On 12/10/2012 5:59 PM, John Gordon wrote: > def encode(plain): > '''Return a substituted version of the plain text.''' > encoded = '' > for ch in plain: > encoded += key[alpha.index(ch)] > return encoded The turns an O(n) problem into a slow O(n*n) solution. Much better to build a list of chars and then join them. -- Terry Jan Reedy