python - After groupby, how to flatten column headers?

Question

I'm trying to left join multiple pandas dataframes on a single Id column, but when I attempt the merge I get warning:

KeyError: 'Id'.

I think it might be because my dataframes have offset columns resulting from a groupby statement, but I could very well be wrong. Either way I can't figure out how to "unstack" my dataframe column headers. None of the answers at this question seem to work.

My groupby code:

step1 = pd.DataFrame(step3.groupby(['Id', 'interestingtabsplittest2__grp'])['applications'].sum())
step1.sort('applications', ascending=False).head(3)

Returns:

How to get those offset headers into the top level?

question from:https://stackoverflow.com/questions/33004573/after-groupby-how-to-flatten-column-headers

Answer 1

You're looking for .reset_index().

In [11]: df = pd.DataFrame([[2, 3], [5, 6]], pd.Index([1, 4], name="A"), columns=["B", "C"])

In [12]: df
Out[12]:
   B  C
A
1  2  3
4  5  6

In [13]: df.reset_index()
Out[13]:
   A  B  C
0  1  2  3
1  4  5  6

---

Note: That you can avoid this step by using as_index=False when doing the groupby.

step1 = step3.groupby(['Id', 'interestingtabsplittest2__grp'], as_index=False)['applications'].sum()