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

    f = get_ipython().display_formatter
    catcher = []
    class SelfDisplaying(object, ):
        def _ipython_display_(self):
            catcher.append(self)
    class NotSelfDisplaying(object, ):
        def __repr__(self):
            return 'NotSelfDisplaying'
        def _ipython_display_(self):
            raise NotImplementedError
    save_enabled = f.ipython_display_formatter.enabled
    f.ipython_display_formatter.enabled = True
    yes = SelfDisplaying()
    no = NotSelfDisplaying()
    (d, md) = f.format(no)
    nt.assert_equal(d, {'text/plain': repr(no)})
    nt.assert_equal(md, {})
    nt.assert_equal(catcher, [])
    (d, md) = f.format(yes)
    nt.assert_equal(d, {})
    nt.assert_equal(md, {})
    nt.assert_equal(catcher, [yes])
    f.ipython_display_formatter.enabled = save_enabled

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

def test_ipython_display_formatter():