Example of append, concat and combine_first:

import pandas as pd

a = {'A': 10, 'B': 20}
b = {'B': 30, 'C': 40, 'D': 50}

df1 = pd.DataFrame(a, index=[0])
df2 = pd.DataFrame(b, index=[1])

d1 = pd.DataFrame()
d1 = d1.append(df1)
d1 = d1.append(df2).fillna(0)
print("\n------------ append ----------------\n")
print(d1)

d2 = pd.concat([df1, df2]).fillna(0)
print("\n------------ concat ----------------\n")
print(d2)

d3 = pd.DataFrame()
d3 = d3.combine_first(df1).combine_first(df2).fillna(0)
print("\n------------ combine_first ----------------\n")
print(d3)

C:\pandas>python example37.py
 
------------ append ----------------
 
      A   B     C     D
0  10.0  20   0.0   0.0
1   0.0  30  40.0  50.0
 
------------ concat ----------------
 
      A   B     C     D
0  10.0  20   0.0   0.0
1   0.0  30  40.0  50.0
 
------------ combine_first ----------------
 
      A     B     C     D
0  10.0  20.0   0.0   0.0
1   0.0  30.0  40.0  50.0