'Return the inventory record from the list of Inventory records that
matches the supplied resource class, or None.
:param res_class: An integer or string representing a resource
class. If the value is a string, the method first
looks up the resource class identifier from the
string.'

def find(self, res_class):

    if (not isinstance(res_class, six.string_types)):
        raise ValueError
    for inv_rec in self.objects:
        if (inv_rec.resource_class == res_class):
            return inv_rec
