Turn Pandas Multi-Index into column

Question

I have a dataframe with 2 index levels:

                         value
Trial    measurement
    1              0        13
                   1         3
                   2         4
    2              0       NaN
                   1        12
    3              0        34

Which I want to turn into this:

Trial    measurement       value

    1              0        13
    1              1         3
    1              2         4
    2              0       NaN
    2              1        12
    3              0        34

How can I best do this?

I need this because I want to aggregate the data as instructed here, but I can't select my columns like that if they are in use as indices.

Duplicate: http://stackoverflow.com/questions/18624039/pandas-reset-index-on-series-to-remove-multiindex You want the first suggestion. `.reset_index()` — TomAugspurger, Nov 21 '13 at 01:51
many thanks, I actually browsed around for this a lot, but "make multiindex to column" and similar queries always got me threads which wanted to pivot their dataframes... — TheChymera, Nov 21 '13 at 03:49

score 318 · Accepted Answer · edited Oct 07 '21 at 09:56

318

The reset_index() is a pandas DataFrame method that will transfer index values into the DataFrame as columns. The default setting for the parameter is drop=False (which will keep the index values as columns).

All you have to do call .reset_index() after the name of the DataFrame:

df = df.reset_index()

edited Oct 07 '21 at 09:56

cs95

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answered Sep 08 '14 at 21:42

CraigSF

3,212
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For my case where I had 3 index levels inplace reset did not work. Alternative is assigning newly resetted dataframe to a new one: df2 = df.reset_index() – Gorkem Mar 15 '18 at 13:30
24

To reset only a particular level(s), use `df.reset_index(level=[...])` – cs95 Jan 24 '19 at 10:54
1

Or the side-effect (probably quicker) way: `df.reset_index(inplace=True)` – Owen Jun 21 '22 at 06:36
1

`df.reset_index(names=['a', 'b'])` to provide names/alternative names to the produced columns. – kva1966 May 19 '23 at 07:00

Karl Anka · Answer 2 · 2020-02-03T07:37:47.587

This doesn't really apply to your case but could be helpful for others (like myself 5 minutes ago) to know. If one's multindex have the same name like this:

                         value
Trial        Trial
    1              0        13
                   1         3
                   2         4
    2              0       NaN
                   1        12
    3              0        34

df.reset_index(inplace=True) will fail, cause the columns that are created cannot have the same names.

So then you need to rename the multindex with df.index = df.index.set_names(['Trial', 'measurement']) to get:

                           value
Trial    measurement       

    1              0        13
    1              1         3
    1              2         4
    2              0       NaN
    2              1        12
    3              0        34

And then df.reset_index(inplace=True) will work like a charm.

I encountered this problem after grouping by year and month on a datetime-column(not index) called live_date, which meant that both year and month were named live_date.

How to have your Trial values to repeat themselves? I had the same problem and it works except my values don't repeat themselves. — Rich, Aug 17 '18 at 18:57

score 25 · Answer 3 · answered Oct 27 '20 at 08:39

25

There may be situations when df.reset_index() cannot be used (e.g., when you need the index, too). In this case, use index.get_level_values() to access index values directly:

df['Trial'] = df.index.get_level_values(0)
df['measurement'] = df.index.get_level_values(1)

This will assign index values to individual columns and keep the index.

See the docs for further info.

answered Oct 27 '20 at 08:39

Alex

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This is soooooooooo useful! It should be possible to do this using much clearer language, e.g. `df['measurement'] = df.index.values(1)`. – Zizzipupp Sep 17 '21 at 13:45

score 19 · Answer 4 · answered May 20 '19 at 00:06

19

As @cs95 mentioned in a comment, to drop only one level, use:

df.reset_index(level=[...])

This avoids having to redefine your desired index after reset.

answered May 20 '19 at 00:06

sameagol

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score 5 · Answer 5 · answered Sep 28 '20 at 16:37

5

I ran into Karl's issue as well. I just found myself renaming the aggregated column then resetting the index.

df = pd.DataFrame(df.groupby(['arms', 'success'])['success'].sum()).rename(columns={'success':'sum'})

df = df.reset_index()

answered Sep 28 '20 at 16:37

kevin_theinfinityfund

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score 2 · Answer 6 · answered May 16 '22 at 13:42

2

Short and simple

df2 = pd.DataFrame({'test_col': df['test_col'].describe()})
df2 = df2.reset_index()

answered May 16 '22 at 13:42

whitetiger1399

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Andrew · Answer 7 · 2023-08-14T11:18:44.043

Similar to Alex solution in a more generalized form. It keeps the indexes untouched and adds the index levels as new columns with its name.

for i in df.index.names:
    df[i] = df.index.get_level_values(i)

which gives the new columns 'Trial' and 'measurement'

                   value Trial    measurement
Trial measurement             
    1           0     13     1              0     
                1      3     1              1     
                2      4     1              2     
  ...

score 1 · Answer 8 · answered Jan 17 '23 at 13:07

1

A solution that might be helpful in cases when not every column has multiple index levels:

df.columns = df.columns.map(''.join)

answered Jan 17 '23 at 13:07

Rafal Plaza

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Turn Pandas Multi-Index into column

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