Flask: Custom Response force_type is now never called

[jaos]

Since #3214 I am no longer able to use a custom Response class to catch and format internal objects.

Is there a more appropriate way to accomplish the same thing now that 1.1.0 is being picked up in my CI env?

Expected Behavior

from flask import Flask, Response, Blueprint                                    
import json                                                                     

class BizModel():                                                               
    def __init__(self, name):                                                   
        self.name = name                                                        
    def to_jsonable(self):                                                      
        return {"name": self.name}                                              

class BizResponse(Response):                                                    
    @classmethod                                                                
    def force_type(cls, rv, environ=None):                                      
        if isinstance(rv, BizModel):                                            
            return Response(json.dumps(rv.to_jsonable(), indent=4), mimetype='application/json', status=200)
        return super(BizResponse, cls).force_type(rv, environ)                  

app = Flask(__name__)                                                           
app.response_class = BizResponse                                                

@app.route('/biz')                                                              
def showbiz():                                                                  
    return BizModel('showbiz')                                                  

if __name__ == "__main__":                                                      
    test = app.test_client()                                                    
    r = test.get('/biz')                                                        
    assert r.is_json                                                            
    assert r.json['name'] == 'showbiz'

Actual Behavior

[2019-07-08 12:24:58,237] ERROR in app: Exception on /biz [GET]
Traceback (most recent call last):
  File "/private/tmp/py3venv/lib/python3.7/site-packages/Flask-1.1.0-py3.7.egg/flask/app.py", line 2446, in wsgi_app
    response = self.full_dispatch_request()
  File "/private/tmp/py3venv/lib/python3.7/site-packages/Flask-1.1.0-py3.7.egg/flask/app.py", line 1952, in full_dispatch_request
    return self.finalize_request(rv)
  File "/private/tmp/py3venv/lib/python3.7/site-packages/Flask-1.1.0-py3.7.egg/flask/app.py", line 1967, in finalize_request
    response = self.make_response(rv)
  File "/private/tmp/py3venv/lib/python3.7/site-packages/Flask-1.1.0-py3.7.egg/flask/app.py", line 2130, in make_response
    " {rv.__class__.__name__}.".format(rv=rv)
TypeError: The view function did not return a valid response. The return type must be a string, dict, tuple, Response instance, or WSGI callable, but it was a BizModel.                                                                                                                     
Traceback (most recent call last):
  File "./flask3214.py", line 28, in <module>
    assert r.is_json
AssertionError

Environment

• Python version: 3.7.2
• Flask version: 1.1.0
• Werkzeug version: 0.15.4

[davidism]

You are misusing force_type, which is intended for coercing WSGI responses from WSGI callables. A model is not (typically) a WSGI callable. force_type is still called on Response subclasses and callables, as intended. You should override Flask.make_response:

class SpecialFlask(Flask):
    def make_response(self, rv):
        if isinstance(rv, BizModel):
            return Response(...)
        return super().make_response(rv)