Return the n-th (n = ``depth``) context containing attribute named ``name``.

def get_containing(self, name, depth = 0):
        """Return the n-th (n = ``depth``) context containing attribute named ``name``."""
        ctx_dict = object.__getattribute__(self, '__dict__')
        if name in ctx_dict:
            if depth <= 0:
                return self
            depth -= 1
        parent = ctx_dict.get('_parent')
        if parent is None:
            return None
        return parent.get_containing(name, depth = depth)