Python/Pandas - Combine two columns with NaN values

Question

Lets say i have a dataframe with two columns: OldValue and NewValue

i want to do some plotting and i want to combine values. If OldValue is empty, NewValue is filled, and the other way around. There is not a single instance both or neither are filled. Lets say my dataframe looks like this:

  OldValue NewValue
0  14.0     NaN 
1  NaN     7.0
2  3.0     NaN
3  NaN     3.0 

I found that i could fillna(0) and then do something along the lines of

df["AllValue"] = df["NewValue"] + df["OldValue"]

Is this the most efficient way?

Answer 1

Your solution should be change with Series.add and fill_value=0:

df["AllValues"] = df["Newvalues"].add(df["OldValues"], fill_value=0)

Or use Series.fillna:

df["AllValue"] = df["NewValue"].fillna(df["OldValue"])

Answer 2

I think this will work:

df["AllValue"] = df["OldValue"]  # make a copy
df.loc[df["AllValue"].isna(), "AllValue"] = df.loc[~df["NewValue"].isna(), "NewValue"]