8000 nth() mixes column order · Issue #20760 · pandas-dev/pandas · GitHub
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sursu opened this issue Apr 20, 2018 · 1 comment · Fixed by #22811
Closed

nth() mixes column order #20760

sursu opened this issue Apr 20, 2018 · 1 comment · Fixed by #22811
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Bug Groupby Reshaping Concat, Merge/Join, Stack/Unstack, Explode
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0.24.0

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@sursu
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sursu commented Apr 20, 2018

Consider the following dataframe:

df = pd.DataFrame([[179293473,'2016-06-01 00:00:03.549745','http://www.dr.dk/nyheder/',39169523],[179293473,'2016-06-01 00:04:22.346018','https://www.information.dk/indland/2016/05/hvert-tredje-offer-naar-anmelde-voldtaegt-tide', 39125224],
 [179773461, '2016-06-01 22:13:16.588146', 'https://www.google.dk', 31658124],
 [179773461, '2016-06-01 22:14:04.059781', 'https://www.google.dk', 31658124],
 [179773461, '2016-06-01 22:16:37.230587', np.nan, 31658124],
 [179773461, '2016-06-01 22:23:09.847149', 'https://www.google.dk', 32718401],
 [179773461, '2016-06-01 22:23:55.158929', np.nan, 32718401],
 [179773461, '2016-06-01 22:27:00.857224', np.nan, 32718401]],
columns=['SessionID', 'PageTime', 'ReferrerURL', 'PageID'])

which looks like this:

 SessionID PageTime ReferrerURL PageID
179293473 2016-06-01 00:00:03.549745 http://www.dr.dk/nyheder/ 39169523
179293473 2016-06-01 00:04:22.346018 https://www.information.dk/ 39125224
179773461 2016-06-01 22:13:16.588146 https://www.google.dk 31658124
179773461 2016-06-01 22:14:04.059781 https://www.google.dk 31658124
179773461 2016-06-01 22:16:37.230587 NaN 31658124
179773461 2016-06-01 22:23:09.847149 https://www.google.dk 32718401
179773461 2016-06-01 22:23:55.158929 NaN 32718401
179773461 2016-06-01 22:27:00.857224 NaN 32718401

Run:
df.groupby('SessionID').nth(-1)

Out:

 SessionID PageID PageTime ReferrerURL
179293473 39125224 2016-06-01 00:04:22.346018 https://www.information.dk/
179773461 32718401 2016-06-01 22:27:00.857224 NaN

Question: Why has nth() mixed the order of my columns?
@ron819
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ron819 commented Aug 9, 2018

@mroeschke @jreback
Not sure if this is a bug but it would be nice if the nth() won't change the orders of the columns.

@reidy-p reidy-p mentioned this issue Sep 23, 2018
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@jreback jreback added this to the 0.24.0 milestone Sep 23, 2018
@jreback jreback added the Reshaping Concat, Merge/Join, Stack/Unstack, Explode label Sep 23, 2018
@wence- wence- mentioned this issue Nov 21, 2023
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BUG: Maintain column order with groupby.nth reidy-p/pandas
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@jreback @sursu @ron819
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