Björn Pollex Björn Pollex - 2 months ago 7
Python Question

How to create a read-only class property in Python?

Essentially I want to do something like this:

class foo:
x = 4
@property
@classmethod
def number(cls):
return x


Then I would like the following to work:

>>> foo.number
4


Unfortunately, the above doesn't work. Instead of given me
4
it gives me
<property object at 0x101786c58>
. Is there any way to achieve the above?

Answer

The property descriptor always returns itself when accessed from a class (ie. when instance is None in its __get__ method).

If that's not what you want, you can write a new descriptor that always uses the class object (owner) instead of the instance:

>>> class classproperty(object):
...     def __init__(self, getter):
...         self.getter= getter
...     def __get__(self, instance, owner):
...         return self.getter(owner)
... 
>>> class Foo(object):
...     x= 4
...     @classproperty
...     def number(cls):
...         return cls.x
... 
>>> Foo().number
4
>>> Foo.number
4
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