Overriding class function with instance function in python [duplicate]

Question

Consider this example:

class master:
    @classmethod 
    def foo(cls):
        cls.bar()

class slaveClass( master ):
    @classmethod 
    def bar(cls):
        print("This is class method")

slaveType = slaveClass
slaveType.foo()

class slaveInstance( master ):
    #def foo(self):
    #    self.foo()
    def __init__(self,data):
        self.data=data
        print("Instance has been made")
    def bar(self):
        print("This is "+self.data+" method")


slaveType = slaveInstance("instance")
slaveType.foo()

I know it works when last definition of foo is uncommented, but is there any other way to use this foo function without changing the usage. I have large project where classes defined the way things worked and I was able to change the way with slaveType but there happen to be a case where instance is needed, and there is bit too many foo like functions to be overridden for instance behavior.

Thank you stackers!

How could that work? The original foo is a classmethod, so simply never receives the instance - only the class. There's no way it could then call an instance method. — Daniel Roseman
– Daniel Roseman, Commented Sep 9, 2011 at 13:21
I edited the example a little, so it is bit more clear, what I need to do. If I uncommented the last foo definition I will never call the classmethod, as it is overridden by instance method. I want to know, if there is any other way. I am able to change the master class but I need to keep the way slaveType was used. — Johu
– Johu, Commented Sep 9, 2011 at 13:28
I have to recommend biting the bullet and getting rid of the classmethods altogether. Anything else will produce complicated code which you'll regret down the road. — Winston Ewert
– Winston Ewert, Commented Sep 10, 2011 at 14:19
Trying to decode this question into something Pythonic. slaveClass is a child class that inherits classmethod foo() from Master. slaveInstance is a grandchild class (not an instance) that then overrides foo() as an instance method. (Btw, style points to avoid confusion: class names should be CamelCase, and never include 'Class' (this ain't Java) or Instance or Type (just plain wrong, in a class name). So: slaveClass should simply be Slave, and 'master` be Master. slaveType is not a type, it's an instance... — smci
– smci, Commented Dec 15, 2020 at 5:15
By "I want to know, if there is any other way." I think you meant "Can I subclass and override a classmethod as an instance method, but still also be able to call it as classmethod from the grandchild class?" — smci
– smci, Commented Dec 15, 2020 at 5:16

krs1 · Accepted Answer · 2011-09-09 13:40:56Z

0

Look closer at the doc for classmethod. Instance methods pass an instance as the first argument, and class methods pass a class as the first argument. Calling slaveType.foo() passes an instance, slaveType, as the first argument of foo(). foo() is expecting a class as the first argument.

answered Sep 9, 2011 at 13:40

krs1

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1 Comment

Johu Over a year ago

I know it. I was trying to come up with a way to bypass this as cls.bar() would work also if cls was actually an instance not a type.

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