Fix oneOf handling for lists of primitives in model_utils

[MintsInc]

Context

PR https://github.com/DataDog/datadog-api-spec/pull/4191 introduced a new model CustomAttributeValuesUnion with a oneOf schema containing primitive list types:

oneOf: [str, [str], float, [float]]

This caused test failures in the generated Python client (#2759):

• CI Run: https://github.com/DataDog/datadog-api-client-python/actions/runs/18286712780/job/52063527051
• Failing test: test_update_case_custom_attribute_returns_not_found_response

The issue was that the generator tried to instantiate primitive types using dict unpacking (str(**arg, **kwargs)), which fails with a TypeError.

This is because of two bugs in the model_utils.j2 template:

1. get_oneof_instance function: When handling list types in oneOf schemas, the code didn't distinguish between:

   • Lists of primitives (e.g., [str], [float])
   • Lists of ModelSimple objects
   • Lists of complex objects (ModelNormal, ModelComposed)

   That's because checking cls in PRIMITIVE_TYPES doesn't catch a list of string for example.

   It treated all lists the same way, using dict unpacking which only works for complex objects.

2. composed_model_input_classes function: Called issubclass([str], ModelSimple) which fails with TypeError: issubclass() arg 1 must be a class when the oneOf schema contains list types like [str].

Changes

Updated .generator/src/generator/templates/model_utils.j2 with two fixes:

1. Type Guard in composed_model_input_classes (lines 73-92)

Added a guard to handle list types before calling issubclass:

if isinstance(cls, list):
    return [cls]

This prevents TypeError: issubclass() arg 1 must be a class when encountering list types like [str] or [float] in oneOf schemas.

2. Three-Way List Handling in get_oneof_instance (lines 1558-1602)

Distinguished between three types of list contents:

• Lists of primitives ([str], [float]): Use validate_and_convert_types to properly handle primitive values
• Lists of ModelSimple: Use single-arg constructor ModelSimple(arg)
• Lists of complex objects: Use dict unpacking (existing behavior preserved)

Tests

I cherry-picked the changes from this PR onto #2759 and ran the tests (I unskipped the problematic test before)

Related Work

Similar issues were fixed in the Java client:

• fix oneof methods name datadog-api-client-java#3177
• Call the correct method of oneof examples datadog-api-client-java#3180

[MintsInc]

• Fix oneOf handling for lists of primitives in model_utils #2880 👈 (View in Graphite)
• master

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[Kaycell]

This LGTM but if I'm correct we run test scenarios against these models? Can we add one with oneOf?