python class with mixed @classmethod and methods

Question

Hi I'm developing a class in a project with different collaborators. I've been able to implement the class with different method and all is working properly. Basically my present situation is the following

class MyClass(object):
    def __init__(self,a,b,c):
        self.a = a
        self.b = b
        self.c = c  

    def one(self,**kwargs):
        d = self.two()
        e = self.three()
        # make something using a an b like 
        return 2*d + 3*e

    def two(self,**kwargs):
        # this uses a for example 
        return self.a*2 + self.b**self.c
    def three(self, **kwargs):
        # this uses b and c
        return self.b/self.c - self.a/3

These are clearly examples and there are more complicated stuff going on. The problem is that this class can be called only through an instance

[1]: x=MyClass(a,b,c)
[2]: y=x.one()

The class is inserted in a larger project and the other collaborators would like to call one without the istance directly as

[1]: y = MyClass.one(a,b,c)
[2]: z = MyClass.two(a,b,c)
[3]: x = MyClass.three(a,b,c)

I know that I can obtain this by using decorators, like @classmethod. For example for one I could do like

@classmethod
def one(cls, a, b, c):
    d = self.two()
    e = self.three()
    cos(2*d+3*e)

but this actually don't work because it raises an error as self is not defined. My problem is that I do not understand how a @classmethod can call another method pertaining in the same class if I did not make an instance. BTW I'm working on python 2.7 Thanks for any clue. I've tried to search on the various @classmethod question but did not find the answer (or maybe I did not understand it)

"I do not understand how a @classmethod can call another method pertaining in the same class" - by accessing it via cls. self isn't part of a class method (as you can see from the signature...), you have access to the class, not an instance. — jonrsharpe
– jonrsharpe, Commented Dec 2, 2015 at 21:33
If you call it without an instance, where do you expect a, b, and c to come from? — user2357112
– user2357112, Commented Dec 2, 2015 at 21:38
you are right I know. Sorry if the question was too simple and for the misspelling — Nicola Vianello
– Nicola Vianello, Commented Dec 2, 2015 at 21:50
Will you have one, only one instance of MyClass in your application? — Ozan
– Ozan, Commented Dec 2, 2015 at 21:58
No, in principle I can call MyClass different times, (with different values of a,b and c) — Nicola Vianello
– Nicola Vianello, Commented Dec 3, 2015 at 7:30

Kijewski · Accepted Answer · 2015-12-02 21:34:29Z

1

You renamed the parameter into cls, so you have to change self in the function body into cls, too. The names self and cls are just normal identifiers, a convention, unlike e.g. C++'s this.

@classmethod
def one(cls, a, b, c):
    d = cls.two()
    e = cls.three()
    cos(2*d+3*e)

works the same as

@classmethod
def one(we_need_more_unicorns, a, b, c):
    d = we_need_more_unicorns.two()
    e = we_need_more_unicorns.three()
    cos(2*d+3*e)

Or a more reasonable identifer like Cls (with an uppercase C to denote that it is a class), which is seen sometimes, too.

answered Dec 2, 2015 at 21:34

Kijewski

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