Return two SqlAlchemy Columns concatenated

Use the hybrid extension:

from sqlalchemy.ext.hybrid import hybrid_property

class Person(Base):
    # ...
    @hybrid_property
    def name(self):
        return '{0} {1}'.format(self.first_name, self.last_name)

    @name.setter
    def name(self, value):
        self.first_name, self.last_name = value.split(' ', 1)

    @name.expression
    def name(cls):
        return db.func.concat(cls.first_name, ' ', cls.last_name)

@davidism suggested using hybrid extension, whilst I say, why not use SQLAlchemy's column_property function?

Example:

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    firstname = Column(String(50))
    lastname = Column(String(50))
    fullname = column_property(firstname + " " + lastname)