Python descriptors.

gistfile1.py

class Desc(object):

  def __get__(self, obj, type=None):
    return 'd'

  def __set__(self, obj, value):
    pass

class A(object):
  pass

a = A()

A.d1 = Desc()
a.d2 = Desc()

# object.__getattribute__ prioritizes instance variables over descriptors
assert(a.d1 == 'd')
assert(type(a.d2) == Desc)

# but type.__getattribute__ doesn't
assert(A.d1 == 'd')
assert(type(A.__dict__['d1']) == Desc)

# functions are (non-data) descriptors, this is where the method transformation comes from

class C(object):
  pass

def bar(*args):
  return len(args)

C.baz = bar
baz = C.baz
assert(baz != bar)

c = C()
c.bax = bar
bax = c.bax
assert(bax == bar)

# similarly

A.bax = bar
a.baz = bar

assert(a.bar() == 1)
assert(a.baz() == 0)

# descriptors added later override instance variables

class B(object):
  d = 0

b = B()
assert(b.d == 0)

b.d = 1
assert(b.d == 1)

B.d = Desc()
assert(b.d == 'd')