[Python] Fix allOf inheritance by using **kwargs throughout inheritance chain

[ericfitz]

Summary

When a child model extends a parent via allOf, the generated __init__ method was consuming parameters from kwargs, preventing them from reaching parent classes. This caused deserialization failures with errors like:

• ValueError: Invalid value for 'name' (None), must not be 'None'
• TypeError: Parent.__init__() got an unexpected keyword argument 'child_field'

The Problem

Consider this OpenAPI spec with inheritance:

components:
  schemas:
    Parent:
      type: object
      required: [name, type]
      properties:
        name:
          type: string
        type:
          type: string
          enum: [typeA, typeB]
    
    Child:
      allOf:
        - $ref: '#/components/schemas/Parent'
        - type: object
          properties:
            child_field:
              type: string

The old generated code had two problems:

class Child(Parent):
    def __init__(self, name=None, type=None, child_field=None, *args, **kwargs):
        self._name = None
        self._type = None
        self._child_field = None
        self.name = name          # consumed from kwargs
        self.type = type          # consumed from kwargs  
        self.child_field = child_field
        Parent.__init__(self, *args, **kwargs)  # BUG: name, type NOT in kwargs anymore!

class Parent:
    def __init__(self, name=None, type=None):  # No **kwargs - rejects unknown params!
        self.name = name  # Gets None because Child consumed it
        self.type = type  # Gets None because Child consumed it

Problem 1: Python extracts named parameters from **kwargs into local variables, so when Parent.__init__ is called, those parameters are no longer in kwargs.

Problem 2: Root classes didn't accept **kwargs, so if a child class has fields the parent doesn't know about, it fails with "unexpected keyword argument".

The Fix

For child classes (with parent): Use **kwargs for all parameters and extract with kwargs.get():

class Child(Parent):
    def __init__(self, **kwargs):
        self._name = None
        self._type = None
        self._child_field = None
        name = kwargs.get('name')
        type = kwargs.get('type')
        child_field = kwargs.get('child_field')
        self.name = name
        self.type = type
        self.child_field = child_field
        Parent.__init__(self, **kwargs)  # Full kwargs passed through!

For root classes (without parent): Add **kwargs to accept extra parameters:

class Parent:
    def __init__(self, name=None, type=None, **kwargs):  # Accepts extra kwargs
        self.name = name
        self.type = type

This ensures the full kwargs dict flows through the entire inheritance chain, and each class extracts what it needs without breaking the chain.

Test Plan

• Generated a Python client with multi-level allOf inheritance
• Verified child class instances correctly populate parent class attributes
• Verified parent classes correctly ignore unknown child-specific fields
• Tested deserialization of API responses with inherited models
• In my project: all unit tests pass
• In my project: all integration tests pass (against production API)

Diagnosis Notes

This bug manifests as errors like:

• ValueError: Invalid value for 'property_name' (None), must not be 'None'
• ValueError: Invalid value for 'property_name' (None), must be one of [...]
• Parent class validation failures during deserialization of child objects

Repro Notes

• My OpenAPI schema is at here
• My OpenAPI client generation code is here
• I generated my client with swagger-codegen 3.0.75
• Tested with Python 3.14.2

🤖 Generated with Claude Code
🧍🏻Manually reviewed and edited by a real human, not just AI slop