths.

The example below is an illustrative function that doesn’t generate guaranteed-unique IDs and doesn’t handle method names without an underscore. However, it demonstrates how you can add a function that generates IDs based on an operation’s method name.

Add Webhooks for Real-Time Notifications

Starting with OpenAPI version 3.1.0, it is possible to specify webhooks for your application in OpenAPI.

Here’s how to add a webhook to FastAPI:

FastAPI generates the following top-level webhooks section in openapi.yaml.

Integrating Speakeasy

Now that we have a customized OpenAPI spec, we can use Speakeasy to generate SDKs based on it. Let’s take a look at how the information we detailed in the OpenAPI specification affects how Speakeasy generates SDKs.

After we added our local server information, this is how it generates in the openapi.yaml file.

After Speakeasy generates the SDK, this leads to the following abbreviated code in sdk/src/sdk/sdkconfiguration.py.

You’ll find calls to SDKConfiguration.get_server_details() when the SDK builds API URLs.

Speakeasy adds the version to the SDKConfiguration in sdk/src/sdk/sdkconfiguration.py.

Eventually, when your users call your API using the generated SDK, this version makes its way into the headers passed with requests. For example, in sdk/src/sdk/burger.py, we see that headers['user-agent'] includes a reference to SDKConfiguration.openapi_doc_version.

This way, your server logs will include the FastAPI App version along with other helpful details about the SDK.

However, after using the custom function we defined previously, the read_burger operation gets a much friendlier operation ID: readBurger. And the usage example becomes much easier to read.

In addition to the two methods described earlier for customizing the operation_id, there is a third way. We can add the top-level x-speakeasy-name-override extension to our OpenAPI spec, allowing Speakeasy to override these generated names when it generates SDK code.

To add this extension, follow the Speakeasy guide on changing method names.

Add Retries to Your SDK With

Speakeasy can generate SDKs that follow custom rules for retrying failed requests. For instance, if your server fails to return a response within a specified time, you may want your users to retry their request without clobbering your server.

To add retries to SDKs generated by Speakeasy, add a top-level x-speakeasy-retries schema to your OpenAPI spec. You can also override the retry strategy per operation by adding x-speakeasy-retries to each operation.

To add global retries, we need to customize the schema generated by the FastAPI get_openapi function.

Keep in mind you’ll need to add this customization after declaring your operation routes.

main.py

from fastapi import FastAPI

app = FastAPI(
    servers=[
        {"url": "http://127.0.0.1:8000", "description": "Local server"},
    ],
)

Summary

In this post, we’ve explored how you can set up a FastAPI-based SDK generation pipeline without hand-editing or updating OpenAPI specifications. By using existing FastAPI methods for extending and customizing OpenAPI specifications, you can improve the usability of your generated client SDKs.