Python-сообщество

        l_Select =  select( [attr.c.column_id], 
            and_(attr.c.expression == None, 
                attr.c.column_id > 0)
            )

filters = []
if f1():
    filters.append(attr.c.expression == None)
if f2():
    filters.append(attr.c.column_id > 0)
l_Select =  select( [attr.c.column_id], and_(*filters))

>>> from sqlalchemy import MetaData, Table, Column, Integer, or_
>>> m = MetaData()
>>> t = Table('foo', m, Column('id', Integer, primary_key = True))

>>> clause = or_()
>>> clause.append(t.c.id == 1)
>>> clause.append(t.c.id == 2)

>>> print clause.compile()
foo.id = :id_1 OR foo.id = :id_2
>>> print clause.compile().params
{'id_2': 2, 'id_1': 1}
>>> print t.select(clause)
SELECT foo.id 
FROM foo 
WHERE foo.id = :id_1 OR foo.id = :id_2