Please assist me with learning python

all i need is a simple python script that allows a user to enter a string, then takes each character in the strign, and converts it do predefined variables.

For example, i tell the program that a = 01 and b = 02
and if the user types in ab, he gets returned 0102.

this shouldent be to hard. Its for a new encryption cypher i developed.

%variables=qw;a 01 b 02;;
s/(.)/$variables{$1}/g,print while<>