Python/Pandas - Query a MultiIndex Column

Question

I'm trying to use query on a MultiIndex column. It works on a MultiIndex row, but not the column. Is there a reason for this? The documentation shows examples like the first one below, but it doesn't indicate that it won't work for a MultiIndex column.

I know there are other ways to do this, but I'm specifically trying to do it with the query function

import numpy as np
import pandas as pd

df = pd.DataFrame(np.random.random((4,4)))
df.index = pd.MultiIndex.from_product([[1,2],['A','B']])
df.index.names = ['RowInd1', 'RowInd2']
# This works
print(df.query('RowInd2 in ["A"]'))

df = pd.DataFrame(np.random.random((4,4)))
df.columns = pd.MultiIndex.from_product([[1,2],['A','B']])
df.columns.names = ['ColInd1', 'ColInd2']
# query on index works, but not on the multiindexed column
print(df.query('index < 2'))
print(df.query('ColInd2 in ["A"]'))

Have you read this [answer](https://stackoverflow.com/a/37644056/4819376)? — rpanai, Jul 24 '18 at 16:20
Yes, I know there is xs and other methods, as I indicated in my question in bold. I'm specifically looking for a way to do this with the query function. The answer might be that it isn't possible. I thought that maybe there is a slightly different syntax that can be used to query with multiindex columns. — Scott B, Jul 24 '18 at 16:44

Scott B · Accepted Answer · 2018-07-24T18:24:21.890

1

To answer my own question, it looks like query shouldn't be used at all (regardless of using MultiIndex columns) for selecting certain columns, based on the answer(s) here:

Select columns using pandas dataframe.query()

edited Jul 24 '18 at 18:24

answered Jul 24 '18 at 18:12

Scott B

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score 0 · Answer 2 · answered Jul 24 '18 at 16:10

0

You can using IndexSlice

df.query('ilevel_0>2')
Out[327]: 
ColInd1         1                  2          
ColInd2         A         B        A         B
3        0.652576  0.639522  0.52087  0.446931
df.loc[:,pd.IndexSlice[:,'A']]
Out[328]: 
ColInd1         1         2
ColInd2         A         A
0        0.092394  0.427668
1        0.326748  0.383632
2        0.717328  0.354294
3        0.652576  0.520870

answered Jul 24 '18 at 16:10

BENY

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I am aware of this, but I'm specifically looking for a way to do this with the query function. The answer might be that it isn't possible, but I thought that maybe there is a slightly different syntax that can be used to query with multiindex columns. – Scott B Jul 24 '18 at 16:46

Python/Pandas - Query a MultiIndex Column

2 Answers2