Pandas overlay one dataframe on another

Question

I have two data frames which have identical columns and the exact same number of rows

Data frame 1 has the original data and data frame 2 contains the fields which have changed (changed fields contain the new value, unchanged fields are nan).

I want to essentially overlay the "new data" in df 2 over the data in df 1 and get the result below.

df1:

Key   a    b

123   6    1
124   7    6
125   3    5

df2:

Key   a    b

123   nan  nan
124   8    nan
125   nan  4

Result df:

Key   a    b

123   6    1
124   8    6
125   3    4

Answer 1

You need combine_first:

df2.combine_first(df1)

Output:

Key    a    b       
123  6.0  1.0
124  8.0  6.0
125  3.0  4.0