[python] Turn Pandas Multi-Index into column

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.

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.

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()

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.

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.