def convert(s):
r, i = '', 0
for c in s:
if c.isalpha():
r += [str.upper, str.lower](c)
i = (i + 1) % 2
else: r += c
return r
print(convert(input())) А можно сократить до такого: def convert(s, upper = True):
return ''.join([str.upper, str.lower][upper := upper != c.isalpha()](c) for c in s)
print(convert(input()))