'Convert a M:SS or numeric string to a float.
Return None if `s` is empty.
Raise an InvalidQueryError if the string cannot be converted.'

def _convert(self, s):

    if (not s):
        return None
    try:
        return util.raw_seconds_short(s)
    except ValueError:
        try:
            return float(s)
        except ValueError:
            raise InvalidQueryArgumentValueError(s, u'a    M:SS    string    or    a    float')
