Python object instance is not being saved in Flask application context

I've made simple Flask extension that wraps Google API Client for Python:

class ProximityBeaconAPI:
    '''
    Simple Flask extension for accessing Google Proxmity Beacon API.
    https://developers.google.com/resources/api-libraries/documentation/proximitybeacon/v1beta1/python/latest/
    '''

    def __init__(self, app=None, credentials=None):
        if app:
            self.init_app(app)
        if credentials:
            self.init_api(credentials)

    def init_app(self, app):
        '''
        Initialize Flask app for extension
        '''
        self.app = app
        self._project_id = app.config.PROJECT_ID

    @property
    def is_initialized(self):
        '''
        Checks if Proximity Beacon API is initialized.

        :return: bool
        '''
        ctx = _app_ctx_stack.top
        return hasattr(ctx, 'proximitybeaconapi')


    def init_api(self, credentials):
        ''' Initialize Proximity Beacon API

        :param credentials: google.oauth2.credentials.Credentials object  
        '''
        proximitybeaconapi = discovery.build(
            OAUTH2.API_NAME, OAUTH2.API_VERSION, credentials=credentials)
        ctx = _app_ctx_stack.top
        if ctx is not None:
            if not hasattr(ctx, 'proximitybeaconapi'):
                ctx.proximitybeaconapi = proximitybeaconapi

    def get_utoken(self):
        ''' Checks if for beacon with given name u_token attachment
        is set.

        :param beacon_name: name of the beacon
        :param namespace: namespace
        '''
        beacon_name = self.get_default_auth_beacon_name()
        ctx = _app_ctx_stack.top
        namespace = self.get_default_project_namespace().split('/')[1]
        namespaced_type = '{}/u_token'.format(namespace)
        query = {
            'beaconName': beacon_name,
            'namespacedType': namespaced_type
        }
        resp = ctx.proximitybeaconapi.beacons().attachments().list(**query).execute()
        b64_data = resp['attachments'][0]['data']
        return self._base64_to_str(b64_data)

    def is_utoken_valid(self, u_token):
        ''' Checks if recieved token is valid with current
        u_token attachment.

        :param u_token: incomming u_token
        :return: True or False
        '''
        incomming_token = self._base64_to_str(u_token)
        current_token = self.get_utoken()
        return incomming_token == current_token

In my oauth2 callback view I initialize Google API by calling init_api method that based on credentials creates an API client instance:

def init_api(self, credentials):
     ''' Initialize Proximity Beacon API
     :param credentials: google.oauth2.credentials.Credentials object  
     '''
     proximitybeaconapi = discovery.build(
         OAUTH2.API_NAME, OAUTH2.API_VERSION, credentials=credentials)
     ctx = _app_ctx_stack.top
     if ctx is not None:
         if not hasattr(ctx, 'proximitybeaconapi'):
             ctx.proximitybeaconapi = proximitybeaconapi

Then in my other endpoint I call method get_token that uses initialized earlier client library. My endpoint

def post(self):
    data = api.payload
    u_token = data.pop('u_token')
    token = beaconapi.get_utoken()
    # other code ...

get_utoken method that makes use of client:

def get_utoken(self):
    ''' Checks if for beacon with given name u_token attachment
    is set.
    :param beacon_name: name of the beacon
    :param namespace: namespace
    '''
    beacon_name = self.get_default_auth_beacon_name()
    ctx = _app_ctx_stack.top # getting client from application context
    # ...
    response = ctx.proximitybeaconapi.beacons().attachments().list(**query)
    return response

But I get AttributeError: 'AppContext' object has no attribute 'proximitybeaconapi' error dispite I set this attribute in app context earlier:

  File "/home/devaerial/Source/automoticz-server/automoticz/utils/beacons.py", line 61, in get_default_auth_beacon_name
    response = ctx.proximitybeaconapi.beacons().list(q=query).execute()
AttributeError: 'AppContext' object has no attribute 'proximitybeaconapi'

It looks like ctx context did not saved my proximitybeaconapi client instance when I set it using init_api when calling last endpoint

What am I doing wrong here?

P.S: I was following this example from official Flask docs.

Found solution to problem without using _app_ctx_stack. Used this approach using app.extensions object where Flask application stores all references to extensions and slightly rebuild my code:

def init_api(self, credentials):
    ''' Initialize Proximity Beacon API

    :param credentials: google.oauth2.credentials.Credentials object  
    '''
    proximitybeaconapi = discovery.build(
           OAUTH2.API_NAME, OAUTH2.API_VERSION, credentials=credentials)
    self.app.extensions['api'] = proximitybeaconapi

Now if I want to call Goolge API client I simply get its reference from extensions

def get_pin(self):
    ''' Checks if for beacon with given name pin attachment
    is set.

    :param beacon_name: name of the beacon
    :param namespace: namespace
    '''
    beacon_name = self.get_default_auth_beacon_name()
    api = self.app.extensions.get('api')
    namespace = self.get_default_project_namespace().split('/')[1]
    namespaced_type = '{}/pin'.format(namespace)
    query = {'beaconName': beacon_name, 'namespacedType': namespaced_type}
    resp = api.beacons().attachments().list(**query).execute()
    b64_data = resp['attachments'][0]['data']
    return self._base64_to_str(b64_data)