Defining our own exceptions


class Stack:
 def __init(self):
  self.stack = []

 def pop(self):
  try:
    return self.stack.pop()
  except IndexError:
    raise "PopEmpty", "pop from empty stack" 

>>> s = Stack()
>>> s.pop()
Traceback (innermost last):
  File "<pyshell#10>", line 1, in ?
    s.pop()
  File "<pyshell#8>", line 12, in pop
    raise "PopEmpty", "pop from empty stack"
PopEmpty: pop from empty stack

Exceptions may also be classes or instances