[python] multipart-form with multiple properties generates a None for non-binary properties in python-flask

Description

Multipart-form with multiple properties generates a None for non-binary properties in python-flask

with the given yaml, the following code was generated

def test(job_id=None, file=None):  # noqa: E501
    return 'do some magic!'

But the non-binary fields (ie. job_id) returns a None always!

workaround:
add a 'body' field to the method and the job_id can be accessed using the body dict

def test(body, job_id=None, file=None):  # noqa: E501
    print(job_id) 
    print(file)
    print(body) # job_id is in this dict
    return 'do some magic!'

Swagger-codegen version

3.0.11

Swagger declaration file content or url

https://gist.github.com/nirandaperera/dba9a70f7f8e189b0b035e4a040d7c82

Command line used for generation

java -jar ~/bin/swagger-codegen-cli-3.0.11.jar generate -i bug.yaml -l python-flask -o bug

Steps to reproduce

• generate the code
• add the following snippet to the generated code in
  bug/swagger_server/controllers/default_controller.py

def test(job_id=None, file=None):  # noqa: E501
    print(job_id)
    print(file)
    return 'do some magic!'

• add a dummy file aaa.txt
• use this curl cmd

curl -X POST "http://localhost:8080/bug/test" -H "accept: text/csv" -H "Content-Type: multipart/form-data" -F "job_id=aaa" -F "file=@aaa.csv;type=text/csv"

Related issues/PRs

I am not sure if these are related, but I found this issue in another project
https://github.com/OpenAPITools/openapi-generator/issues/1666

and this is where I found the workaround!

Suggest a fix/enhancement

please see the given workaround

Start by generating the python-flask server with Swagger Codegen 3.0.11 using the YAML linked in the issue, then reproduce the request with the shown curl command. Inspect bug/swagger_server/controllers/default_controller.py and the generated multipart handling to determine why job_id is None while file is supplied. Done means a multipart request populates both the non-binary job_id and binary file arguments without requiring a body workaround.