express constraints on types

 Variables in Python are just that - variable. Sometimes
 though you want to make sure that a variable is of a
 particular type, like an integer, a string or a something
 to iterate (loop) through.
 .
 To express such constraints on the type that a variable
 may have in the code, such that the program can react
 to it at runtime, this small modules provides a set of
 routines.