def make_feature_label(row,polarity_attr='polarity',mz_attr='mz',rt_attr='rt_peak'):
    """
    For consistency these are my preferred way to do this:
    * polarity: a "+" or "-" sign
    * mz: float (typically mz_centroid)
    * rt: float (typically rt_peak)

    Returns:
    * string that looks like "+1234.1234@1.23" for
    positive mode, m/z=1234.1234, & rt=1.23
    """
    return '%s%.4f@%.2f'%(row[polarity_attr],row[mz_attr],row[rt_attr])