accessor: Overload operators to allow in-place assignments on m_base

[nicholasjng]

This enables users to write syntactic sugar for in-place ops as

nb::object obj = ...;
// increment a (mutable) counter value inline.
obj.attr("count") += nb::int(1);

This is useful in bindings and wrappers because it avoids the duplication of obj.attr() calls. Templating the operator declaration allows their use in all access implementations.

---

Hey! This is a change I used for a small personal project lately. We previously discussed this briefly in #688 (comment), and I'd be happy about a review.

If you are interested, I will add more tests on other Impl specializations. Thanks!

[wjakob]

This looks good. I can't think of any substantially better way of implementing it. Can you increase test coverage? It would also be good to cover the other accessors (e.g. dict key-based or index-based indexing).

[nicholasjng]

I added tests for str_item, num_item_list, and obj_item - I could not think of a test for num_item, since num_item_list is the specialization for list, and I don't know of other mutable sequence types.

Note that nb::dict with int keys does not work, because it is interpreted as a sequence:

// test_accessor.cpp
    m.def("test_num_item_accessor_inplace_mutation", []() {
        nb::dict d;
        d[0] = nb::int_(0);
        d[0] += nb::int_(1);
        return d;
    });

// test_accessor.py
def test_05_num_item_inplace_mutation():
    """
    Similar to test 01, but tests obj[n] (indexed access)
    on the C++ side.
    """
    d = t.test_num_item_accessor_inplace_mutation()
    assert len(d) == 1
    assert d.keys() == {0}
    assert d[0] == 1  # dict lookup

[...]

// raises: >      d = t.test_num_item_accessor_inplace_mutation()
//                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
//         E       TypeError: dict is not a sequence

And I did not test num_item_tuple, since you protected its setter with a compile-time assert, see:

nanobind/include/nanobind/nb_accessor.h

Lines 177 to 180 in 879bca4

	template <typename...Ts> static void set(Ts...) {
	static_assert(false_v<Ts...>, "tuples are immutable!");
	}
	};

[wjakob]

Thanks!