[BUG][Python-Flask] Missing import of `re` when using `string` with `pattern`

Description

• We use a pattern in string schemas.
• python-flask uses re.match for the pattern matching in the generated model code, but it misses the import of re.

Note: I noticed that there seems to be another point in the generated server code where the pattern validation is done. You can see this by calling the following command when the server runs:

curl -X PUT "http://localhost:8080/data" -H  "accept: */*" -H  "Content-Type: application/json" -d "{\"myItem\":\"Hello There\"}"

It will properly return an error:

{
  "detail": "'Hello There' does not match '^[a-zA-Z0-9]+$'",
  "status": 400,
  "title": "Bad Request",
  "type": "about:blank"
}

while the server log the message:

http://localhost:8080/data validation error: 'Hello There' does not match '^[a-zA-Z0-9]+$'
127.0.0.1 - - [13/Dec/2018 12:22:11] "PUT /data HTTP/1.1" 400 -

openapi-generator version

I used OpenAPI generator CLI version 4.0.0-SNAPSHOT:
https://oss.sonatype.org/content/repositories/snapshots/org/openapitools/openapi-generator-cli/4.0.0-SNAPSHOT/openapi-generator-cli-4.0.0-20181210.103357-85.jar

OpenAPI declaration file content or url

See python-flask-string-pattern.yaml in the attached zip-file:
python-flask-string-pattern.zip

Command line used for generation

java -jar openapi-generator-cli-4.x.jar generate -i ./python-flask-string-pattern.yaml -g python-flask -o ./python-flask-string-pattern/

Steps to reproduce

1. Generate the server code

   ./python-flask-string-pattern.sh
   

2. Start the server

   (cd python-flask-string-pattern && python3 -m openapi_server)
   

3. Perform a client request

   curl -X PUT "http://localhost:8080/data" -H  "accept: */*" -H  "Content-Type: application/json" -d "{\"myItem\":\"HelloThere\"}"
   

   {
     "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"
   }
   

4. The server now logs an error message

   [2018-12-13 12:22:03,113] ERROR in app: Exception on /data [PUT]
   Traceback (most recent call last):
     File "/home/tom/.local/lib/python3.5/site-packages/flask/app.py", line 2292, in wsgi_app
       response = self.full_dispatch_request()
   ...
     File "/home/tom/issues/python-flask-string-pattern/openapi_server/util.py", line 111, in deserialize_model
       setattr(instance, attr, _deserialize(value, attr_type))
     File "/home/tom/issues/python-flask-string-pattern/openapi_server/models/data.py", line 63, in my_item
       if my_item is not None and not re.search(r'^[a-zA-Z0-9]+$', my_item):  # noqa: E501
   NameError: name 're' is not defined
   127.0.0.1 - - [13/Dec/2018 12:22:03] "PUT /data HTTP/1.1" 500 -
   

Related issues/PRs

• #1755: It is not a model object, but an implementation requirement though.
• #1256

Suggest a fix

Two things to cover here:

1. Add missing import of re
2. Check whether duplicate check of the pattern is really required?
   Probably setting the field value in the model object is used somewhere else too? Not only when verifying (remote) user input?

Start with the generated openapi_server/models/data.py and the python-flask-string-pattern.yaml input, then reproduce the failure with python-flask-string-pattern.sh and the documented curl request. Trace the generated model validation and compare it with openapi_server/util.py. Done means pattern validation no longer raises NameError, the request returns the expected validation response, and duplicate validation is assessed.