Using unstack with Pandas - python-3.x

Question

I am getting an exception when applying unstack, and would like to understand it.
For a reproducible example:
(to load the data: pd.DataFrame(json.loads(titanic)))
titanic
'{"home.dest":{"0":"St Louis, MO","1":"Montreal, PQ \\/ Chesterville, ON","2":"Montreal, PQ \\/ Chesterville, ON","3":"Montreal, PQ \\/ Chesterville, ON","4":"Montreal, PQ \\/ Chesterville, ON","5":"New York, NY","6":"Hudson, NY","7":"Belfast, NI","8":"Bayside, Queens, NY","9":"Montevideo, Uruguay","10":"New York, NY","11":"New York, NY","12":"Paris, France","13":null,"14":"Hessle, Yorks","15":"New York, NY","16":"Montreal, PQ","17":"Montreal, PQ","18":null,"19":"Winnipeg, MN"},"pclass":{"0":1,"1":1,"2":1,"3":1,"4":1,"5":1,"6":1,"7":1,"8":1,"9":1,"10":1,"11":1,"12":1,"13":1,"14":1,"15":1,"16":1,"17":1,"18":1,"19":1},"survived":{"0":1,"1":1,"2":0,"3":0,"4":0,"5":1,"6":1,"7":0,"8":1,"9":0,"10":0,"11":1,"12":1,"13":1,"14":1,"15":0,"16":0,"17":1,"18":1,"19":0},"name":{"0":"Allen, Miss. Elisabeth Walton","1":"Allison, Master. Hudson Trevor","2":"Allison, Miss. Helen Loraine","3":"Allison, Mr. Hudson Joshua Creighton","4":"Allison, Mrs. Hudson J C (Bessie Waldo Daniels)","5":"Anderson, Mr. Harry","6":"Andrews, Miss. Kornelia Theodosia","7":"Andrews, Mr. Thomas Jr","8":"Appleton, Mrs. Edward Dale (Charlotte Lamson)","9":"Artagaveytia, Mr. Ramon","10":"Astor, Col. John Jacob","11":"Astor, Mrs. John Jacob (Madeleine Talmadge Force)","12":"Aubart, Mme. Leontine Pauline","13":"Barber, Miss. Ellen \\"Nellie\\"","14":"Barkworth, Mr. Algernon Henry Wilson","15":"Baumann, Mr. John D","16":"Baxter, Mr. Quigg Edmond","17":"Baxter, Mrs. James (Helene DeLaudeniere Chaput)","18":"Bazzani, Miss. Albina","19":"Beattie, Mr. Thomson"},"sex":{"0":"female","1":"male","2":"female","3":"male","4":"female","5":"male","6":"female","7":"male","8":"female","9":"male","10":"male","11":"female","12":"female","13":"female","14":"male","15":"male","16":"male","17":"female","18":"female","19":"male"},"age":{"0":29.0,"1":0.92,"2":2.0,"3":30.0,"4":25.0,"5":48.0,"6":63.0,"7":39.0,"8":53.0,"9":71.0,"10":47.0,"11":18.0,"12":24.0,"13":26.0,"14":80.0,"15":null,"16":24.0,"17":50.0,"18":32.0,"19":36.0},"sibsp":{"0":0,"1":1,"2":1,"3":1,"4":1,"5":0,"6":1,"7":0,"8":2,"9":0,"10":1,"11":1,"12":0,"13":0,"14":0,"15":0,"16":0,"17":0,"18":0,"19":0},"parch":{"0":0,"1":2,"2":2,"3":2,"4":2,"5":0,"6":0,"7":0,"8":0,"9":0,"10":0,"11":0,"12":0,"13":0,"14":0,"15":0,"16":1,"17":1,"18":0,"19":0},"ticket":{"0":"24160","1":"113781","2":"113781","3":"113781","4":"113781","5":"19952","6":"13502","7":"112050","8":"11769","9":"PC 17609","10":"PC 17757","11":"PC 17757","12":"PC 17477","13":"19877","14":"27042","15":"PC 17318","16":"PC 17558","17":"PC 17558","18":"11813","19":"13050"},"fare":{"0":211.3375,"1":151.55,"2":151.55,"3":151.55,"4":151.55,"5":26.55,"6":77.9583,"7":0.0,"8":51.4792,"9":49.5042,"10":227.525,"11":227.525,"12":69.3,"13":78.85,"14":30.0,"15":25.925,"16":247.5208,"17":247.5208,"18":76.2917,"19":75.2417},"cabin":{"0":"B5","1":"C22 C26","2":"C22 C26","3":"C22 C26","4":"C22 C26","5":"E12","6":"D7","7":"A36","8":"C101","9":null,"10":"C62 C64","11":"C62 C64","12":"B35","13":null,"14":"A23","15":null,"16":"B58 B60","17":"B58 B60","18":"D15","19":"C6"},"embarked":{"0":"S","1":"S","2":"S","3":"S","4":"S","5":"S","6":"S","7":"S","8":"S","9":"C","10":"C","11":"C","12":"C","13":"S","14":"S","15":"S","16":"C","17":"C","18":"C","19":"C"},"boat":{"0":"2","1":"11","2":null,"3":null,"4":null,"5":"3","6":"10","7":null,"8":"D","9":null,"10":null,"11":"4","12":"9","13":"6","14":"B","15":null,"16":null,"17":"6","18":"8","19":"A"},"body":{"0":null,"1":null,"2":null,"3":135.0,"4":null,"5":null,"6":null,"7":null,"8":null,"9":22.0,"10":124.0,"11":null,"12":null,"13":null,"14":null,"15":null,"16":null,"17":null,"18":null,"19":null}}'
I create a multi index with the following command:
titanic = titanic.set_index(['name', 'home.dest'])
Then I want to unstack.
titanic.unstack(level = 'home.dest')
I get the following exception message:
ValueError: Index contains duplicate entries, cannot reshape

Answer 1

The error is saying that your choice of columns in which you built the MultiIndex is not unique and therefore it has problems unstacking because there are ambiguities.
One way to fix this is to guarantee the uniqueness by adding a counter.
counts = titanic.gropuby(['name', 'home.dest']).cumcount().rename('Counter')
titanic = titanic.set_index(['name', 'home.dest', counts])
Then your unstack will work
titanic.unstack(level = 'home.dest')
But I'd advise maybe
titanic.unstack(['home.dest', 'Counter'])
Otherwise, you'll have to aggregate with a groupby
titanic.groupby(['name', 'home.dest']).first().unstack()