@staticmethod and @classmethod

[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.