python weird codes

weird.py

"""
Why line 21 raise ??
"""


def error():
    print 'foo'


class OK(object):
    pass


class Hehe():
    impl_ok = OK
    impl_err = error

    def __init__(self):
        self.implement = self.__class__.impl_ok()
        print self.implement

        self.implement = self.__class__.impl_err()
        print self.implement


he = Hehe()

<__main__.OK object at 0x7f75bc03b750>
Traceback (most recent call last):
  File "a.py", line 26, in <module>
    he = Hehe()
  File "a.py", line 22, in __init__
    self.implement = self.__class__.impl_err()
TypeError: unbound method error() must be called with Hehe instance as first argument (got nothing instead)