[BUG] missing 1 required positional argument

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

openapi-generator is generating invalid python-flask code.

openapi-generator version

4.0.3

OpenAPI declaration file content or url

openapi: 3.0.0
servers:
  - url: 'http://localhost:8080'

info:
  version: 0.0.1
  title: ratbag-emu
  license:
    name: MIT
    url: 'https://raw.githubusercontent.com/libratbag/ratbag-emu/master/LICENSE'

tags:
  - name: device
    description: Control interface for the simulated device

paths:
  /devices:
    get:
      tags:
        - device
      summary: List of simulated devices
      description: 'Provides the list of devices that are being currently simulated by ratbag-emu'
      operationId: list_devices
      parameters: []
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Device'
  '/devices/add/{shortname}':
    get:
      tags:
        - device
      summary: Creates a simulated device
      description: 'Tells ratbag-emu to create a new simulated device'
      operationId: add_device
      parameters:
        - name: shortname
          in: path
          description: Short name name of the device to add
          required: true
          schema:
            type: string
      responses:
        '200':
          description: OK
        '400':
          description: Can't add device
  '/devices/{device_id}':
    get:
      tags:
        - device
      summary: Returns a simulated device
      description: 'Returns one the of devices currently simulated by ratbag-emu'
      operationId: get_device
      parameters:
        - name: device_id
          in: path
          description: ID of the device to return
          required: true
          schema:
            type: integer
            format: int32
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Device'
        '404':
          description: Device not found
  '/devices/{device_id}/event':
    post:
      tags:
        - device
      summary: Send an event to a simulated device
      operationId: device_event
      description: 'Send HID event data to the target device'
      parameters:
        - name: device_id
          in: path
          description: ID of the device to use as the event source
          required: true
          schema:
            type: integer
            format: int32
      requestBody:
        description: Event data
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/EventData'
      responses:
        '200':
          description: OK
        '400':
          description: Error sending data to the device
        '404':
          description: Device not found

components:
  requestBodies:
    Device:
      description: Simulated device
      required: true
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Device'
  schemas:
    Device:
      title: Device
      description: Simulated device
      type: object
      required:
        - id
      properties:
        id:
          type: integer
          format: int32
        name:
          type: string
    EventData:
      title: Event data
      description: Raw HID event data
      type: object
      properties:
        x:
          type: integer
          format: int32
        y:
          type: integer
          format: int32

Command line used for generation

openapi-generator generate \
    -i src/ratbag_emu/openapi/ratbag-emu.yaml \
    --package-name ratbag_emu_server \
    -g python-flask \
    -o src/server_gen/

Steps to reproduce

• Run the command above
• Run the server

$ curl -X POST "http://localhost:8080/devices/0/event" -H  "accept: */*" -H  "Content-Type: application/json" -d "{\"x\":0,\"y\":6}"
{
  "detail": "The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.",
  "status": 500,
  "title": "Internal Server Error",
  "type": "about:blank"
}

And in the server log

[2019-07-12 10:45:14,479] ERROR in app: Exception on /devices/0/event [POST]
Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/flask/app.py", line 2311, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/lib/python3.7/site-packages/flask/app.py", line 1834, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/lib/python3.7/site-packages/flask/app.py", line 1737, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/lib/python3.7/site-packages/flask/_compat.py", line 36, in reraise
    raise value
  File "/usr/lib/python3.7/site-packages/flask/app.py", line 1832, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/lib/python3.7/site-packages/flask/app.py", line 1818, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/root/.local/lib/python3.7/site-packages/connexion/decorators/decorator.py", line 73, in wrapper
    response = function(request)
  File "/root/.local/lib/python3.7/site-packages/connexion/decorators/uri_parsing.py", line 132, in wrapper
    response = function(request)
  File "/root/.local/lib/python3.7/site-packages/connexion/decorators/validation.py", line 165, in wrapper
    response = function(request)
  File "/root/.local/lib/python3.7/site-packages/connexion/decorators/validation.py", line 339, in wrapper
    return function(request)
  File "/root/.local/lib/python3.7/site-packages/connexion/decorators/decorator.py", line 44, in wrapper
    response = function(request)
  File "/root/.local/lib/python3.7/site-packages/connexion/decorators/parameter.py", line 126, in wrapper
    return function(**kwargs)
TypeError: device_event() missing 1 required positional argument: 'event_data'

Start by running the provided openapi-generator command with the YAML declaration and inspect the generated handler for device_event and its request-body parameter handling. Reproduce the POST request and trace why event_data is not passed; done means the generated python-flask server accepts the payload without the missing-argument TypeError.