Using key while sorting values for just one column

Question

Lets say we have a df like below:

df = pd.DataFrame({'A':['y2','x3','z1','z1'],'B':['y2','x3','a2','z1']})

    A   B
0   y2  y2
1   x3  x3
2   z1  a2
3   z1  z1

if we wanted to sort the values on just the numbers in column A, we can do:

df.sort_values(by='A',key=lambda x: x.str[1])

    A   B
3   z1  z1
2   z1  a2
0   y2  y2
1   x3  x3

If we wanted to sort by both columns A and B, but have the key only apply to column A, is there a way to do that?

df.sort_values(by=['A','B'],key=lambda x: x.str[1])

Expected output:

The easiest way would be to generate an extra column/separate Series with the output of your key function — sholderbach
– sholderbach, Commented Feb 19, 2021 at 14:06

Quang Hoang · Accepted Answer · 2021-02-19 14:11:02Z

3

You can sort by B, then sort by A with a stable method:

(df.sort_values('B')
   .sort_values('A', key=lambda x: x.str[1], kind='mergesort')
)

Output:

    A   B
2  z1  a2
3  z1  z1
0  y2  y2
1  x3  x3

answered Feb 19, 2021 at 14:11

Quang Hoang

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