Methods not defined within class don't pass self as first arg

cls_test.py

class X(object):
    def __str__(self):
        return 'x'

    def my_print_arg(*args):
        print args[0]

def print_arg(*args):
    print args[0]

if __name__ == '__main__':
    x = X()
    x.y = print_arg
    x.y('z') # prints 'z'
    x.my_print_arg('z') # prints 'x'