YippieMove has been running into some spurious problems with SQLAlchemy. When upgrading Python from 2.5 to 2.6 the errors became a lot more frequent. The error is “Set changed size during iteration” and happens in all kinds of random code paths when using the ORM layer. Today I saw it happen when getting a result object from a query by index, and then again when iterating over the result of another query.

The end of the traceback looks something like this:

  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/SQLAlchemy-0.5.5-py2.6.egg/sqlalchemy/orm/dynamic.py", line 192, in __iter__
    sess = self.__session()
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/SQLAlchemy-0.5.5-py2.6.egg/sqlalchemy/orm/dynamic.py", line 181, in __session
    sess.flush()
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/SQLAlchemy-0.5.5-py2.6.egg/sqlalchemy/orm/session.py", line 1354, in flush
    self._flush(objects)
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/SQLAlchemy-0.5.5-py2.6.egg/sqlalchemy/orm/session.py", line 1359, in _flush
    if (not self.identity_map.check_modified() and
  File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/SQLAlchemy-0.5.5-py2.6.egg/sqlalchemy/orm/identity.py", line 56, in check_modified
    for state in self._mutable_attrs:
RuntimeError: Set changed size during iteration

The code in question looks like this:

    for state in self._mutable_attrs:
        if state.modified:
            return True

The program is single threaded and it sure didn’t look like “state.modified” would cause the iterated over set to change. Weird. After snooping around for a while I found this post which suggests an async GC being the problem.

Sure enough the problem appears to have gone away when automatic GC is disabled. It’s not a nice workaround because the garbage collector in Python collects cyclic garbage. But it did solve the problem for now so here it is if anyone else needs it:

import gc
gc.disable()

Unless you want to be buried in garbage, make sure to collect it once in a while in a safe spot in your code using gc.collect().

Hopefully this will be fixed in a future version.