pandas sort_values is working outside a for loop, but not inside?

Question

I am trying to write a for loop that will take a dataframe with census data, count the populations of the three largest counties of each state, and write the sum to a new series. Here is the function that is not working:

import numpy as np
import pandas as pd

##created a dataframe earlier with a census csv file called 'census_df'


def bad_function():
    only_counties = census_df.set_index(['STNAME'])

    ser = pd.Series(index = only_counties.index)
    ser = ser.index.drop_duplicates() ##get a unique list of all 50 states from the dataframe

    state_name = pd.Series(index = ser)


    for i in state_name.index:
        a = only_counties.loc[i, 'CENSUS2010POP']
        a = a.sort_values(ascending=False)

        population = np.sum(a[0:3])

        state_name.loc[i] = population

    return state_name

When I call this function, I get the following error:

AttributeError                            Traceback (most recent call last)
<ipython-input-59-dc2686648261> in <module>()
     26     return state_name
     27 
---> 28 answer_six()

<ipython-input-59-dc2686648261> in answer_six()
     18     for i in state_name.index:
     19         a = only_counties.loc[i, 'CENSUS2010POP']
---> 20         a = a.sort_values(ascending=False)
     21 
     22         population = np.sum(a[0:3])

AttributeError: 'numpy.int64' object has no attribute 'sort_values'

HOWEVER, when I ditched the loop for testing purposes and selected one item('Alabama') from the index of what I am trying to iterate over, and use the same sort_values method in the same way, it works just fine. Like this:

def bad_function():
    only_counties = census_df.set_index(['STNAME'])

    ser = pd.Series(index = only_counties.index)
    ser = ser.index.drop_duplicates()

    state_name = pd.Series(index = ser)

    a = only_counties.loc['Alabama', 'CENSUS2010POP']
    a = a.sort_values(ascending=False)

    b = np.sum(a[0:3])

    return a, b

It returns exactly what I want, which is a: a list of counties in the state sorted by population and b: the sum of the three highest population counties. So what is happening?

Answer 1

Are you the following:

for i in state_name.index:
    print (I)

prints the state names or index?