How to generate an OpenAPI document 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 document.

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

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

Generating an OpenAPI document with FastAPI

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

The process is fairly straightforward: FastAPI builds the OpenAPI document 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 document. 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 document. 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 document 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 framework-fastapi  folder of the Speakeasy Examples repository.

The framework-fastapi directory consists of two directories: app and sdk.

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

The sdk directory and the two OpenAPI documents, 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.

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 documents, which, in turn, influences SDK code.

Scalar API documentation

FastAPI automatically generates API documentation using Swagger UI. This example also has Scalar  API documentation. Scalar is an alternative to Swagger UI. Its standout feature, as explained in our blog post, Choosing a docs vendor: Mintlify vs Scalar vs Bump vs ReadMe vs Redocly , is that it can be used as a standalone API client. Scalar is also easy to use and has a modern UI that can be customized to create branded documentation. We use it for our Speakeasy docs .

We added Scalar to the FastAPI app by installing the Scalar FastAPI plugin .

After installing the plugin, we added a “/scalar” route, in app/main.py, that returns the Scalar API Reference:

Server configuration

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 document needs to list servers.

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

This leads to the following generated output in openapi.yaml:

Application information

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

FastAPI generates the following YAML in our openapi.yaml file:

Route customizations

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

Typed responses

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:

FastAPI adds a schema for our specific error message to openapi.yaml:

Operation tags

As your API develops and grows bigger, you’re likely to split it into separate files. FastAPI provides conveniences  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:

Tag metadata

You can add metadata to your tags to further improve the developer experience.

FastAPI accepts a parameter called openapi_tags, which we can use to add metadata, such as a description and a list of external documentation links.

Here’s how to add metadata to tags:

When we add metadata to tags, FastAPI adds a top-level tags section to our OpenAPI document:

Each tagged path in our OpenAPI document also gets a list of tags:

Operation ID customization

When FastAPI outputs an OpenAPI document, 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:

1. Using a custom generate_unique_id_function
2. Specifying operation_id per operation

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:

With FastAPI, you can also specify the operationId per operation. For our example, we’ll add a new parameter called operation_id to the operation decorator:

Webhooks

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:

Speakeasy integration

Now that we have a customized OpenAPI document, we can use Speakeasy to generate SDKs based on it. You can generate an SDK by running the gen.sh bash script, which runs the speakeasy generate command. Alternatively, you can use the Speakeasy quick start command for a guided SDK setup:

Following the prompts, provide the OpenAPI document location, name your SDK, and select the SDK language. In the terminal, you’ll see the steps taken by Speakeasy to create the SDK:

Speakeasy validates the OpenAPI document to check that it’s ready for code generation. Validation issues will be printed in the terminal and the generated SDK will be saved as a folder in your project. Speakeasy also suggests improvements for your SDK using Speakeasy Suggest, which is an AI-powered tool in Speakeasy Studio. You can see suggestions by opening the link to your Speakeasy Studio workspace in the terminal.

Let’s take a look at how the information we detailed in the OpenAPI document affects how Speakeasy generates SDKs.

Adding the local server information leads to the following generated output in the openapi.yaml file:

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

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

Speakeasy uses the title, summary, and descriptions we provided earlier to add helpful text to the generated SDK documentation, including comments in the SDK code. For example, in sdk/src/openapi/sdk.py:

Speakeasy adds the version to the SDKConfiguration in sdk/src/openapi/sdkconfiguration.py. It also uses this version to construct the user agent (user_agent), which contains the version of the SDK, the version of the Speakeasy generator build, and the version of the OpenAPI documentation:

When users call your API using the generated SDK, the user_agent from SDKConfiguration is automatically added to the user-agent header. The _build_request_with_client method in BaseSDK constructs the HTTP request and sets the header using headers[user_agent_header] = self.sdk_configuration.user_agent:

Operation ID issues and solutions

The unique operation_id generated by FastAPI does not translate well into an SDK. We need to customize the unique operation_id that FastAPI generates for better readability.

For instance, in the operation that returns a burger by burger_id, the default unique ID would be read_burger_burger__burger_id__get. This makes its way into SDK code, leading to class names such as ReadBurgerBurgerBurgerIDGetRequest or function names like read_burger_burger_burger_id_get.

Here’s a usage example after generating an SDK without customizing the operationId:

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

In addition to the two methods described earlier, there is a third way to customize the operation_id. We can add the top-level x-speakeasy-name-override Speakeasy extension  to our OpenAPI document, allowing Speakeasy to override the generated names when it generates SDK code. To add this extension, follow the Speakeasy guide to changing method names.

Adding retry functionality

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 document. 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 that you’ll need to add this customization after declaring your operation routes.

This change adds the following top-level section to openapi.yaml:

To add x-speakeasy-retries to a single operation, update the operation and add the openapi_extra parameter as follows:

Authentication and security

FastAPI supports several authentication mechanisms that can easily be integrated into your API.

The example below demonstrates adding an API key authentication scheme to the /burger/ endpoint of our API. We use the APIKeyHeader dependency to validate the API key passed in the Authorization header:

We can pass a key parameter to the list_burgers function, retrieve the API key from the header, and perform validation:

Now when generating the OpenAPI document, the API key authentication scheme will be included and only required for the listing on the /burger/ endpoint.

Handling form data

Form data is a common way to receive information from clients, particularly in web applications. FastAPI provides robust support for form data through its Form class, and it correctly documents these form fields in the OpenAPI schema.

When working with form data in FastAPI, you need to use the Form class from the fastapi module. This ensures that FastAPI correctly adds the appropriate schema information to your OpenAPI document.

In the OpenAPI document, FastAPI correctly identifies this endpoint as accepting form data:

When Speakeasy generates an SDK from this OpenAPI document, it will correctly handle form data submissions. The generated SDK will provide a clean interface for submitting form data:

File uploads

FastAPI also supports file uploads, both as individual files or as multiple files. The OpenAPI schema will correctly document these endpoints as accepting multipart form data.

This endpoint will be documented in the OpenAPI schema with multipart/form-data content type, allowing Speakeasy to generate appropriate SDK code for handling file uploads.

Advanced form validation

For form data that needs validation beyond simple type checking, you can combine Pydantic models with the Form class. First, define your model with the validation rules:

Then use the model fields with form data:

FastAPI will generate an OpenAPI document that includes these validation constraints, allowing SDK users to understand the requirements before submitting the form.

Summary

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