right = pd.DataFrame({'key': ['K0', 'K1', 'K2', 'K3'], 'C': ['C0', 'C1',
    'C2', 'C3'], 'D': ['D0', 'D1', 'D2', 'D3']})
result = pd.merge(left, right, on='key')
print(result)
How to merge two pandas dataframe in parallel (multithreading or multiprocessing)