Read-Only Python Properties

Following onto yesterday’s post about Python Properties and @property, there is a neat little trick to create a read-only property of a Python class. All you have to do is omit the setter. This Hero class has a read-write property “name”

class Hero(object):
    def __init__(self, **kwargs):
        self.name = kwargs['name']

    @property
    def name(self):
        return '%s %s' % (self.firstname, self.lastname)

    @name.setter
    def name(self, name):
        self.firstname, self.lastname = name.split(' ', 2)

if '__main__' == __name__:
    superman = Hero(name='Kal El')
    print superman.name
    print superman.firstname
    superman.name = 'Clark Kent'
    print superman.name
    print superman.firstname

# Kal El
# Kal
# Clark Kent
# Clark

You can see it used in the the constructor and in the “superman.name=” statement at the bottom.

We can easily make the name attribute read-only like this:

class Hero(object):
    def __init__(self, **kwargs):
        self.firstname, self.lastname = kwargs['name'].split(' ', 2)

    @property
    def name(self):
        return '%s %s' % (self.firstname, self.lastname)

if '__main__' == __name__:
    superman = Hero(name='Kal El')
    print superman.name
    print superman.firstname
    superman.name = 'Clark Kent'

# Kal El
# Kal
# Traceback (most recent call last):
#   File "/home/art/PycharmProjects/hello/Hero.py", line 14, in <module>
#     superman.name = 'Clark Kent'
# AttributeError: can't set attribute

Note that I had to change the constructor, directly setting self.firstname and self.lastname, since it is no longer valid to write “self.name =”.