Problem: leading _ in name is removed by PythonClientCodegen

[dkliban]

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Description of the PR

Solution: move leading _ to the end of the name

Double leading underscores in Python attribute names cause them to have their names
mangled as described in PEP 8[0]. That's why they were being removed completely before
this patch.

Due to the implementation details of the python models that are generated, leaving a
single leading underscore causes a conflict between setters of variables name that differ
only by a leading underscore. e.g.: 'name' and '_name'.

This patch changes how leading underscores in variables and parameters are handled. All
leading underscores are moved to the end of the name.

[0] https://www.python.org/dev/peps/pep-0008/

[jimschubert]

cc python technical community for review:
@taxpon @frol @mbohlool @cbornet @kenjones-cisco @tomplus @Jyhess

The target spec for this work defined type as well as _type, and this caused issues. I provided some feedback in Slack, but I've only used Python minimally. The approach in this PR looks good to me, but does anyone else have any concerns with merging it?

[dkliban]

This patch resolves some of the concerns raised here #3168

[wing328]

@dkliban Please add a few tests to https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/java/org/openapitools/codegen/python/PythonClientCodegenTest.java

A good reference is https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/test/java/org/openapitools/codegen/typescript/typescriptnode/TypeScriptNodeClientCodegenTest.java#L20 (you can simply copy all these tests to start with)

[wing328]

Looks like this is a breaking change without fallback?

[dkliban]

Looks like this is a breaking change without fallback?

Yes, this is a breaking change. However, I don't think the existing behavior was correct.

How can we make this change less painful for existing users?

[dkliban]

I decided to change the parameter names in my OpenAPI schema and no longer have this problem. I am closing this PR.