def values_of(choices):
    """
    Returns a tuple of values from choices options represented as a tuple of
    tuples (value, label). For example:

    .. sourcecode:: python

        >>> values_of((
        ...         ('1', 'One'),
        ...         ('2', 'Two'),))
        ('1', '2')

    :rtype: tuple
    """
    return tuple([value for value, label in choices])