if 'MICHAEL89'.casefold() in (name.casefold() for name in USERNAMES): Or: if 'MICHAEL89'.casefold() in map(str.casefold, USERNAMES): As per the docs: Casefolding is similar to lowercasing but more aggressive because it is intended to remove all case distinctions in a string. For example, the German lowercase letter 'ß' is equivalent to "ss". Since it is already lowercase, lower() would do nothing to 'ß'; casefold() converts it to "ss".