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

Created by: alexras

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?
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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.