[BUG][Python] UUID properties with default values trigger ClassCastException

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

If I define a schema that has a UUID property, and that property has a default value, the Python code generator throws a ClassCastException.

openapi-generator version

4.0.3

OpenAPI declaration file content or url

---
components:
  schemas:
    MyModel:
      properties:
        id:
          default: "00000000-0000-4000-8000-000000000000"
          format: uuid
          type: string
      title: MyModel
      type: object

info:
  title: 'My API'
  version: 0.1.0dev

openapi: 3.0.2

paths: {}

Command line used for generation

docker run --rm -v ${PWD}:/local --env 'PYTHON_POST_PROCESS_FILE="/usr/local/bin/yapf -i"' openapitools/openapi-generator-cli:v4.0.3 generate -i /local/swagger.yml -g python -o /local/build

Steps to reproduce

Run the generator with the above declaration file; it will throw an exception that looks like this:

Exception in thread "main" java.lang.RuntimeException: Could not process model 'MyModel'.Please make sure that your schema is correct!
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:470)
        at org.openapitools.codegen.DefaultGenerator.generate(DefaultGenerator.java:920)
        at org.openapitools.codegen.cmd.Generate.run(Generate.java:396)
        at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:60)
Caused by: java.lang.ClassCastException: java.util.UUID cannot be cast to java.lang.String
        at org.openapitools.codegen.languages.PythonClientCodegen.toDefaultValue(PythonClientCodegen.java:643)
        at org.openapitools.codegen.DefaultCodegen.fromProperty(DefaultCodegen.java:1981)
        at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:3627)
        at org.openapitools.codegen.DefaultCodegen.addVars(DefaultCodegen.java:3575)
        at org.openapitools.codegen.DefaultCodegen.fromModel(DefaultCodegen.java:1845)
        at org.openapitools.codegen.DefaultGenerator.processModels(DefaultGenerator.java:1197)
        at org.openapitools.codegen.DefaultGenerator.generateModels(DefaultGenerator.java:465)
        ... 3 more

If you remove the default and run the generator again, code generation will succeed.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/2209 - similar exception, with a different cause.

Suggest a fix

Rather than just casting to a string, perhaps just calling p.getDefault().toString() would work.

Start with PythonClientCodegen.java at the toDefaultValue method around line 643, using the minimal OpenAPI YAML and Docker generation command from the issue to reproduce the exception. Done means the Python generator completes successfully for a UUID property with a default value and produces the client model without the ClassCastException.