Fill missing values in a subset of columns

Using pandas the missing values in a data frame df can be replaced (with e.g. 0) in place using

df.fillna(0, inplace=True)

To replace missing values only in a subset of columns (e.g. col 1 and col 3), use

df.loc[:,["col 1", "col 3"]] = df.loc[:,["col 1", "col 3"]].fillna(0)