double square brackets not required

[iamhassaan]

We do not need the double square brackets. I have replicated this function and can see similar results of: .join(df2[[df2_key, df2_value]], df1[df1_key] == df2[df2_key], 'left') and .join(df2[df2_key, df2_value], df1[df1_key] == df2[df2_key], 'left').