The function either returned None or ended without a return statement while using with flask_restful

[paurakhsharma]

flask_restful supports returning None from the view function, but I am getting error this error when I use metrics for endpoint.

e.g

from flask_restful import Resource

by_path_counter = metrics.counter(
    'by_path_counter', 'Request count by request paths',
    labels={'path': lambda: request.path}
)

class FoodApi(Resource):
    @by_path_counter
    def post(self):
        create_food()
        return None, 200

It works fine if I don't use @by_path_counter decorator, and returns null in the response.

But when I add the decorator, I get this error.

Traceback (most recent call last):
  File "/home/wartner/.local/share/virtualenvs/config_service-TDer8cy_/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/wartner/.local/share/virtualenvs/config_service-TDer8cy_/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/wartner/.local/share/virtualenvs/config_service-TDer8cy_/lib/python3.8/site-packages/flask_restful/__init__.py", line 468, in wrapper
    resp = resource(*args, **kwargs)
  File "/home/wartner/.local/share/virtualenvs/config_service-TDer8cy_/lib/python3.8/site-packages/flask/views.py", line 89, in view
    return self.dispatch_request(*args, **kwargs)
  File "/home/wartner/.local/share/virtualenvs/config_service-TDer8cy_/lib/python3.8/site-packages/flask_restful/__init__.py", line 583, in dispatch_request
    resp = meth(*args, **kwargs)
  File "/home/wartner/.local/share/virtualenvs/config_service-TDer8cy_/lib/python3.8/site-packages/prometheus_flask_exporter/__init__.py", line 633, in func
    response = make_response(response)
  File "/home/wartner/.local/share/virtualenvs/config_service-TDer8cy_/lib/python3.8/site-packages/flask/helpers.py", line 223, in make_response
    return current_app.make_response(args)
  File "/home/wartner/.local/share/virtualenvs/config_service-TDer8cy_/lib/python3.8/site-packages/flask/app.py", line 2097, in make_response
    raise TypeError(
TypeError: The view function did not return a valid response. The function either returned None or ended without a return statement.

Is there any way, I can work around this? (I have to return None)

Any kind of help will be highly appreciated, thank you 🙂

Note: It works fine on other view functions where I return not None values

[paurakhsharma]

I am using prometheus-flask-exporter version 0.14.1

[rycus86]

Thanks for the report! I'll have a look, sounds like a bug potentially.

[paurakhsharma]

Thank you for the quick reply 🙂

[rycus86]

If you switch to RESTfulPrometheusMetrics in the new 0.15.0 version, this should now work out of the box.

[paurakhsharma]

@rycus86 thank you so much, this does solve the problem.
But I have one problem I need to implement application factory method because of circular dependency issue,

Previously I used to do

metrics = PrometheusMetrics.for_app_factory()
# and later
metrics.init_app(app)

Any tip on how I can achieve something similar while using RESTfulPrometheusMetrics

[rycus86]

That might need some changes to support it nicely. 🤔
Until then, you should be able to use it like this:

metrics = RESTfulPrometheusMetrics(app=None, api=api)
# and later
metrics.init_app(app)

[rycus86]

Started building a new 0.15.1 release, in that you can do it like this:

metrics = RESTfulPrometheusMetrics.for_app_factory(api)
# and later
metrics.init_app(app)

[paurakhsharma]

This looks great, is it possible we pass api later as well?
like,

metrics = RESTfulPrometheusMetrics.for_app_factory()
# and later
metrics.init_app(app, api)

[rycus86]

Yeah, good point!
In 0.15.4 you'll be able to do:

app = Flask(__name__)
restful_api = Api(app)

metrics = RESTfulPrometheusMetrics.for_app_factory()
# and later
metrics.init_app(app, restful_api)

[paurakhsharma]

You are just awesome. Thank you so much this helps me a lot ☺️

[Curiouspaul1]

Hi @rycus86 thanks for all your effort, i have one small question about this. Which do i use between GunicornPrometheusMetrics and RESTfulPrometheusMetrics for when I'm using both gunicorn and Flask-restX

[rycus86]

Good question @Curiouspaul1 , not sure if we have an exact test for that yet.
Have you tried either way? If so, did it behave as you needed it?
If it doesn't work, perhaps a sample app & setup might be good then we can test how the metrics setup composition should look like there.