def construct_format_field_str(fname, fspec, conv):
    """
    Constructs a format field string from the field name, spec, and
    conversion character (``fname``, ``fspec``, ``conv``). See Python
    String Formatting for more info.
    """
    if fname is None:
        return ""
    ret = "{" + fname
    if conv:
        ret += "!" + conv
    if fspec:
        ret += ":" + fspec
    ret += "}"
    return ret