#!/usr/local/bin/python3
from types import MethodType

class C(object):
  def __init__(self):
    self.__setattr__ = MethodType(s, self)

def s(self, key, value):
  print('set')
  if key == 'a':
    raise Error
  object.__setattr__(self, key, value)

c = C()
c.a = 1
print(c.a)