>>> class Point:
...   def __init__(self, x, y):
...     self.x = x
...     self.y = y
...   def move_by(self, x, y):
...     self.x += x
...     self.y += y
... 
>>> p = Point(10, -4)
>>> p
<__main__.Point instance at 0x6fee0>
>>> q = Point(10, -4)
>>> q
<__main__.Point instance at 0x6ff08>
>>> p.x,p.y
(10, -4)
>>> q.x,q.y 
(10, -4)
>>> p is q
False
>>> r = p
>>> r.x,r.y
(10, -4)
>>> p is r
True
>>> p.move_by(20,20)
>>> p.x,p.y
(30, 16)
>>> q.x,q.y
(10, -4)
>>> r.x,r.y
(30, 16)
>>> p=q  
>>> p.x,p.y
(10, -4)
>>> r.x,r.y
(30, 16)
>>> p2 = Point(p.x, p.y)
>>> class CopyablePoint(Point):
...   def copy(self):
...     return CopyablePoint(self.x, self.y)
... 
>>> q = CopyablePoint(10, 4)
>>> q2 = q.copy()   
>>> q.move_by(10,10)
>>> q.x,q.y
(20, 14)
>>> q2 
<__main__.CopyablePoint instance at 0x6ffa8>
>>> q2.x,q2.y
(10, 4)
>>> q is q2
False