def extract_attr(param, field):
    """
    Extract a particular field from a parameter object

    Parameters
    ----------
    param : parameter object
       Various parameter objects defined by Google messages.
    field : string
       Parameter field.

    Returns
    -------
    string or None
        Field value or None if parameter or field doesn't exist.

    """
    tmpval = getattr(param, field, None)
    if tmpval is not None:
        return tmpval
    else:
        return None
