Adding custom code (Python) | Fern Documentation

Enterprise feature

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

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

Adding custom logic

To get started adding custom code:

These steps cover adding a new custom file to the SDK. To preserve line-level edits to a generated file, use Replay instead.

Create a new file and add your custom logic

src/<package>/helper.py

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

Add your file to `.fernignore`

.fernignore

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

Consume the helper

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

from package.helper import my_helper
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

- name: fern-python-sdk
  version: "..."
  config:
    client:
      class_name: BaseClient        # The name of the generated client you will extend
      filename: base_client.py      # The name of the file the generated client will live in
      exported_class_name: YourClient   # The name of the class you will be creating that extends the generated client
      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

  from .base_client import BaseClient // import generated client
  
  class YourClient(BaseClient): // extend generated client
      def my_helper(self) -> None:
          print("Hello World!")
          def my_helper(self) -> None:
              print("Hello World")

See an example client.py from ElevenLabs.

Update `.fernignore`

Add the client.py to .fernignore.

.fernignore

+ 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.

client.my_helper()

Adding custom dependencies

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