human
Solve the following python programming problem given the following code:
def Update(self, client, callback) : 
    'Call the base class "Update" method in order to persist modified
    columns to the db. But also ensure that this device has a unique
    push token.
two Viewfinder devices might share the same push token
    if a phone has been given or sold to another person without
    re-installing the OS. Also, ensure that the device is added to the
    secondary index used for alerting (alert_user_id), for fast
    enumeration of all devices that need to be alerted for a particular
    user.
    ' 
    def _DoUpdate() : 
        super (Device, self).Update (client, callback) 
def _OnQueryByPushToken(devices) : 
        'Disable alerts for all other devices.' 
        with util.Barrier (_DoUpdate) as b : 
            for device in devices : 
                if (device.device_id != self.device_id) : 
                    device.push_token = None 
                    device.alert_user_id = None 
                    super (Device, device).Update (client, b.Callback ()) 
self.last_access = util.GetCurrentTimestamp () 
    if self._IsModified ('push_token') : 
        if (_OnQueryByPushToken.push_token is None) : 
            self.alert_user_id = None 
            _DoUpdate () 
else : 
            self.alert_user_id = self.user_id 
            query_expr = ('device.push_token={t}', { 
                't' : self.push_token, 
}) 
            Device.IndexQuery (client, query_expr, None, _OnQueryByPushToken) 
else : 
        _DoUpdate () 

    
    

    Fix the buggy line: if (_OnQueryByPushToken.push_token is None) :
assistant
#Here is Python code to fix the buggy line: if (_OnQueryByPushToken.

if (self.push_token is None) :