updating row data within apply from other rows doesn't work

[lamakaha]

I'm grouping rows by column investor id. within apply, when a group has more than 1 row, i'm copying a value in the second row in the group into the a column of the first row in that group. I'm using groupby with apply but the assembled result is incorrect - all rows except row 1 show the same investor B - seems like a bug?

In [1]: import pandas as pd
In [2]: import numpy as np

In [3]: df=pd.DataFrame({'InvestorID':[6,6,17,17,19,19,30,40],
   ...:                  'Investor':['AAA','AAA','BBB','BBB','CCC','CCC','EEE','FFF'],
   ...:                  'CurrentPeriodEnd':['7/31/2014','5/31/2014','7/31/2014','5/31/2014','7/31/2014','5/31/2014','7/31/2014','7/31/2014'],
   ...:                  'PriorPeriodEnd':np.nan*8})

In [4]: df
Out[4]: 
  CurrentPeriodEnd Investor  InvestorID  PriorPeriodEnd
0        7/31/2014      AAA           6             NaN
1        5/31/2014      AAA           6             NaN
2        7/31/2014      BBB          17             NaN
3        5/31/2014      BBB          17             NaN
4        7/31/2014      CCC          19             NaN
5        5/31/2014      CCC          19             NaN
6        7/31/2014      EEE          30             NaN
7        7/31/2014      FFF          40             NaN

this function updates the first positional row in a group with values from the second row
where a group has 2 rows or more

In [5]: def f(x):
   ...:     if len(x) >1:
   ...:         x.iloc[0,x.columns.get_loc('PriorPeriodEnd')]=x.iloc[1,x.columns.get_loc('CurrentPeriodEnd')] 
   ...:     return x.iloc[0]

In [6]: df.groupby(['InvestorID']).apply(f).reset_index(drop=True)

this is not an expected result - the investor column should not have repeating BBB values

Out[6]: 
  CurrentPeriodEnd Investor  InvestorID PriorPeriodEnd
0        7/31/2014      AAA           6      5/31/2014
1        7/31/2014      BBB          17      5/31/2014
2        7/31/2014      BBB          19      5/31/2014
3        7/31/2014      BBB          30      5/31/2014
4        7/31/2014      BBB          40      5/31/2014

the function below copies the first row first before making updates and it works as expected

In [7]: def f(x):
   ...:     top_row=pd.Series(x.iloc[0])
   ...:     if len(x) >1:
   ...:         top_row.iloc[x.columns.get_loc('PriorPeriodEnd')]=x.iloc[1,x.columns.get_loc('CurrentPeriodEnd')] 
   ...:     return top_row

this is what the result should look like

In [8]: df.groupby(['InvestorID']).apply(f).reset_index(drop=True)
Out[8]: 
  CurrentPeriodEnd Investor  InvestorID PriorPeriodEnd
0        7/31/2014      AAA           6      5/31/2014
1        7/31/2014      BBB          17      5/31/2014
2        7/31/2014      CCC          19      5/31/2014
3        7/31/2014      EEE          30            NaN
4        7/31/2014      FFF          40            NaN

[jreback]

Not really sure what you are trying to do. This will get you started.

In [52]: dfs = df.sort(['InvestorID','CurrentPeriodEnd'],ascending=[1, 0])

In [53]: dfs
Out[53]: 
  CurrentPeriodEnd Investor  InvestorID  PriorPeriodEnd
0        7/31/2014      AAA           6             NaN
1        5/31/2014      AAA           6             NaN
2        7/31/2014      BBB          17             NaN
3        5/31/2014      BBB          17             NaN
4        7/31/2014      CCC          19             NaN
5        5/31/2014      CCC          19             NaN
6        7/31/2014      EEE          30             NaN
7        7/31/2014      FFF          40             NaN

In [54]: indexer = dfs.groupby(['Investor']).head(1).index

In [55]: dfs.groupby(['Investor']).head(1)      
Out[55]: 
  CurrentPeriodEnd Investor  InvestorID  PriorPeriodEnd
0        7/31/2014      AAA           6             NaN
2        7/31/2014      BBB          17             NaN
4        7/31/2014      CCC          19             NaN
6        7/31/2014      EEE          30             NaN
7        7/31/2014      FFF          40             NaN

In [56]: indexer = dfs.groupby(['Investor']).head(1).index

In [57]: dfs.loc[indexer,'PriorPeriodEnd'] = dfs.CurrentPeriodEnd

In [58]: dfs
Out[58]: 
  CurrentPeriodEnd Investor  InvestorID PriorPeriodEnd
0        7/31/2014      AAA           6      7/31/2014
1        5/31/2014      AAA           6            NaN
2        7/31/2014      BBB          17      7/31/2014
3        5/31/2014      BBB          17            NaN
4        7/31/2014      CCC          19      7/31/2014
5        5/31/2014      CCC          19            NaN
6        7/31/2014      EEE          30      7/31/2014
7        7/31/2014      FFF          40      7/31/2014

Your function doesn't work because you have duplicates in x so the indexing of the column returns returns multiple values.

[lamakaha]

when i change my function not to update back the first row but rather a copy of the first row it works as expected but shouldn't it work even when i update the first row directly in apply?

def f(x):
top_row=pd.Series(x.iloc[0])
if len(x) >1:
top_row.iloc[x.columns.get_loc('PriorPeriodEnd')]=x.iloc[1,x.columns.get_loc('CurrentPeriodEnd')]
return top_row

CurrentPeriodEnd Investor InvestorID PriorPeriodEnd
0 7/31/2014 AAA 6 5/31/2014
1 7/31/2014 BBB 17 5/31/2014
2 7/31/2014 CCC 19 5/31/2014
3 7/31/2014 EEE 30 NaN
4 7/31/2014 FFF 40 NaN

[jreback]

their is some detection logic to see if you have mutated the return from groupby-apply. in general this is not a good (and is completely a non-performant soln). This might be an edge case, but since you have duplicates and you are not handling them this is not a bug.

You don't want to mutate things, its bad practice and can lead to very subtle issues.

[jreback]

tell you what. I'll mark it as a bug. can you update the top of the issue with 2 examples both copy-pastable that should yield the same results?

and if you want to look at it would be great! (those 2 examples are basically the tests). I won't have time for a while.

[lamakaha]

thanks for looking into this. I have updated the top comment with both versions of the function - one which mutates and doesn't work and the other one which copies and works. I still don't understand where the duplicates come into play? Once the dataframe is split into groups by investor id, I don't have any duplicates in x - neither indexes nor columns. I'm using positional reference iloc to set values of the first row with values of the second row. Also I debugged this code and it picks up singular values (in my case dates) both from the assignor and assignee (once the assignment of the value is made). I also used the print statements within the apply function and everything is printing as expected. If you could clarify or demonstrate where I have duplicates in x, it would be great

[jreback]

on 2nd thought, I think this is almost the same bug as #8467 (but in a different case). So should be straightforward to fix (I think).

want to give a shot?

basically step thru the code with the working one, and note what is happening (in the reassembly portion).
Then do the same for the non-working. It is probably something simple. If its not or you get stuck, lmk.

[rhshadrach]

Closing in favor of #12653