For SQLAlchemy 0.7.9 and newer
Please use the answer from @jsnow. !!!
For SQLAlchemy 0.7.8 and older
(as indicated by @augurar): Because sqlalchemy uses magic methods (operator overloading) to create SQL constructs, it can only handle operator such as != or ==, but is not able to work with is (which is a very valid Python construct).
Therefore, to make it work with sqlalchemy, you should use:
...filter(or_(people.marriage_status!='married', people.marriage_status == None))
, basically replace the is None with == None. In this case your query will be translated properly to the following SQL:
SELECT people.name AS people_name, people.marriage_status AS people_marriage_status
FROM people
WHERE people.marriage_status IS NULL OR people.marriage_status != ?
See IS NULL in the documentation.