AttributeError: у объекта 'NoneType' нет атрибута 'db'

Question

Traceback (most recent call last):
  File "/Users/knutjorgenbjuland/PycharmProjects/assignment_7_1/main.py", line 13, in <module>
    Column('course_id',Integer,ForeignKey('courses.id')))
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/sqlalchemy/testing/schema.py", line 26, in Table
    if exclusions.against(config._current, "mysql"):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/sqlalchemy/testing/exclusions.py", line 452, in against
    config
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/sqlalchemy/testing/exclusions.py", line 356, in __call__
    if pred(config):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/sqlalchemy/testing/exclusions.py", line 273, in __call__
    engine = config.db
AttributeError: 'NoneType' object has no attribute 'db'

Я получаю сообщение об ошибке выше и ниже мой код.

 from sqlalchemy import Table, Column, Integer, String, ForeignKey
 from sqlalchemy import create_engine
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import relationship, sessionmaker

 # declarative_base() returns the base class used for table descriptions
 from sqlalchemy.testing.schema import Table

 Base = declarative_base()

 students_course_table = Table('student_course',Base.metadata,
                          Column('student_id',Integer,ForeignKey('students.id')),
                          Column('course_id',Integer,ForeignKey('courses.id')))

Я добавил все столбцы и таблицу в базу данных Sqlite. Что я могу сделать, чтобы исправить эту ошибку?