Adding custom code | Fern Documentation

Fern-generated SDKs are designed to be extended with custom code. Your custom code can add additional functionality to the SDK and live in harmony with the generated code.

This page explains how to configure custom logic using a .fernignore file, create custom SDK methods, and add additional dependencies to your SDK.

Adding custom logic

If you want your SDK to do more than just make basic API calls (like combining multiple calls, processing data, adding utilities), you can use .fernignore to protect your custom code from being overwritten during regeneration.

Simply add your custom files to the SDK repository and list them out in .fernignore. Fern won’t override any files that you add in .fernignore.

To get started adding custom code:

Create a new file and add your custom logic

src/<package>/helper.py

1 def my_helper() -> None:
2   print("Hello World!")

Add your file to `.fernignore`

A .fernignore file is automatically created in your SDK repository when you use GitHub publishing.

.fernignore

1 # Specify files that shouldn't be modified by Fern
2 
3 src/<package>/helper.py

Consume the helper

Now your users can consume the helper function by importing it from the SDK.

1 from package.helper import my_helper
2 
3 my_helper()

Adding custom SDK methods

Fern also allows you to add custom methods to the SDK itself (e.g. client.my_method() ) by inheriting the Fern generated client and then extending it.

See an example from ElevenLabs using this process in their Python SDK.

Update `generators.yml` configuration

Name your Fern-generated client something like BaseClient to reflect that this client will be extended. Configure the generator to output the client in a file called base_client.py.

generators.yml

1 - name: fernapi/fern-python-sdk
2   version: "..."
3   config:
4     client:
5       class_name: BaseClient        # The name of the generated client you will extend
6       filename: base_client.py      # The name of the file the generated client will live in
7       exported_class_name: YourClient   # The name of the class you will be creating that extends the generated client
8       exported_filename: client.py

Generate the SDK

Trigger SDK generation by running fern generate:

$ fern generate --group sdk

Import and extend the generated client

First, import the Fern generated base clients from .base_client.py and extend them to create your custom clients. Then, add whatever methods you want.

src/<package>/client.py

1   from .base_client import BaseClient // import generated client
2   
3   class YourClient(BaseClient): // extend generated client
4       def my_helper(self) -> None:
5           print("Hello World!")
6           def my_helper(self) -> None:
7               print("Hello World")

See an example client.py from ElevenLabs.

Update `.fernignore`

Add the client.py to .fernignore.

.fernignore

1 + src/<package>/client.py

See an example .fernignore from ElevenLabs.

Consume the method

Now your users can consume the helper function by importing it from the SDK.

1 client.my_helper()

Adding custom dependencies

Pro Feature

This feature is only available on paid plans. Please schedule a demo or email us to get started.

To add packages that your custom code requires, update your generators.yml.