human
Coalesce values from 2 columns into a single column in a pandas dataframe
assistant
Here is some code to combine values from column 'b' and column 'a' of dataframe `df`  into column 'c' of datafram `df`

df['c'] = np.where(df['a'].isnull, df['b'], df['a'])

#Warning: this code generation is experimental. Please inspect the code for any bugs before executing.