def get_extinct_coeff(filter_name):
    """
    Given a filter name, return an extinction coefficient. Numbers are approximate.
    V-band value is used if the filter name is 'none'

    :type filter_name: str
    :param filter_name: Filter name string
    :return: Extinction coefficient

    >>> get_extinct_coeff("none")
    0.2
    >>> get_extinct_coeff("U")
    0.6
    >>> get_extinct_coeff("other")
    Traceback (most recent call last):
        ...
    ValueError: Bad filter name: other
    """
    coeff_values = {"none": 0.2, "U": 0.6, "B": 0.4, "V": 0.2, "R": 0.1, "I": 0.08}
    try:
        return coeff_values[filter_name]
    except KeyError:
        raise ValueError("Bad filter name: {filter_name}".format(filter_name=filter_name))