def get_a_by_b(adata, a, b, norm=False):
    """
    Get A x B.
    ----------------
    Number of each .obs b per .obs a
        returns pd.Dataframe
    """
    hm = adata.obs.groupby([a,b]).size().reset_index().set_index(a).pivot(columns=b)

    if norm:
        hm = hm.div(hm.sum(1), 0)

    hm.columns = hm.columns.droplevel()
    hm.columns.name = None
    return hm