human
Here's a good function header given the following:
I wrote the below code

    def np_mean(x):
        return np.mean(x)
    def np_sum(x):
        return np.sum(x)
    def np_add(x):
        return np.add(x)
    t1 = T1[('a', 'c', 'd')]
    tg = t1.group_by('a')
    tga_r = tg.groups.aggregate(np.sum)
    tga_a = tg.groups.aggregate(np.add)
    tga_n = tg.groups.aggregate(np_sum)
    assert np.all((tga_r == tga_n))
    assert np.all((tga_a == tga_n))
    assert (tga_n.pformat() == ['    a            c                d    ', '---    ----    ---', '        0        0.0            4', '        1        6.0        18', '        2    22.0            6'])
    tga_r = tg.groups.aggregate(np.mean)
    tga_n = tg.groups.aggregate(np_mean)
    assert np.all((tga_r == tga_n))
    assert (tga_n.pformat() == ['    a            c            d    ', '---    ---    ---', '        0    0.0    4.0', '        1    2.0    6.0', '        2    5.5    1.5'])
    t2 = T1[('a', 'c')]
    tg = t2.group_by('a')
    with catch_warnings(Warning) as warning_lines:
        tga = tg.groups.aggregate(np_add)
        assert (warning_lines[0].category == AstropyUserWarning)
        assert ('Cannot    aggregate    column' in str(warning_lines[0].message))
    assert (tga.pformat() == ['    a    ', '---', '        0', '        1', '        2'])

What's a good function header?
assistant
#Here's a good function header.

def test_table_aggregate_reduceat(T1):