In [1]: import pandas as pd

In [2]: left = pd.DataFrame(dict(b=[None]), dtype=float).set_index(['b']).index

In [3]: right = pd.DataFrame(dict(b=[None]), dtype=float).set_index(['b']).index

In [4]: sum(left.isin(right))
Out[4]: 1

In [5]: len(left.intersection(right))
Out[5]: 1

In [6]: left = pd.DataFrame(dict(a=[1], b=[None]), dtype=float).set_index(['a', 'b']).index

In [7]: right = pd.DataFrame(dict(a=[1], b=[None]), dtype=float).set_index(['a', 'b']).index

In [8]: sum(left.isin(right))
Out[8]: 0

In [9]: len(left.intersection(right))
Out[9]: 1