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> Consider here:
> http://www.sqlalchemy.org/trac/browser/lib/sqlalchemy/engine/base.py?rev=6884:b181f1e53603#L1329
> the py3k code uses the "raise ... from" syntax which isn't legal in Python 2. 
In this case, I would write
 error = exc.DBAPIError.instance(statement,
 parameters,
 e,
 connection_invalidated=is_disconnect)
 if sys.version_info < (3,):
 raise error, None, sys.exc_info()[2]
 else:
 error.__cause__ = e
 raise error
You don't *have* to use the from syntax to set the cause.
> Approach B isn't a general solution because it requires you to replace entire functions at a time. 
I don't claim that. However, I claim that there will be always an
appropriate solution using existing techniques, so that such a macro
processing wouldn't be necessary. IOW, I'm not aware of a case where
using such preprocessing would be appropriate and better than what
you can do without.