Are classes'objects, in python 3.9, passed by reference?

Question

I have read, multiple times, that python passes arguments by value. However, I was testing the following simple code, and it looks like the class c objects are passed by reference (a modification inside the function modifies the object). Could someone please explain why so?

Code:

class c:
    def __init__(self,i):
        self.i=i

def incr_obj(obj):
    obj.i+=1

oo=c(1)
incr_obj(oo)
print(oo.i)

it prints 2.