[python-flask] the operationId is not incorrectly sanitized

Hello,

Checklist from your CONTRIBUTING files :

• I have not found a matching issue, neither in swagger-api/swagger-codegen nor swagger-codegen-generators
• I am using the latest master
  • kinda, I'm using the Docker version of 2 weeks ago : swaggerapi/swagger-codegen-cli-v3:3.0.54
• test locally with the most recent jar
  • I'll try to do it soon

Using the python-flask generator, I noticed that some functions generated for the controller, when containing an underscore followed by a digit, have a name different than the operationId.

Minimal reproducible example :

openapi.yml

openapi: 3.0.3
info:
  title: Minimal Reproducible Example
  description: "."
  version: 0.0.0
paths:
  /foo:
    get:
      summary: Get
      responses:
        "200":
          description: Status details
      operationId: foo_123

generate_server.bash

set -euo pipefail  # check for errors

echo '(re-)create output directory'
rm -Rf ./output && mkdir ./output
echo 'copy the OpenAPI file into it'
cp ./openapi.yml ./output
echo 'generate the server files from the OpenAPI file'
docker run \
    --rm \
    -u "$(id -u):$(id -g)" \
    -v "${PWD}/output:/local" \
    swaggerapi/swagger-codegen-cli-v3:3.0.54 \
    generate \
    -l python-flask \
    -i /local/openapi.yml \
    -o /local
echo 'check the name of the function defined in the controller'
grep 'foo' < ./output/swagger_server/controllers/default_controller.py

It produces :

check the name of the function defined in the controller
def foo123():  # noqa: E501

Here we can see that the function is named foo123. This is different to the operationId foo123.

I suspect the cause is this line : permalink

public String toOperationId(String operationId) {
    // ... snip
    return underscore(sanitizeName(operationId));
}

I don't have a Java development environment at hand, so I can't find where these 2 functions are defined (being imported by *) and so can't review them.

For reference, here is the rule for valid Python identifiers (cf Python doc on lexical analysis)) but I find it much inscrutable (with all the Unicode linguo), a dumber (but inexact) version would be (according to this StackOverflow answer) :

identifier ::=  (letter|"_") (letter | digit | "_")*

(then applies the rules of keywords and dunder methods, clearly expressed in the Python doc on lexical analysis linked above)

Expected result : the function generated should have the same name than the operationId

Actual result : the function generated have a different name than the operationId

Start with PythonFlaskConnexionCodegen.java at the linked toOperationId method, then reproduce the issue using the provided openapi.yml and generate_server.bash files. Compare the generated default_controller.py function name with operationId: foo_123; done means the generated function retains the operationId name.