Is the cls keyword equivalent to the class name?

Question

Are the following methods always equivalent within a class? In other words, within MyClass can I interchangeably use cls in a class method and MyClass in a static method?

class MyClass:

    @classmethod
    def my_class_method(cls):
        cls.attribute = "a"

    @staticmethod
    def my_non_class_method():
        MyClass.attribute = "b"

If it needs access to the class, it probably shouldn’t be a static method. — jonrsharpe, Mar 19 '21 at 22:53
`import keyword; keyword.iskeyword('cls')` → `False`. Both methods accomplish the same thing but the `classmethod` has the advantage of eliminating the need to hardcode the class name into its code. — martineau, Mar 20 '21 at 00:58
@martineau they accomplish the same thing only if you call `my_class_method` on `MyClass` and not a subclass. — Ryan Haining, Mar 20 '21 at 01:14
@martineau right, which is why there's `@staticmethod`. My point is that `cls.attr` and `MyClass.attr` are not always equivalent, and the goal of `@classmethod` isn't simply to avoid repeating a typename — Ryan Haining, Mar 20 '21 at 02:52

Ryan Haining · Accepted Answer · 2021-03-19T22:54:21.153

2

cls isn't a keyword, just like self isn't a keyword.

No, cls and MyClass aren't interchangeable unless you are positive that MyClass doesn't have any subclasses.

The point of a @classmethod is to get the right class type if you call it through a subclass. for example

class Base:
    @classmethod
    def f(cls):
        print(f'class is {cls}')

class Sub(Base):
    pass

Sub.f()  # calls Base.f with cls=Sub

If you don't need the actual class type, then you can use @staticmethod instead.

edited Mar 19 '21 at 22:54

answered Mar 19 '21 at 22:52

Ryan Haining

32,906
11
104
160

Ups, you are right, it's not a keyword. – user1315621 Mar 19 '21 at 22:53

score -2 · Answer 2 · answered Mar 19 '21 at 22:46

-2

Yes. Heck, you could use MyClass in the class method, too, if you really felt like it.

answered Mar 19 '21 at 22:46

Tim Roberts

34,376
3
17
24

Thanks. Could you comment on the edit I just added to the question? – user1315621 Mar 19 '21 at 22:52
1

@user1315621 please don’t start tacking on extra questions once you get an answer. – jonrsharpe Mar 19 '21 at 22:53

Is the cls keyword equivalent to the class name?

2 Answers2