Make method can be called from class or instance

Question

I want to make a method to be called from class or instance.
For example :

class SomeClass:
    a = 10

    def __init__(self, val):
        self.a = val

    def print_a(self):
        print(self.a)

SomeClass(20).print_a()    # 20
SomeClass.print_a()    # Error!

Here I want to make print_a can be called by class either.
If I use classmethod, the result is wrong.

class SomeClass:
    a = 10

    def __init__(self, val):
        self.a = val

    @classmethod
    def print_a(cls):
        print(cls.a)
        
SomeClass(20).print_a()    # 10  (wrong!)
SomeClass.print_a()    # 10

I hope the result is like this:

SomeClass(20).print_a()    # 20
SomeClass.print_a()    # 10

How can I achieve this?

This question is similar to: Same name for classmethod and instancemethod. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. — Daraan
– Daraan, Commented Aug 12, 2024 at 14:17

juanpa.arrivillaga · Accepted Answer · 2020-11-10 07:41:36Z

4

classmethod is simply a descriptor object, you can read about how it could be implemented using pure python in the Descriptor HOWTO. Using that implementation as an inspiration:

from types import MethodType
class HybridMethod:
    def __init__(self, f):
        self.f = f

    def __get__(self, obj, cls=None):
        if obj is None:
            return MethodType(self.f, cls)
        else:
            return MethodType(self.f, obj)

class SomeClass:
    a = 10

    def __init__(self, val):
        self.a = val

    @HybridMethod
    def print_a(self):
        print(self.a)

SomeClass(20).print_a()    
SomeClass.print_a()

answered Nov 10, 2020 at 7:41

juanpa.arrivillaga

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2 Comments

Chun-Ye Lu Over a year ago

Very thanks! I think I need some time to figure out how this code works.

juanpa.arrivillaga Over a year ago

@Chun-YeLu read the HOWTO from the beginning. Also, more details in the data model documentation

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