SQLAlchemy raises None, causes TypeError - Test

gistfile1.txt

Traceback (most recent call last):
  File "test_case_9525220.py", line 41, in <module>
    session.commit()
  File "/usr/local/Cellar/python/2.7.2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 645, in commit
    self.transaction.commit()
  File "/usr/local/Cellar/python/2.7.2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 313, in commit
    self._prepare_impl()
  File "/usr/local/Cellar/python/2.7.2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 297, in _prepare_impl
    self.session.flush()
  File "/usr/local/Cellar/python/2.7.2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1547, in flush
    self._flush(objects)
  File "/usr/local/Cellar/python/2.7.2/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1635, in _flush
    raise
TypeError: exceptions must be old-style classes or derived from BaseException, not NoneType

test_case_9525220.py

# For http://stackoverflow.com/questions/9525220/sqlalchemy-raises-none-causes-typeerror

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String
from sqlalchemy.orm import sessionmaker
import MySQLdb

Base = declarative_base()

CONNECTION_TYPE = 'mysql_db_pool'

class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True, autoincrement=True)
    userid = Column(String(50), nullable=False)

if __name__ == '__main__':
    url = 'mysql://root:root@127.0.0.1:3306/testdb'

    if CONNECTION_TYPE == 'sqlite':
        engine = create_engine('sqlite:///:memory:', echo=True)
    elif CONNECTION_TYPE == 'mysql':
        engine = create_engine(url)
    elif CONNECTION_TYPE == 'mysql_db_pool':
        from eventlet.db_pool import ConnectionPool
        pool = ConnectionPool(MySQLdb, host='127.0.0.1', user='root', passwd='root', db='testdb')
        engine = create_engine(url,
            creator=pool.create,
            pool_size=pool.max_size,
        )

    session = sessionmaker(bind=engine)()

    User.metadata.drop_all(bind=engine)
    User.metadata.create_all(bind=engine)

    u = User()
    session.add(u)
    session.commit()