OpenAPI spec with a default value of object/dict causes issues with Pydantic Model

[joel-wright]

I'm trying to generate Pydantic V2 models from the Kubernetes OpenAPI Batch spec, but I'm running into issues when the default value in the OpenAPI spec is given as {}. This default value is copied directly into the model code, which generates warnings & results in invalid models when loading & relying on defaults.

One example:

UserWarning: Pydantic serializer warnings:
    Expected `ObjectMeta` but got `dict` with value `{}` - serialized value may not be as expected
    Expected `ObjectMeta` but got `dict` with value `{}` - serialized value may not be as expected
    Expected `ObjectMeta` but got `dict` with value `{}` - serialized value may not be as expected
    Expected `ObjectMeta` but got `dict` with value `{}` - serialized value may not be as expected

It seems it may be possible to solve this problem by adding the validate_default = True argument to the generated Field whenever the default value is an object/dict (in fact, I have worked around the issue locally by doing exactly this), but it would be nice to have a better solution for this problem.

To Reproduce

Generate a Pydantic V2 model from the kubernetes spec here: https://github.com/kubernetes/kubernetes/tree/master/api/openapi-spec

Used commandline:

$ datamodel-codegen --use-default-kwarg --use-union-operator --use-standard-collections --strict-nullable --use-exact-imports --input-file-type openapi --output-model-type pydantic_v2.BaseModel --target-python-version 3.10 --input kubernetes/api/openapi-spec/v3/apis__batch__v1_openapi.json --output k8s_openapi

Expected behavior
Model code that produces a correct validated model when loading JobSpecs from JSON files.

Version:

• OS: linux
• Python version: 3.10
• datamodel-code-generator version: [e.g. 0.26.4]

[joel-wright]

I wonder if it's as simple as tweaking the jinja template for pydantic v2...