class Vector2D(object):
 def __init__(self, x, y):
 self.__x = float(x)
 self.__y = float(y)

 @property
 def x(self):
 return self.__x
 @property
 def y(self):
 return self.__y

if __name__ == "__main__":
 v = Vector2D(3, 4)
 print(v.x, v.y)
 v.x = 8 # error will be raised.

(3.0, 4.0)
Traceback (most recent call last):
 File ...., line 16, in <module>
 v.x = 8 # error will be raised.
AttributeError: can't set attribute

class MyCls():
 def __init__(self):
 pass

 def __setattr__(self, f, v):
 print 'setting %r = %r'%(f, v)
if __name__ == '__main__':
 obj = MyCls()
 obj.new_field = 1

# encoding=utf8
class MyCls(object):
 readonly_property = 'readonly_property' 
 def __init__(self):
 pass
 def __setattr__(self, f, v):
 if f == 'readonly_property':
  raise AttributeError('{}.{} is READ ONLY'.     format(type(self).__name__, f))

 else:
  self.__dict__[f] = v

if __name__ == '__main__':
 obj = MyCls()

 obj.any_other_property = 'any_other_property'
 print(obj.any_other_property)

 print(obj.readonly_property)
 obj.readonly_property = 1

any_other_property
readonly_property
Traceback (most recent call last):
 File "...", line 21, in <module>
 obj.readonly_property = 1
 ...
 AttributeError: MyCls.readonly_property is READ ONLY