def rank_obj_data(serData):
    """
    Parameters
    #---------
    serData    pandas.Series containing one Column from data set
    
    Returns
    #------
    serFirstTwentyData    pandas.Series with first twenty highest counts
        of classes from column.
    
    Description
    #----------
    Groupby classes in column. Count number per class. Sort values descending.
    Take only first twenty. Helper method for plotting only most important
    classes of a feature. Helps keeping overview.
    """
    serCount = serData.groupby(by = serData).count()
    serSortData = serCount.sort_values(ascending = False)
    serFirstTwentyData = serSortData[:20]
    return serFirstTwentyData