This *may* be a duplicate of 729913, but there's either additional
info here, or this is actually different.

The issue is that any special method (e.g. __call__, __str__, etc.)
defined for a new-style (i.e. object-based) class seems to be static
(i.e. unchangeable) with some special lookup applied for that method,
but this doesn't seem to be the case for regular methods.

class A:
  def foo(self): return 1
  def __call__(self): return 2
  def bar(self): return 3
  def adjust(self):
    self.foo = self.bar
    self.__call__ = self.bar

a = A()
print a.foo(), a()
a.adjust()
print a.foo(), a()

Will print:
1 2
3 3

But if the A is turned into a new-style class by changing the
first line:

class A(object):

then the printed results are:
1 2
3 2

To the best of my understanding of the migration from classic classes 
to new-style classes (and metaclassing), this shouldn't occur.  I have 
also tried various name munging for the special method (e.g. setting 
_B__call__, using setattr, etc.), but I haven't found the special trick 
yet.

The attached script shows the example in more detail.