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

You can use the combine function.

import pandas as pd

# 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(16).reshape(4,4)*2, columns='A B C D'.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))

Out[248]: 
                      A   B   C   D
2015-07-09 12:00:00 NaN NaN NaN NaN
2015-07-09 13:00:00 NaN NaN NaN NaN
2015-07-09 14:00:00   2   2   2   2
2015-07-09 15:00:00   2   2   2   2
2015-07-09 16:00:00   2   2   2   2
2015-07-09 17:00:00   2   2   2   2

combiner = lambda x, y: np.where(x.isnull(), y, x)

# use df1 to update result
result.combine(df1, combiner)

Out[249]: 
                     A  B  C   D
2015-07-09 12:00:00  1  1  1 NaN
2015-07-09 13:00:00  1  1  1 NaN
2015-07-09 14:00:00  2  2  2   2
2015-07-09 15:00:00  2  2  2   2
2015-07-09 16:00:00  2  2  2   2
2015-07-09 17:00:00  2  2  2   2

# maybe fillna(method='ffill') if you like