Why return a mutable attribute might be dangerous

return_mutable_attributes_is_dangerous.py

#!/usr/bin/env python

""" Why return a mutable attribute might be dangerous """

class Car(object):
    def __init__(self, owners):
        self._owners = owners

    def get_owners(self):
        """ Return a list of owners. Getter/setters are evil, but that
        is the story for another day.
        This is dangerous since someone could modify that list
        """
        return self._owners


if __name__ == '__main__':
    car = Car(['John', 'Cindy'])
    print 'This car belongs to', car.get_owners()  # John, Cindy

    owners = car.get_owners()
    owners.append('Hai')                           # Danger!
    print 'This car belongs to', car.get_owners()  # John, Cindy, Hai