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

[tusharsadhwani]

BPO	44973
Nosy	@rhettinger, @tusharsadhwani

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2021-08-22.17:25:36.323>
created_at = <Date 2021-08-22.06:39:43.790>
labels = ['type-feature', '3.9', '3.10', '3.11']
title = '@classmethod can be stacked on @property, but @staticmethod cannot'
updated_at = <Date 2021-08-22.17:38:02.202>
user = 'https://github.com/tusharsadhwani'

bugs.python.org fields:

activity = <Date 2021-08-22.17:38:02.202>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2021-08-22.17:25:36.323>
closer = 'rhettinger'
components = []
creation = <Date 2021-08-22.06:39:43.790>
creator = 'tusharsadhwani'
dependencies = []
files = []
hgrepos = []
issue_num = 44973
keywords = []
message_count = 2.0
messages = ['400051', '400081']
nosy_count = 2.0
nosy_names = ['rhettinger', 'tusharsadhwani']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44973'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[tusharsadhwani]

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.

[rhettinger]

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