[Python] Why does Python pass an instance to a class attribute that is a function?

I define a class attribute and give it a function. When I call this function, the instance is passed on as the first argument (as if it's an instance function call with a self). I would not expect an instance to care that the attribute fn is a function, let alone for it to pass itself as an argument. Why does it do that?

class BStatic:
fn = None
@staticmethod
def static_fn(*args, **kwargs):
print(args, kwargs)

BStatic.fn = BStatic.static_fn

bs = BStatic()

bs.fn()
# (<__main__.BStatic object at 0x0000022D63214590>,) {}

BStatic.fn()
# () {}


In another example, I assigned a class attribute to a function defined on a module. When I call this function on an instance of a class, the instance is again passed as the first argument.

class B:
fn = None
def instance_fn(self, *args, **kwargs):
print(self, args, kwargs)

def fn(*args, **kwargs):
print(args, kwargs)

B.fn = fn

B.fn()
# () {}

b = B()

b.fn()
# (<__main__.B object at 0x0000022D643D23F0>,) {}


This is not the case if I assign the class attribute on an instance.

b.fn = fn

b.fn()
# () {}

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