You are not returning a JSON string
I am using Flask on python 2.7 and I want to return a value as a json value
PS: As correctly pointed out by @bereal in his answer, the above approach wont add Content-Type: application/json header in the response. You'd want to use jsonify(..) as shown in his answer.
You are returning a python object. When Flask sees a python object, it tries to call it. When you return a dictionary, it assumes it is a callable and tries to call it.