Generate all combinations of a string, uppercase and lowercase.

combinations.py

def recurse (pref,suff):
    # If no characters left, just print prefix.

    if suff == "":
            print pref
            return

    # Otherwise add lowercase of first suffix letter to prefix
    # and recur with that and the remainder of the suffix.
    # Then do the same for uppercase.
    # If you wanted smarts, this is where you'd detect if the
    # upper and lower were the same and only recurse once.

    recurse (pref + suff[0:1].lower(), suff[1:])
    recurse (pref + suff[0:1].upper(), suff[1:])

# Test the function with "macOS".

recurse ("","macos")