Build ExpresionAttributeValues from a value or kwargs

def build_expression_values(dynamizer, expr_values, kwargs):
    """ Build ExpresionAttributeValues from a value or kwargs """
    if expr_values:
        values = expr_values
        return dynamizer.encode_keys(values)
    elif kwargs:
        values = dict(((':' + k, v) for k, v in six.iteritems(kwargs)))
        return dynamizer.encode_keys(values)