'Deserialize to a UsageKey instance: for now it\'s a location missing the run'

def to_python(self, location):

    assert isinstance(location, (NoneType, basestring, UsageKey))
    if (location == ''):
        return None
    if isinstance(location, basestring):
        location = super(UsageKeyField, self).to_python(location)
        return Location.from_deprecated_string(location)
    else:
        return location
