I would not differentiate needlessly between the terms function and ["dunder", "instance", "module", "class"] method, this isn't practical at the current point in time, and arguably not pythonic. They are all callables and behave very similar for all practical purposes, all quacking like a duck.

If you want to deep-dive into the intricacies, there's technically a bunch of alternative "method types" with slightly differing calling and argument passing conventions. There are class methods, static methods, class properties, and module-scope functions off the top of my head, all very similar, but particularly suitable under specific circumstances.

If I wanted a String utility class, for example, I could have that:

class String:
  @staticmethod
  def len(string: str) -> int:
    if not isinstance(string, str):
      raise TypeError("Cannot determine length of non-string object")
    return len(string)

s = "foobar"
assert String.len(s) == 6

In Python I generally find static methods hardly as relevant as in other languages, and tend to create utility modules instead, with plain functions and maybe dataclasses for complex arguments or return values, which can then be imported and called pretty much the same way as a static method, but without the extra organisational class layer. I'd argue that's very much a matter of taste, and how much of a purebred OOP jockey you are, for better or worse.

Class properties are cool, however, you might want to look into that. It's getters/setters on crack, masquerading as their respective property, so that something like this "just works":

s = MyString("foobar")
assert s.len == 6