Adding two pandas dataframes

How about x.add(y, fill_value=0)?

import pandas as pd

df1 = pd.DataFrame([(1,2),(3,4),(5,6)], columns=['a','b'])
Out: 
   a  b
0  1  2
1  3  4
2  5  6

df2 = pd.DataFrame([(100,200),(300,400),(500,600)], columns=['a','b'])
Out: 
     a    b
0  100  200
1  300  400
2  500  600

df_add = df1.add(df2, fill_value=0)
Out: 
     a    b
0  101  202
1  303  404
2  505  606

If I understand you correctly, you want something like:

(x.reindex_like(y).fillna(0) + y.fillna(0).fillna(0))

This will give the sum of the two dataframes. If a value is in one dataframe and not the other, the result at that position will be that existing value (look at B0 in X and B0 in Y and look at final output). If a value is missing in both dataframes, the result at that position will be zero (look at B1 in X and B1 in Y and look at final output).

>>> x
   A   B   C
0  1   2 NaN
1  3 NaN   4
>>> y
    A   B   C
0   8 NaN  88
1   2 NaN   5
2  10  11  12
>>> (x.reindex_like(y).fillna(0) + y.fillna(0).fillna(0))
    A   B   C
0   9   2  88
1   5   0   9
2  10  11  12

For making more general the answer... first I will take the common index for synchronizing both dataframes, then I will join each of them to my pattern (dates) and I will sum the columns of the same name and finally join both dataframes (deleting added columns in one of them),

you can see an example (with google's stock prices taken from google) here:

import numpy as np
import pandas as pd
import datetime as dt

prices = pd.DataFrame([[553.0, 555.5, 549.3, 554.11, 0],
                       [556.8, 556.8, 544.05, 545.92, 545.92],
                       [545.5, 546.89, 540.97, 542.04, 542.04]],
                       index=[dt.datetime(2014,11,04), dt.datetime(2014,11,05), dt.datetime(2014,11,06)],
                       columns=['Open', 'High', 'Low', 'Close', 'Adj Close'])

corrections = pd.DataFrame([[0, 555.22], [1238900, 0]],
                    index=[dt.datetime(2014,11,3), dt.datetime(2014,11,4)],
                    columns=['Volume', 'Adj Close'])

dates = pd.DataFrame(prices.index, columns = ['Dates']).append(pd.DataFrame(corrections.index, columns = ['Dates'])).drop_duplicates('Dates').set_index('Dates').sort(axis=0)
df_corrections = dates.join(corrections).fillna(0)
df_prices = dates.join(prices).fillna(0)

for col in prices.columns:
    if col in corrections.columns:
        df_prices[col]+=df_corrections[col]
        del df_corrections[col]

df_prices = df_prices.join(df_corrections)

Both the above answers - fillna(0) and a direct addition would give you Nan values if either of them have different structures.

Its Better to use fill_value

df.add(other_df, fill_value=0)

Adding two pandas dataframes

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