df = pd.DataFrame({'year': [2013, 2014, 2014, 2015, 2013, 2013], 'class': [
    'a', 'b', 'b', 'b', 'b', 'a'], 'value': [13, 25, 32, 12, 23, 54]})
df
Filtering rows from dataframe on multiple conditions, getting ValueError