How to check if a value in one dataframe is present in keys in the other dataframe

Question

I have two dataframes:

df_1:

         Letters Boolean
          a         Nan
          b         Nan
          c         Nan

df_2:

Answer 1

Use numpy.where with isin:

df1['Boolean'] = np.where(df1['Letters'].isin(df2.columns), 'x', np.nan)