[Feature]: Support for injecting custom input in custom openapi error response format

Start with FlaskOpenAPIErrorsHandler and compare its behavior around commit 22ace6d, using the custom handler in server.py as the reproduction. Read test.yaml to understand the validation request and response, then verify that custom injected fields and OpenAPI-Core error details can be returned together in the current release.

Suggested Behavior

I would like to add a way to customize error response format and would like to add additional information in the error response generated by OpenAPI-Core

currently the error response generated by OpenAPI-Core is as follows

{
    "errors": [
        {
            "class": "<class 'openapi_core.validation.schemas.exceptions.InvalidSchemaValue'>",
            "status": 400,
            "title": "Value {'flag': 'testF'} not valid for schema of type object: (<ValidationError: \"'testF' is too short\">,)"
        }
    ]
}

I want to customize the response in following format

{
 "title": "Failed to execute your favorite API"                       \\ Custom String provided by code
"causedBy": [
        "Value {'flag': 'testF'} not valid for schema of type object: (<ValidationError: \"'testF' is too short\"     \\ Error Resposne generated by OpenAPI Core
    ]
}

As you can see in the comments, the message in causedBy field includes error messages generated by OpenAPI Core module
whereas the message in title field includes data generated and injected by actual code

Why is this needed?

This is required to make OpenAPI core more flexible and customizable and become a better choice for openapi linters and validators.

Also If OpenAPI does not support cutomization on the content of response, then it would be a deal breaker for most of the implementation as most of the projects tries to maintain some standard format of response

References

I came across FlaskOpenAPIErrorsHandler class and created a custom handler class which inherits from FlaskOpenAPIErrorsHandler class

I used constructor of this class to inject custom fields and generate custom response accordingly.

following is my Flask code server.py

#!/usr/bin/python3
"""Test server."""

from flask import Flask, request, jsonify
from openapi_core.contrib.flask.decorators import FlaskOpenAPIViewDecorator, FlaskOpenAPIErrorsHandler
from openapi_core import Spec

# Custom Error Handler block
class MyCustomErrorHandler(FlaskOpenAPIErrorsHandler):
    """"Custom Error Handler"""
    def __init__(self, title:str):
        self.title =  title

    def handle(self, errors:list):
        response_object =  jsonify({
            "title": self.title,
            "causedBy" : [self.handle_error(error) for error in errors]
        })
        response_object.error_code = 400
        return response_object

    def handle_error(self, error):
        """
        Converts error object into error string message

        :param error: Error object which stores exception message
        :type error: Exception object
        :return: Error message string corresponding to error object
        :rtype: str
        """
        if error.__cause__ is not None:
            error = error.__cause__
        return str(error)

SPEC = "test.yaml"
app = Flask(__name__)

spec_object = Spec.from_file_path(SPEC)
my_error_handler_object = MyCustomErrorHandler("Failed to execute test API")
openapi_decorator = FlaskOpenAPIViewDecorator.from_spec(spec_object, openapi_errors_handler=my_error_handler_object)

@app.route("/test", methods=["POST"])
@openapi_decorator
def read_permission():
    """Test function"""
    temp =  jsonify({
                    "stri_normal_json": request.json.get("flag", 1)
                })
    print(dir(temp))
    return temp

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=345, debug=True)

#Valid request: curl -X POST http://localhost:345/test --data '{"flag":"rohana"}' -H 'Content-Type: application/json'

#Invalid Request: curl -X POST http://localhost:345/test --data '{"flag":"rohan"}' -H 'Content-Type: application/json'

Following is OpenAPI Spec

test.yaml

openapi: '3.0.2'
info:
  title: Test Title
  version: '1.0'
servers:
  - url: http://localhost:345/
paths:
  /test:
    post:
      requestBody:
        content:
          application/json:
            schema:
              type: object
              required:
                - flag
              properties:
                flag:
                  x-pii: true
                  type: string
                  pattern: "^[\\w.-]*$"
                  minLength: 6
                  maxLength: 20
      responses:
        200:
          description: Sample response
          content:
            application/json:
              schema:
                type: object
                properties:
                  stri_normal_json:
                    type: string
                    minLength: 6
                    maxLength: 20

As we can see in code, I am creating object of custom error handler and injecting custom string as part of constructor in above example following is the line

my_error_handler_object = MyCustomErrorHandler("Failed to execute test API")

Following is the output of curl request

Invalid Curl Request

root@ip-10-31-1-220:~/playground# curl -X POST http://localhost:345/test --data '{"flag":"rohan"}' -H 'Content-Type: application/json'
{
  "causedBy": [
    "Value {'flag': 'rohan'} not valid for schema of type object: (<ValidationError: \"'rohan' is too short\">,)"
  ],
  "title": "Failed to execute test API"
}

Valid Curl Request

root@ip-10-31-1-220:~/playground# curl -X POST http://localhost:345/test --data '{"flag":"rohana"}' -H 'Content-Type: application/json'
{
  "stri_normal_json": "rohana"
}

In this way I was able to customize the response body and also inject my custom string when I create object of custom error handler. which was working fine till OpenAPI 0.16.x but it started failing since OpenAPI 0.18.x

this issue happened due to following pull request
https://github.com/python-openapi/openapi-core/commit/22ace6d39607ef769ba0a36e8a24dc6df704a620

Would you like to implement a feature?

Yes