def is_letter(char_code):
    """Return True if char_code is a letter character code from the ASCII 
    table. Otherwise return False.
    """
    if isinstance(char_code, str) or isinstance(char_code, bytes):
        char_code = ord(char_code)

    if char_code >= 65 and char_code <= 90:  # uppercase letters
        return True

    if char_code >= 97 and char_code <= 122: # lowercase letters
        return True

    return False