[Python] sqlalchemy OperationalError

I want to just create a database table and write some data into it. But I found some errors; how to resolve them?

# coding=utf-8
from sqlalchemy import Column, Integer, String, BigInteger
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()


class User(Base):

__tablename__ = 'users'

id = Column(Integer(), primary_key=True)
username = Column(String(30), unique=True, nullable=False)
password = Column(String, default='123456', nullable=False)
name = Column(String(30), nullable=False)
grade = Column(String(30))
emp_no = Column(BigInteger, unique=True, nullable=False)

print User.__table__


from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()
Session = sessionmaker()
engine = create_engine('sqlite:///:memory:', echo=True)
Session.configure(bind=engine)
session = Session()
Base.metadata.create_all(engine)

ed_user = User(name='ed', username='jack', password='123', emp_no=1, grade='1')
#
session.add(ed_user)
#
with session.no_autoflush:
session.query(User).filter_by(name='ed').first()


Here are result and errors output:

/usr/bin/python2.7 /home/pajamas/PycharmProjects/untitled5/data_generator.py
users
2017-07-30 17:56:28,903 INFO sqlalchemy.engine.base.Engine SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
2017-07-30 17:56:28,904 INFO sqlalchemy.engine.base.Engine ()
2017-07-30 17:56:28,904 INFO sqlalchemy.engine.base.Engine SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
2017-07-30 17:56:28,904 INFO sqlalchemy.engine.base.Engine ()
2017-07-30 17:56:28,908 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such table: users [SQL: u'SELECT users.id AS users_id, users.username AS users_username, users.password AS users_password, users.name AS users_name, users.grade AS users_grade, users.emp_no AS users_emp_no \nFROM users \nWHERE users.name = ?\n LIMIT ? OFFSET ?'] [parameters: ('ed', 1, 0)]
2017-07-30 17:56:28,909 INFO sqlalchemy.engine.base.Engine SELECT users.id AS users_id, users.username AS users_username, users.password AS users_password, users.name AS users_name, users.grade AS users_grade, users.emp_no AS users_emp_no
FROM users
WHERE users.name = ?
LIMIT ? OFFSET ?
2017-07-30 17:56:28,909 INFO sqlalchemy.engine.base.Engine ('ed', 1, 0)

Process finished with exit code 1

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