counting randoms letters(Python)

The program will ask a user for a string. The program will then ask the user for a single letter. If the user enters a letter, then the program will call a function to count the number of occurrences of the supplied letter in the string. If the user does not supply a letter, then the program will randomly pick one letter from the supplied string and call a function to count that letter. The program will output the string, the letter to be counted, and the number of occurrences of that letter