Adding custom code | Fern Documentation

This page covers how to add custom logic, methods, and dependencies to your Python SDK.

Before getting started, read about how Fern SDKs use custom code and learn about the .fernignore file.

Adding custom logic

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`

.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 and Enterprise feature

This feature is available only for the Pro and Enterprise plans. To get started, reach out to support@buildwithfern.com.

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