@staticmethod and @classmethod

[akbarza]

hi
the below code is in address:
https://python101.pythonlibrary.org/chap...ators.html

# decorator in python: @staticmethod & @classmethod usage
# from address:https://python101.pythonlibrary.org/chapter25_decorators.html


class DecoratorTest(object):
    """
    Test regular method vs @classmethod vs @staticmethod
    """

    def __init__(self):
        """Constructor"""
        pass

    def doubler(self, x):
        """"""
        print("running doubler")
        return x*2

    @classmethod
    def class_tripler(klass, x):
        """"""
        print("running tripler: %s" % klass)
        return x*3

    @staticmethod
    def static_quad(x):
        """"""
        print("running quad")
        return x*4

if __name__ == "__main__":
    decor = DecoratorTest()
    print(f"decor.doubler(5) results: {decor.doubler(5)}" )
    print("-"*50)
    print(f"decor.class_tripler(3) results: {decor.class_tripler(3)}")
    print("-"*50)
    print(f"DecoratorTest.class_tripler(3) results:{DecoratorTest.class_tripler(3)}" )
    print("-"*50)
    print(f"DecoratorTest.static_quad(2) results: {DecoratorTest.static_quad(2)}")
    print("-"*50)
    print(f"decor.static_quad(3) results: {decor.static_quad(3)}")
    print("-"*50)
    print("-"*50)
    print("-"*50)
    print(f"decor.doubler results: {decor.doubler}")
    print("-"*50)
    print(f"decor.class_tripler results: {decor.class_tripler}")
    print("-"*50)
    print(f"decor.static_quad results: {decor.static_quad}")
    print("-"*50)

my questions :
1)what is the klass argument in def class_tripler(klass, x)? if I write decor.class_tripler('test',3), causes a TypeError error message, but the method has two arguments.
2) On the same page is written that

You will also note that the last print statement shows that decor.static_quad returns a regular function instead of a bound method.

. what are the meanings of regular function and bound function?
thanks

[Gribouillis]

what is the klass argument in def class_tripler(klass, x)?

The klass argument is an implicit argument which value is the calling class

>>> class A:
...     @classmethod
...     def foo(klass, x):
...         return (klass, x)
... 
>>> A.foo(3)
(<class '__main__.A'>, 3)
>>> class B(A):
...     pass
... 
>>> B.foo(4)
(<class '__main__.B'>, 4)

what are the meanings of regular function and bound function?

A bound method is a small box containing an object and a function. The object is the implicit argument of the function. This bound method is callable with the rest of the arguments. In the following example, c is an instance of class C and c.spam is a bound method containing the object c and the method spam.

>>> class C:
...     def spam(self, x):
...         return (self, x)
... 
>>> c = C()
>>> c
<__main__.C object at 0x7fe88a394d90>
>>> c.spam
<bound method C.spam of <__main__.C object at 0x7fe88a394d90>>
>>> c.spam(3)
(<__main__.C object at 0x7fe88a394d90>, 3)

A bound method is very close to a partial function as returned by functools.partial(). It is an example of curried function.

[deanhystad]

1)what is the klass argument in def class_tripler(klass, x)? if I write decor.class_tripler('test',3), causes a TypeError error message, but the method has two arguments.

Always post error messages. When I run your code with "def class_tripler(klass, x)" I get this error:

Error:
TypeError: DecoratorTest.class_tripler() takes 2 positional arguments but 3 were given

If I was confused about what was happpening I would modify the class_tripler() method to take three arguments to see what they are.

    @classmethod
    def class_tripler(klass, x, y):
        print(f"klass {klass}, x {x}, y {y}")
        print("running tripler: %s" % klass)
        return x*3

When run I get this output:

Output:
klass <class '__main__.DecoratorTest'>, x test, y 3
running tripler: <class '__main__.DecoratorTest'>

Where does "<class '__main__.DecoratorTest'>" come from? I didn't pass that. Wondering what could make that happen I google "python @classmethod". Among the many useful hits I pick this:

https://docs.python.org/3/library/functi...lassmethod

@classmethod
Transform a method into a class method.

A class method receives the class as an implicit first argument, just like an instance method receives the instance.

This not only explains why the "extra" argument appears but uses the more conventional "cls" naming of the first argument.

On the same page they discuss @staticmethod

@staticmethod
Transform a method into a static method.

A static method does not receive an implicit first argument.

Unlike an instance method or a class method, a static method is not implicitly passed an instance or a class object as the first argument. That might be what is mean by not being "bound", because the function/method is not bound to any particular instance or class object. The only thing that differentiates a static method from a function is scope. Functions exist in global or enclosed scope. static methods exist in class scope. You need to use the class name to call a static method.