Construct a dictionary out of an iterable, using an attribute name as
    the key. Optionally provide a filter function, to determine what should be
    kept in the dictionary.

def from_keyed_iterable(iterable, key, filter_func=None):
    """Construct a dictionary out of an iterable, using an attribute name as
    the key. Optionally provide a filter function, to determine what should be
    kept in the dictionary."""

    generated = {}

    for element in iterable:
        try:
            k = getattr(element, key)
        except AttributeError:
            raise RuntimeError("{} does not have the keyed attribute: {}".format(
                element, key
            ))

        if filter_func is None or filter_func(element):
            if k in generated:
                generated[k] += [element]
            else:
                generated[k] = [element]

    return generated