from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

def comparator(a, b):
    return a > b

class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)
    data = Column(PickleType(comparator=comparator))


e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)
s.add(A(data='some data'))
s.commit()

a1 = s.query(A).first()
a1.data = func.foo("im a SQL expression")
s.commit()

#!

Traceback (most recent call last):
  File "test.py", line 27, in <module>
    s.commit()
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 790, in commit
    self.transaction.commit()
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 392, in commit
    self._prepare_impl()
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 372, in _prepare_impl
    self.session.flush()
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 2004, in flush
    self._flush(objects)
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 2122, in _flush
    transaction.rollback(_capture_exception=True)
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/util/langhelpers.py", line 60, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/session.py", line 2086, in _flush
    flush_context.execute()
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/unitofwork.py", line 373, in execute
    rec.execute(self)
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/unitofwork.py", line 532, in execute
    uow
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/persistence.py", line 170, in save_obj
    mapper, table, update)
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/persistence.py", line 613, in _emit_update_statements
    lambda rec: (
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/persistence.py", line 456, in _collect_update_commands
    value, state.committed_state[propkey]):
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/sql/elements.py", line 2726, in __bool__
    raise TypeError("Boolean value of this clause is not defined")
TypeError: Boolean value of this clause is not defined