Join an empty pandas DataFrame with a Multiindex DataFrame

Question

I want to build a large pandas DataFrame in a loop. In the first iteration the DataFrame df1 is still empty. When I join df1 with df2 that has a MultiIndex, the Index gets squashed somehow.

df1 = pd.DataFrame(index=range(6))
df2 = pd.DataFrame(np.random.randn(6, 3),
                   columns=pd.MultiIndex.from_arrays((['A','A','A'], 
                                                      ['a', 'b', 'c'])))
df1[df2.columns] = df2

df1
     (A, a)    (A, b)    (A, c)
0 -0.673923  1.392369  1.848935
1  1.427368  0.042691  0.130962
2 -0.258589  0.216157  0.196101
3 -1.022283  1.312113 -0.770108
4  0.511127 -0.633477 -0.229149
5 -1.364237  0.713107  2.124274

I was hoping for a DataFrame with the MultiIndex intact like this:

          A                    
          a         b         c
0 -0.673923  1.392369  1.848935
1  1.427368  0.042691  0.130962
2 -0.258589  0.216157  0.196101
3 -1.022283  1.312113 -0.770108
4  0.511127 -0.633477 -0.229149
5 -1.364237  0.713107  2.124274

What am I doing wrong?

Answer 1

The multiple index will not always recognized when we do assign for a simple index , so

df1 = pd.DataFrame(index=range(6),columns=pd.MultiIndex.from_arrays([[],[]]))
df1[df2.columns] = df2
df1
Out[697]: 
          A                    
          a         b         c
0 -0.755397  0.574920  0.901570
1 -0.165472 -1.865715  1.583416
2 -0.403287  1.358329  0.706650
3  0.028019  1.432543 -0.586325
4 -0.414851  0.825253  0.745090
5  0.389917  0.940657  0.125837