Why should functions always return the same type?

Question

I read somewhere that functions should always return only one type so the following code is considered as bad code:

def x(foo):
 if \'bar\' in foo:
  return          


        

Answer 1

I personally think it is perfectly fine for a function to return a tuple or None. However, a function should return at most 2 different types and the second one should be a None. A function should never return a string and list for example.