succ.py

def succ(s):
    a = list(s)
    for i in reversed(range(len(a))):
        a[i] = chr(ord(a[i]) + 1)
        if a[i] <= 'z':
            break
        a[i] = 'a'
    else:
        a.insert(0, 'a')
    return ''.join(a)

succ('aaa')  # 'aab'
succ('a')    # 'b'
succ('xyz')  # 'xza'
succ('zzz')  # 'aaaa'
succ('ayx')  # 'ayy'
succ('lman') # 'lmao'