'Helper method to get the operators from the URL query
@param get_vars: the GET vars (a dict)
@param selector: field selector
@return: query operator - None, str or list'

@classmethod
def _operator(cls, get_vars, selector):

    variables = [('%s__%s' % (selector, op)) for op in cls.alternatives]
    slen = (len(selector) + 2)
    operators = [k[slen:] for (k, v) in get_vars.iteritems() if (k in variables)]
    if (not operators):
        return None
    elif (len(operators) == 1):
        return operators[0]
    else:
        return operators
