Pandas: How to add column to multiindexed dataframe?

Change:

flights_unstack = flights_indexed.unstack()

to:

flights_unstack = flights_indexed['passengers'].unstack()

for remove Multiindex in columns.

---

And last is necessary add_categories by new column name:

flights_unstack.columns = flights_unstack.columns.add_categories(['total'])

flights_unstack['total'] = flights_unstack.sum(axis = 1)
print (df)
       January  February  March  April  May  June  July  August  September  \
month                                                                        
1949       112       118    132    129  121   135   148     148        136   
1950       115       126    141    135  125   149   170     170        158   
1951       145       150    178    163  172   178   199     199        184   
1952       171       180    193    181  183   218   230     242        209   
1953       196       196    236    235  229   243   264     272        237   
1954       204       188    235    227  234   264   302     293        259   
1955       242       233    267    269  270   315   364     347        312   
1956       284       277    317    313  318   374   413     405        355   
1957       315       301    356    348  355   422   465     467        404   
1958       340       318    362    348  363   435   491     505        404   
1959       360       342    406    396  420   472   548     559        463   
1960       417       391    419    461  472   535   622     606        508   

       October  November  December  total  
month                                      
1949       119       104       118   1520  
1950       133       114       140   1676  
1951       162       146       166   2042  
1952       191       172       194   2364  
1953       211       180       201   2700  
1954       229       203       229   2867  
1955       274       237       278   3408  
1956       306       305       336   4003  
1957       347       310       337   4427  
1958       359       362       405   4692  
1959       407       362       405   5140  
1960       461       390       432   5714  

Setup:

import pandas as pd

temp=u"""month;January;February;March;April;May;June;July;August;September;October;November;December

1949;112;118;132;129;121;135;148;148;136;119;104;118
1950;115;126;141;135;125;149;170;170;158;133;114;140
1951;145;150;178;163;172;178;199;199;184;162;146;166
1952;171;180;193;181;183;218;230;242;209;191;172;194
1953;196;196;236;235;229;243;264;272;237;211;180;201
1954;204;188;235;227;234;264;302;293;259;229;203;229
1955;242;233;267;269;270;315;364;347;312;274;237;278
1956;284;277;317;313;318;374;413;405;355;306;305;336
1957;315;301;356;348;355;422;465;467;404;347;310;337
1958;340;318;362;348;363;435;491;505;404;359;362;405
1959;360;342;406;396;420;472;548;559;463;407;362;405
1960;417;391;419;461;472;535;622;606;508;461;390;432"""
#after testing replace 'pd.compat.StringIO(temp)' to 'filename.csv'
df = pd.read_csv(pd.compat.StringIO(temp), sep=";", index_col=[0])
print (df)


df.columns = pd.CategoricalIndex(df.columns)

df.columns = df.columns.add_categories(['total'])
df['total'] = df.sum(axis = 1)

I know this is kind of late but I found the answer to your problem in the FAQs section of the course. Here's what it says:

"Q. What are the issues with Pandas categorical data?

A. Since version 0.6, seaborn.load_dataset converts certain columns to Pandas categorical data (see http://pandas.pydata.org/pandas-docs/stable/categorical.html). This creates a problem in the handling of the "flights" DataFrame used in "Introduction to Pandas/Using multilevel indices". To avoid the problem, you may load the dataset directly with Pandas:

flights = pd.read_csv('https://raw.githubusercontent.com/mwaskom/seaborn-data/master/flights.csv')"

I hope this helps.