@classmethod can be stacked on @property, but @staticmethod cannot #89136

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1. tusharsadhwani commented on Aug 22, 2021

   

   tusharsadhwanimannequin

   on Aug 22, 2021

   MannequinAuthor

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   Starting with Python3.9, @classmethod can be stacked on top of @property, but it seems that @staticmethod cannot.

   >>> class C:
   ...     @classmethod
   ...     @property
   ...     def cm(cls):
   ...         return cls.__name__
   
   ...     @staticmethod
   ...     @property
   ...     def magic_number():
   ...         return 42
   ... 
   >>> C.cm
   'C'
   >>> C.magic_number
   <property object at 0x7f0ad3c1ea90>
   >>> 

   This feels like inconsistent behaviour, plus, having staticmethod properties can be useful for creating read-only class attributes, for eg:

   class C:
       @staticmethod
       @property
       def FINE_STRUCTURE_CONSTANT():
           return 1 / 137

   This would make it hard to accidentally modify the constant inside the class.
2. tusharsadhwani

   added

   3.9 (EOL)end of lifeend of life

   3.10 (EOL)end of lifeend of life

   3.11only security fixesonly security fixes

   type-featureA feature request or enhancementA feature request or enhancement

   on Aug 22, 2021
3. rhettinger commented on Aug 22, 2021

   

   rhettinger

   on Aug 22, 2021

   Contributor

   More actions

   The classmethod and staticmethod decorators have somewhat different semantics. Accessing a classmethod with C.cm creates a bound method object which has both __call__ and __get__. In contrast, accessing a staticmethod with C.sm returns the underlying function. So there is no opportunity for a customized __get__ method as we have with classmethod.

   Also, even if C.sm did return a custom descriptor, the code example still can't be made to work because the __get__ method on property objects is hardwired to pass in an instance.¹ That means that the underlying method must have a "self" argument.

   Another issue is that properties have __set__ and __delete__ methods which could not be made to work without a reference to the class or instance.

   To make a read-only class variable, a metaclass would be needed. While it is possible to write a descriptor to make C.FINE_STRUCTURE_CONSTANT call a function that returns 1/137, there is no way for the descriptors to block the assignment: C.FINE_STRUCTURE_CONSTANT = 1 / 100

   Thanks for the suggestion, but I don't think this can be made to work.

   ¹ https://docs.python.org/3/howto/descriptor.html#properties
4. rhettinger

   closed this as completedon Aug 22, 2021
5. rhettinger

   closed this as completedon Aug 22, 2021
6. rhettinger

   assigned

   rhettinger

   on Aug 22, 2021
7. ezio-melotti

   transferred this issue fromon Apr 10, 2022