Class MyContainer

tests/test_traitlets.py:2766–2774

    # observe with names=custom container with iter, contains
    class MyContainer:
        def __init__(self, container):
            self.container = container

        def __iter__(self):
            return iter(self.container)

        def __contains__(self, key):
            return key in self.container

    c.observe(record, names=MyContainer({"i", "s"}))
    c.i = 10