[BUG][Python] Python server string pattern regex improperly escaped.

Created by: daniellehanks

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?
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Description

Using the python-flask generator, string regex patterns have forward slashes somehow additionally escaped, changing the meaning of the pattern.

I found this bug that seems related. Seems like a reoccurrence.

Generated line: Actual:

if bar is not None and not re.search(r'^foo\\/bar$', bar):  # noqa: E501

Expected:

if bar is not None and not re.search(r'^foo\/bar$', bar):  # noqa: E501

With the current implementation, the input body "{"bar": "foo/bar"} passes the jsonschema validation (as it should), but then the model conversion fails with the raised ValueError, causing a 500.

openapi-generator version

Verified on 5.0.0-beta2 and 4.3.1.

OpenAPI declaration file content or url

openapi: 3.0.0

info:
    title: Foo API
    description:
        Test out deepObjects with query
    version: 0.1.0

servers:
  - url: 'http://localhost:8080/v1'

components:
  schemas:
    Foo:
      type: object
      properties:
        bar:
          type: string
          pattern: '^foo\/bar$'
      description:
        A Foo object

paths:
  /foo:
    post:
      operationId: CreateFoo
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Foo'
      responses:
        '200':
          description: OK
        '400':
          description: Bad request

Generation Details

Steps to reproduce

Save the above yaml as openapi.yaml, then run either:

openapi-generator-cli generate -i openapi.yaml -g python-flask -o out/

docker run --rm -t -v $(pwd):/tmp:rw openapitools/openapi-generator-cli:latest generate -i tmp/openapi.yaml -g python-flask -o tmp/out/

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/1823

Suggest a fix

I suspect something somewhere around here or the base class, since that is what the referenced bug added, but I can't quite seem to figure out what.