def alpha(string):
    """Returns a string only containing letters.

    Args:
        string: String containing all kinds of characters.

    Returns:
        The string without any non-alpha characters."""
    return ''.join([x for x in string if x in 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'])