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How to append a column from each dataframe in a dictionary, to another dataframe in another dictionary?

June 7, 2022

Here’s the problematic code:

import pandas as pd

here I create a sample src dictionary containing 3 dataframes:

src = {}
for i in range(1,4):
    src[i] = pd.DataFrame({'a':[i, 2*i, 3*i], 'b':[10*i, 20*i, 30*i], 'c':[100*i, 200*i, 300*i]})
    display(src[i])

here are 3 dataframes created in src dictionary:

    a   b   c
0   1   10  100
1   2   20  200
2   3   30  300
    a   b   c
0   2   20  200
1   4   40  400
2   6   60  600
    a   b   c
0   3   30  300
1   6   60  600
2   9   90  900

here I want to append a column from each dataframe in src dictionary to a dataframe in output dictionary, and b column to b dataframe.

output = {}
for i in src:
    output['a'] = output['a'].concat([output[i]['a'], src[i][a]], axis = 1)
    output['b'] = output['b'].concat([output[i]['b'], src[i][b]], axis = 1)

I got this error message:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Input In [30], in <cell line: 11>()
     10 # retrieve a column from all source dataframes, put them in a new dataframe. and these new dataframes are in a new dictionary.
     11 for i in src:
---> 12     output['a'] = output['a'].concat([output[i]['a'], src[i][a]], axis = 1)
     13     output['b'] = output['b'].concat([output[i]['b'], src[i][b]], axis = 1)

KeyError: 'a'

How can I fix it?

>Solution :

The problem is that on the first loop you do not have a key called 'a' (at this point output is an empty dictionary) – so define the keys at the definition of output – don’t make it an empty dictionary