import pandas as pd
import numpy as np
df1 = pd.DataFrame({'key1' : np.arange(10), 'value1' : np.random.randn(10)})
df2 = pd.DataFrame({'key1' : np.arange(10), 'value1' : np.random.randn(10)})
df3 = pd.DataFrame({'key2' : np.arange(10), 'value2' : np.random.randn(10)})