Style:Exceptions

From 6.00 reference wiki

Elements of Style for Python Programmers: Main | Names | Variable Declarations | Indentation | Exceptions

Functions should either raise an exception or return the special value None when there is no sensible value for them to return.

For example:

     def factorial(n):
         """n: int
              if n >= 0
                 returns n!
              else returns None""" 

or

     def factorial(n):
         """n: int
              if n >= 0
                 returns n!
              else raises negArg""" 
      

is better than a factorial program that returns -1 when called with a negative number.