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)

Answer 2

You need :

df1['Boolean']=df1.Letters.isin(df2.columns).map({True:'x',False:np.nan})
print(df1)

  Letters Boolean
0       a       x
1       b       x
2       c     NaN