How to get the first group under certain level of a groupby of multiple columns?

user_13618007 0
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I am interested in the first group in level 2 and want to get all the rows related to it.

Take a look at the example below:

col1  col2  col3   col4
 1     34   green   10
            yellow  20
            orange  30
       89   green   40
            yellow  50
            orange  60
 2     89   green   15
            yellow  25
            orange  35
       90   green   45
            yellow  55
            orange  65

Please note that the length of row for each level 2 group is not definitely 3.

Now I want to get all the first group under col2, then result is supposed to be:

col1  col2  col3   col4
 1     34   green   10
            yellow  20
            orange  30
 2     89   green   15
            yellow  25
            orange  35

The example and problem are modified from the question: How to get the first group in a groupby of multiple columns?

I have tried the get_group method but it seems not able to address this specific question.

I am wondering if there is any one-line code could solve this kind of question? Thx!

pandas-groupby multi-index pandas
2 Answers

There's a quick stack/unstack solution:

df.unstack('col3').groupby(level=0).head(1).stack('col3')

Output:

                col4
col1 col2 col3      
1    34   g       10
          o       30
          y       20
2    89   g       15
          o       35
          y       25
user_42384080

We can do 

df.groupby(level=[0,2]).head(1)
Out[342]: 
                  col4
col1 col2 col3        
1    34   green     10
          yellow    20
          orange    30
2    89   green     15
          yellow    25
          orange    35
user_79645270
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