Breakpoints inside Flask-RESTful class methods aren't hit on PTVS

Question

I'm using Python Tools for Visual Studio, and I've set up a project with a virtual environment and installed Flask-RESTful there.

Then, I just copied their hello world example

from flask import Flask
from flask.ext.restful import reqparse, abort, Api, Resource

app = Flask(__name__)
app.debug = True
api = Api(app)

TODOS = {
    'todo1': {'task': 'build an API'},
    'todo2': {'task': '?????'},
    'todo3': {'task': 'profit!'},
}


def abort_if_todo_doesnt_exist(todo_id):
    if todo_id not in TODOS:
        abort(404, message="Todo {} doesn't exist".format(todo_id))

parser = reqparse.RequestParser()
parser.add_argument('task', type=str)


# Todo
#   show a single todo item and lets you delete them
class Todo(Resource):
    def get(self, todo_id):
        abort_if_todo_doesnt_exist(todo_id)
        return TODOS[todo_id]

    def delete(self, todo_id):
        abort_if_todo_doesnt_exist(todo_id)
        del TODOS[todo_id]
        return '', 204

    def put(self, todo_id):
        args = parser.parse_args()
        task = {'task': args['task']}
        TODOS[todo_id] = task
        return task, 201


# TodoList
#   shows a list of all todos, and lets you POST to add new tasks
class TodoList(Resource):
    def get(self):
        return TODOS

    def post(self):
        args = parser.parse_args()
        todo_id = 'todo%d' % (len(TODOS) + 1)
        TODOS[todo_id] = {'task': args['task']}
        return TODOS[todo_id], 201

##
## Actually setup the Api resource routing here
##
api.add_resource(TodoList, '/todos')
api.add_resource(Todo, '/todos/<string:todo_id>')

if __name__ == '__main__':
    app.run(debug=True)

Everything works fine, and if I put breakpoints on the lines that are executed before starting the app.run(debug=True) they're hit (with F10 and F11 working great, and local variables being updated as expected)

However, I'd love to debug what happens when a request is processed, but if I add breakpoints to the methods of the Todo or TodoList classes, they're never hit. I added code (like print('here')) to see if they're being processed and they are... also, they're returning what I expect when opened from a browser.

Is there something I'm missing from the setup?

Thanks!

UPDATE: I found out that if I attach VS to the python.exe process that's running my code, I'm able to debug those methods... so I guess that the question is now: can I force VS to attach to the process once it's launched as it does with regular .NET apps?

Answer 1

I had the same exact problem (but with PyCharm on Mac).

I believe this has something to do with the way Flask reloads itself when debug=True. Setting debug to False allowed me to break inside the view methods.

Answer 2

Disabling Debug resolves the issue with the breakpoint hit, but has the disadvantage that you are unable to read exception trace output.

One way to bypass this limitation is to add:

DEBUG = False
PROPAGATE_EXCEPTIONS = True

to your config.

When an exception occurs the browser still displays "Internal Server Error" message, but the console Window will receive the exception trace normally.