# substring_swap.py

# substring_swap.py

def subswap(t,a,z):
    if '{}' not in t:
        for x in a,z: t = t.replace(x,'{}')
        return t.format(z,a)
0
s =  "123 AAA 456 ZZZ 789 (from AAA-to-ZZZ)"
r = subswap(s,' AAA ',' ZZZ ')

print r