import re
some_text = 'xxxxxxx@yyyyyyyyy@zzzzzzzzz@'
substitutions = ['ONE', 'TWO', 'THREE']
pattern = re.compile('@')
x = pattern.sub(lambda i: substitutions.pop(0), some_text)
print x
aW1wb3J0IHJlCgpzb21lX3RleHQgPSAneHh4eHh4eEB5eXl5eXl5eXlAenp6enp6enp6QCcKc3Vic3RpdHV0aW9ucyA9IFsnT05FJywgJ1RXTycsICdUSFJFRSddCnBhdHRlcm4gPSByZS5jb21waWxlKCdAJykKeCA9IHBhdHRlcm4uc3ViKGxhbWJkYSBpOiBzdWJzdGl0dXRpb25zLnBvcCgwKSwgc29tZV90ZXh0KQoKcHJpbnQgeA==