How to shift and cycle characters in python

Question

I want to increase each character's ASCII value, and cycle (wraparound) if value > z.

#small chars:
for char in string.ascii_lowercase:
    print factName + '(' + char + ', ' + chr(((ord(char)+key) % ord('z'))) + ').'

if key = 3 >> a became d but w, x, and y are not correct!

Answer 1

Well, you reduced everything mod ord('z'), as if the alphabet's internal values were zero-based. As you've discovered, this is not the case. Instead, you need to make that overflow check, and then subtract the width of your data set. The basic logic is

if ord(chr) + key > ord('z'):
    result = ord(chr) + key - 26

Is that enough to get you moving?

You can also do this with a simple translation table, building a new alphabet from the old: cut the first 3 characters and append them to the back of your translation string:

blank_code = 'abcdefghijklmnopqrstuvwxyz'
new_code = blank_code[-n:] + blank_code[:n]

This gives you a corresponding "alphabet" to use. Also see the translate method for a quick way to use these.