python - Select NULL Values in SQLAlchemy

Question

Here's my (PostgreSQL) table --

test=> create table people (name varchar primary key,
                            marriage_status varchar) ; 

test=> insert into people values ('Ken', 'married');
test=> insert into people values ('May', 'single');
test=> insert into people values ('Joe', NULL);

I want to select all people that are not known to be married, i.e., including those with NULL marriage_status.

This does not work --

test=> select * from people where marriage_status != 'married' ; 
 name | marriage_status 
------+-----------------
 May  | single
(1 row)

Of course this does --

test=> select * from people where marriage_status != 'married'
       or marriage_status is NULL ; 
 name | marriage_status 
------+-----------------
 May  | single
 Joe  | 

The problem is that I'm accessing it from SQLAlchemy with --

...filter(or_(people.marriage_status!='married',
              people.marriage_status is None))

which gets translated to --

SELECT people.name as name,
       people.marriage_status as marriage_status
FROM people 
WHERE people.marriage_status != %(status_1)s OR False
sqlalchemy.engine.base.Engine.... {'status_1': 'married'}

And does not work --

test=> select * from people where marriage_status != 'married'
       or False; 
 name | marriage_status 
------+-----------------
 May  | single
(1 row)

neither does --

test=> select * from people where marriage_status != 'married'
       or NULL; 
 name | marriage_status 
------+-----------------
 May  | single
(1 row)

How should I select NULL values through SQLAlchemy?

Answer 1

(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.