A = pd.DataFrame([[1, 5, 2], [2, 4, 4], [3, 3, 1], [4, 2, 2], [5, 1, 4]],
    columns=['A', 'B', 'C'], index=[1, 2, 3, 4, 5])
for col1, col2 in IT.combinations(A.columns, 2):

    def tau(idx):
        pass
Using rolling_apply with a function that requires 2 arguments in Pandas