def METHOD_NAME(self):
    dct = dict(a=dict(a1=1, a2=2), b=dict(b1=dict(b11=11, b12=dict(b121=121))))
    self.assertDictEqual(
        {"a": {"a1": 1, "a2": 2}, "b": {"b1": {"b11": 11, "b12": {"b121": 400}}}},
        set_nested_value(dct, 400, ("b", "b1", "b12", "b121")),
    )