SQLAlchemy raises None, causes TypeError

Question

I'm using the declarative extension in SQLAlchemy, and I noticed a strange error when I attempted to save an instance of a mapped class with incorrect data (specifically a column declared with nullable=False with a value of None).

The class (simplified):

class User(Base):
    __tablename__ = 'users'

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

Causing the error (session is a SQLAlchemy session):

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

...

TypeError: exceptions must be old-style classes or derived from BaseException, not NoneType

Looking at the code that causes this exception, I found (in sqlalchemy.orm.session):

except:
    transaction.rollback(_capture_exception=True)
    raise

The exception being caught in this case is a sqlalchemy.exc.OperationalError. If I change these lines to:

except Exception as e:
    transaction.rollback(_capture_exception=True)
    raise e

then the problem goes away, and the OperationalError gets thrown instead of None. Shouldn't the original code work in any recent version of Python though? (I'm using 2.7.2) Is this error somehow specific to my application?

Python 2.7.2

SQLAlchemy 0.7.5

UPDATE: the error seems to be specific to my application in some way. I'm wrapping an eventlet.db_pool with a SQLAlchemy engine, which appears to be the source of the problem somehow. Running my simple test with either in-memory SQLite or basic MySQL engine doesn't have this problem, but with the db_pool it does.

Test case: https://gist.github.com/1980584

The full traceback is:

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

Answer 1

Here is what I've discovered:

• The exception (an OperationalError) is ok until the failed transaction rolls back (in Session._flush()).
• The transaction rollback is handled by mysqldb through eventlet.tpool. Specifically, eventlet.tpool.execute is called, which involves creating an eventlet.Event and calling its wait method.
• While waiting, a few complicated thread-related things happen, one of is checking for an exception and passing it to the Event to handle. It picks up the OperationalError which is still in sys.exc_type, and ultimately clears it in eventlet.event.hubs.hub.BaseHub.switch.
• Control returns to Session._flush, and the exception is re-raised (using raise), but at this point there is no exception so it tries to raise None.

This behaviour can be reproduced with a simple example:

from eventlet import tpool

def m(): 
    pass

try:
    raise TypeError
except:
    tpool.execute(m)
    raise

Its a bit unclear exactly what eventlet should be doing in this situation, so I don't know whether the bug should be reported to sqlalchemy or eventlet, or both.

The easiest way to rectify it is, as you've already noted, to change the last few lines of sqlalchemy.orm.session.Session._flush from

    except Exception:
        transaction.rollback(_capture_exception=True)
        raise

to

    except Exception, e:
        transaction.rollback(_capture_exception=True)
        raise e

Edit: I have raised an issue on eventlet's issue tracker. It might be worth raising it on sqlalchemy too though.