Setting DataFrame values with enlargement

Question

I have two DataFrames (with DatetimeIndex) and want to update the first frame (the older one) with data from the second frame (the newer one).

Answer 1

In addition to previous answer, after reindexing you can use

result.fillna(df1, inplace=True)

so based on Jianxun Li's code (extended with one more column) you can try this

# your data
# ===========================================================
df1 = pd.DataFrame(np.ones(12).reshape(4,3), columns='A B C'.split(), index=pd.date_range('2015-07-09 12:00:00', periods=4, freq='H'))
df2 = pd.DataFrame(np.ones(20).reshape(4,5)*2, columns='A B C D E'.split(), index=pd.date_range('2015-07-09 14:00:00', periods=4, freq='H'))

# processing
# =====================================================
# reindex to populate NaN
result = df2.reindex(np.union1d(df1.index, df2.index))
# fill NaN from df1
result.fillna(df1, inplace=True)

Out[3]:             
                     A  B  C   D   E
2015-07-09 12:00:00  1  1  1 NaN NaN
2015-07-09 13:00:00  1  1  1 NaN NaN
2015-07-09 14:00:00  2  2  2   2   2
2015-07-09 15:00:00  2  2  2   2   2
2015-07-09 16:00:00  2  2  2   2   2
2015-07-09 17:00:00  2  2  2   2   2