Question

I have the following code:

class Test(object):

    _spam = 42

    @classmethod
    def get_spam(cls):
        cls._spam

    @classmethod
    def set_spam(cls, value):
        cls._spam = value

    spam = property(get_spam, set_spam)

print Test.spam
Test.spam = 24
print Test.spam

The output is:

<property object at 0x01E55BD0>
24

Is there any way to prevent the setting of Test.spam from overriding the property? I don’t want to use Test.spam to set the value of Test._spam. The setter and getter have to remain as class methods, and I do not want to have to call Test.set_spam.

The output should be:

<property object at 0x01E55BD0>
<property object at 0x01E55BD0>

Answer 1

I suppose this stops developers from accidentally overwriting Test‘s spam property. Is that why you want this? I not sure that is a good idea (what if a developer wants to override the spam property? why throw up roadblocks?), but…

You could use a metaclass. If you don’t supply a setter for the metaclass’s property, then Test.spam will raise an AttributeError:

class MetaTest(type):
    @property
    def spam(cls):
        return cls._spam

class Test(object):
    __metaclass__=MetaTest
    _spam = 42

    @classmethod
    def get_spam(cls):
        cls._spam

    @classmethod
    def set_spam(cls, value):
        cls._spam = value

    spam = property(get_spam, set_spam)

print Test.spam
# 42

But

Test.spam = 24

raises

AttributeError: can't set attribute