sqlalchemy strange

gistfile1.py

# -*- coding: UTF-8 -*-

"""This module contains all methods to manipulate with database."""

#THIRD_PARTY MODULES
from sqlalchemy import *

#STANDART MODULES
import time
from hashlib import md5

class Database:

    """ Initialize this class to create an instance of database.

        NOTE: don't forget to monkeypatch SA to use with python 3
        http://www.sqlalchemy.org/trac/browser/README.py3k
    """
    
    def __init__(self):
    
        """ Creates connection to database """
        self.engine = create_engine('mysql://root@localhost/bigzypy',
                                    encoding="utf-8")
        #self.engine.echo = True
        meta = MetaData(bind=self.engine, reflect=True)
        self.users_table = meta.tables['users']
        self.videos_table = meta.tables['videos']

    def update_user(self, login, new_values):

        """ @type login str
            @type new_values dict

            Updates user's fields with given values in database.
            new_values must be a dict with db fields as keys and
            new values as correspondent values.
        """
        # TODO: universal update method
        query = 'UPDATE users SET :field = :value WHERE login = :login'

        field, value = new_values.popitem()
        data = {'field': field, 'value': value, 'login': login}

        result = self.engine.execute(text(query), data)
        print(result)

        if result:
            return True

        return False