python

Python Generator

This repository contains the source for the various generators that produce Python artifacts for Fern:

• fernapi/fern-python-sdk
• fernapi/fern-pydantic-model
• fernapi/fern-fastapi-server

The Python generators are written in Python. We strongly emphasize idiomatic code generation that feels hand-written and is friendly to read.

Fern handles transforming an API definition -- either an OpenAPI or Fern specification -- into Fern intermediate representation. IR is a normalized, Fern-specific definition of an API containing its endpoints, models, errors, authentication scheme, version, and more. Then the Python generator takes over and turns the IR into production-ready code.

What is Fern?

Fern is a toolkit for designing, building, and consuming REST APIs. With Fern, you can generate client libraries, API documentation, and boilerplate for your backend server.

Head over to the official Fern website for more information, or head over to our Documentation to dive straight in and find out what Fern can do for you!

Generating Python

This generator is used via the Fern CLI, by defining one of the aforementioned Python artifacts as a generator:

- name: fernapi/fern-python-sdk
  version: 0.6.6
  output:
    location: local-file-system
    path: ../generated/python

By default, Fern runs the generators in the cloud. To run a generator on your local machine, using the --local flag for fern generate. This will run the generator locally in a Docker container, allowing you to inspect its logs and output. Read more.

Configuration

You can customize the behavior of generators in generators.yml:

default-group: local
groups:
  local:
    generators:
      - name: fernapi/fern-python
      version: 0.6.6
      config: # <--
        include_validators: true

SDK Generator Configuration

The Python SDK generator supports the following options:

✨ timeout_in_seconds

Type: integer

Default: 60

By default, the generator generates a client that times out after 60 seconds. You can customize this value by providing a different number or setting to infinity to get rid of timeouts.

✨ client_class_name

Type: boolean

Default: {organization}{api-name}

By default, the generator will concatenate your organization and API name to generate the name of the client class. You can customize it by overriding this value.

✨ skip_formatting

Type: boolean

Default: true

✨ extra_dependencies

Type: map<string, string>

Default: {}

If you want to add custom dependencies to your generated SDK, you can specify them using this configuration. For example, to add a dependency on boto3, your config would look like

config:
  extra_dependencies:
    boto3: 1.28.15

✨ pydantic_config.include_union_utils

Type: boolean

Default: False

When enabled, the generator will output a Pydantic __root__ class that will contain utilities to visit the union. For example, for the following union type:

types:
  Shape:
    union:
      circle: Circle
      triangle: Triangle

you will get a generated Shape class that has a factory and visitor

# Use a factory to instantiate the union
Shape.factory.circle(Circle(...))

# Visit every case in the union
shape = get_shape()
shape.visit(
  circle: lambda circle: do_something_with_circle(circle),
  triangle: lambda triangle: do_something_with_triangle(triangle),
)

When enabled, the python generator will not run Black formatting in the generated code. Black is slow so this can potentially speed up code generation quite a bit.

✨ pydantic_config.wrapped_aliases

Type: boolean

Default: false

When enabled, any alias types defined in your Fern Definition will be generated as an individual class.

✨ pydantic_config.version

Type: "v1" or "v2" or "both"

Default: "both"

By default, the generator generates pydantic models that are v1 and v2 compatible. However you can override them to strictly for v1 or v2.

config:
  pydantic_config:
    version: v1 # or v2 or "both"

FastAPI Generator Configuration

The FastAPI generator supports the following options:

✨ pydantic_config.version

Type: "v1" or "v2" or "both"

Default: "both"

By default, the generator generates pydantic models that are v1 and v2 compatible. However you can override them to strictly for v1 or v2.

config:
  pydantic_config:
    version: v1 # or v2 or "both"

✨ async_handlers

Type: boolean

Default: "false"

When enabled, all generated services will leverage async endpoint handlers, allowing you to leverage async functions within your APIs.

✨ skip_formatting

Type: boolean

Default: true

Pydantic Generator Configuration

The Pydantic generator supports the following options:

✨ version

Type: "v1" or "v2" or "both"

Default: "both"

By default, the generator generates pydantic models that are v1 and v2 compatible. However you can override them to strictly for v1 or v2.

config:
  version: v1 # or v2 or "both"

Versions

Find the latest version number and changelog for this generator in this SDK Generators table. The changelog shows earlier version numbers, if any. You can directly use these version numbers in your generator configuration files.

For instance, if you want to use version 0.3.7 of the Python generator:

default-group: local
groups:
  local:
    generators:
      - name: fernapi/fern-python
        version: 0.3.7 # <--- generator version
        output:
          location: local-file-system
          path: ../generated/python

Fern will handle the rest automatically.