Julius Caesar is said to have hidden messages by shifting every letter a fixed number of places along the alphabet. With a shift of 3, A becomes D, B becomes E, and at the end it wraps round: X becomes A, Y becomes B, Z becomes C.

To shift one capital letter:

Turn it into a position from 0 to 25: ord(ch) - ord("A").
Add the shift, and wrap round with % 26.
Turn it back into a letter: chr(position + ord("A")).

ch = "Y"
shift = 3
position = (ord(ch) - ord("A") + shift) % 26
print(chr(position + ord("A")))