you were close, what you need is to construct the following query in sqla:
root@localhost [inDB]> SELECT * FROM Person;
+-----------+------------+-----------+
| person_id | first_name | last_name |
+-----------+------------+-----------+
| 1 | Bob | Smith |
| 2 | John | Smith |
+-----------+------------+-----------+
2 rows in set (0.00 sec)
root@localhost [inDB]> SELECT * FROM Person WHERE CONCAT(first_name, ' ', last_name) LIKE 'Bob Sm%';
+-----------+------------+-----------+
| person_id | first_name | last_name |
+-----------+------------+-----------+
| 1 | Bob | Smith |
+-----------+------------+-----------+
then it will become clear that you need a filter with concat() function:
from sqlalchemy import func
res = session.query(Person).filter(func.concat(Person.first_name, ' ', Person.last_name).like('Bob Sm%')).all()
len(res) # is 1
res, = res
print res.first_name, res.last_name # 'Bob Smith'
firstNameandlastName