@classmethod and method without self

Question

Let's suppose we have this class:

class Demo:

    def __init__(self, str):
        self.str = str

    def fromBytes1(bytes):
        return Demo(bytes2str(bytes))

    @classmethod
    def fromBytes2(cls, bytes):
        return cls(bytes2str(bytes))

What is the difference between fromBytes1 and fromBytes2, except for the fact that you can't call the 1st method in the following way?

Demo().fromBytes1(bytes)

Is there something more subtle that I cannot see here?

You're missing a self parameter in the first case. Also, could you give the methods slightly different names? — Mad Physicist
– Mad Physicist, Commented Dec 20, 2021 at 16:01
@MadPhysicist I thought that omission was intentional. It lets you use the method as a pseudo-static method (Demo.fromBytes(…) works), but Demo().fromBytes() does something unexpected. — Konrad Rudolph
– Konrad Rudolph, Commented Dec 20, 2021 at 16:05
@igol you can but you shouldn't. Don't write something like your version 1 ever. That should just be a regular function, outside of the class. In any case, the difference is obvious, a classmethod will implicitly get passed the class as it's first positional argument — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 20, 2021 at 16:24
If you really want the first variant, make it an @staticmethod. — MisterMiyagi
– MisterMiyagi, Commented Dec 20, 2021 at 16:46

Mad Physicist · Accepted Answer · 2021-12-20 16:36:14Z

4

Assuming that you only intend to call the methods from a class object, all the important differences are in inheritance. Say you have

class Test(Demo):
    pass

If you do Test.fromBytes1(b'ar'), you get an instance of Demo.

If you do Test.fromBytes2(b'ar'), you get an instance of Test.

The second method is more flexible because you can hard code Demo or __class__ into it directly, but you must hard code it in the first case.

edited Dec 20, 2021 at 16:36

answered Dec 20, 2021 at 15:59

Mad Physicist

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5 Comments

igol Over a year ago

So, with @classmethod, you don't need to override the method?

igol Over a year ago

@MisterMiyagi Yeah I know that. I wrote that you can't call the 1st variant with Demo().fromBytes(some_bytes)

Mad Physicist Over a year ago

@MisterMiyagi. Of course. I have no idea why I didn't write it that way. Will fix momentarily.

Mad Physicist Over a year ago

@igol. I misread. Will fix ASAP

Mad Physicist Over a year ago

@igol. F i x e d

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