TypeError: key <built-in function id> is not a string

Question

I am using flask and its flask-restful extension to make a simple restful-api. I am having trouble stuffing data into Question table with User id (reference).

The model.py file is the following:

class User(UserMixin, SurrogatePK, Model):

    __tablename__ = 'users'
    username = Column(db.String(80), unique=True, nullable=False)
    email = Column(db.String(80), unique=True, nullable=False)
    #: The hashed password
    password = Column(db.String(128), nullable=True)
    created_at = Column(db.DateTime, nullable=False, default=dt.datetime.utcnow)

    def __init__(self, username, email, password=None, **kwargs):
        db.Model.__init__(self, username=username, email=email, **kwargs)
        if password:
            self.set_password(password)
        else:
            self.password = None

    def set_password(self, password):
        self.password = bcrypt.generate_password_hash(password)

    def check_password(self, value):
        return bcrypt.check_password_hash(self.password, value)

    def generate_auth_token(self, expiration = 600):
        s = Serializer('secret_key', expires_in = expiration)
        return s.dumps({ 'id': self.id })

    @staticmethod
    def verify_auth_token(token):
        s = Serializer('secret_key')
        try:
            data = s.loads(token)
        except SignatureExpired:
            return None # valid token, but expired
        except BadSignature:
            return None # invalid token
        user = User.query.get(data['id'])
        return user
    def __repr__(self):
        return '<User({username!r})>'.format(username=self.username)

class Question(SurrogatePK, Model):
    __tablename__ = 'questions'
    text = Column(db.String(400), nullable=True)
    created_at = Column(db.DateTime, nullable=True, default=dt.datetime.utcnow)

    user_id = ReferenceCol('users', nullable=True)
    user = db.relationship('User', backref='question_users')

    def __init__(self, text, created_at, user, **kwargs):
        db.Model.__init__(self, text=text, user=user, **kwargs)

    def __repr__(self):
        return '<Question({text})>'.format(text=self.text)

I used flask-httpauth's HTTPBasicAuth to authenticate user and used following decorator to store user info to global variable g:

@auth.verify_password
def verify_password(username_or_token, password):
    # first try to authenticate by token
    user = User.verify_auth_token(username_or_token)
    if not user:
        # try to authenticate with username/password
        user = User.query.filter_by(username = username_or_token).first()
        if not user or not user.check_password(password):
            return False
    g.user = user
    return True

lastly, my view file. The view file looks like below:

questionlist_fields = {
    'text':fields.String,
    'uri':fields.Url
}

class QuestionListAPI(Resource):
    decorators = [auth.login_required]
    def __init__(self):
        self.reqparse = reqparse.RequestParser()
        self.reqparse.add_argument('text', type=str, required=True,
            help='No Question Title Provided', location='json')
        super(QuestionListAPI, self).__init__()
    def post(self):
        args = self.reqparse.parse_args()

        #----------the code causing error-----------------#
        question = Question.create(text=args.text,
                                    created_at=datetime.utcnow(),
                                    user=g.user)
        #-------------------------------------------------#

        return {id:marshal(question, questionlist_fields)}
api.add_resource(QuestionListAPI, '/api/questions', endpoint='questions')

The error log is the following:

Traceback (most recent call last):
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask_restful\__init__.py", line 265, in error_router
    return original_handler(e)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\_compat.py", line 32, in reraise
    raise value.with_traceback(tb)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask_restful\__init__.py", line 265, in error_router
    return original_handler(e)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\_compat.py", line 32, in reraise
    raise value.with_traceback(tb)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask\app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask_debugtoolbar\__init__.py", line 124, in dispatch_request
    return view_func(**req.view_args)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask_restful\__init__.py", line 450, in wrapper
    return self.make_response(data, code, headers=headers)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask_restful\__init__.py", line 474, in make_response
    resp = self.representations[mediatype](data, *args, **kwargs)
  File "C:\Users\NI\Envs\nektime\lib\site-packages\flask_restful\representations\json.py", line 24, in output_json
    dumped = dumps(data, **local_settings)
  File "C:\Python34\Lib\json\__init__.py", line 237, in dumps
    **kw).encode(obj)
  File "C:\Python34\Lib\json\encoder.py", line 194, in encode
    chunks = list(chunks)
  File "C:\Python34\Lib\json\encoder.py", line 422, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "C:\Python34\Lib\json\encoder.py", line 368, in _iterencode_dict
    raise TypeError("key " + repr(key) + " is not a string")
TypeError: key <built-in function id> is not a string

Sorry for long lines of code. I just don't know what is causing the problem.

Answer 1

Your problem is in this line:

return {id:marshal(question, questionlist_fields)}

I think you wanted this:

return {'id': marshal(question, questionlist_fields)}

In some languages, notably JavaScript, all dict keys are strings, and the syntax allows you to leave the quotes off.

In Python, dict keys can be anything you want.^* If you want a string, you need the quotes. If you just pass id, you're just saying you want the key to be whatever's in the variable id. Since you don't have a local variable named id, what you get is the builtin.

That's still perfectly valid, even if it's not what you want, and not very useful.

But when you return a dict from a Flask post method on a JSON API, it gets encoded to JSON. And JSON only allows strings for dict keys, not functions. Hence the error.

---

_{* Well, not quite. They can be anything hashable, which doesn't include mutable types like list, and some other types that don't want to be used as keys for some other reason.}