@classmethod
def from_defaults(cls, ctx, **defaults):
    """Creates context `ctx` introducing only new keys given in `defaults`.

        The normal behavior when you enter a new level in the context is to
        override the values with the new one.

        Example:

           >>> with context.from_defaults('A', a=1):
           ...    with context.from_defaults('A', a=2, b=1) as c:
           ...        assert c['a'] == 1

        """
    return cls.from_dicts(ctx, defaults=defaults)
