Leaky global scope in Python - not only an issue in older versions?

[dsuedholt]

Hi! While using nanobind, I ran into pretty much the exact issue previously described here.

Just to provide an as-minimal-as-possible example, using the following bindings in C++:

#include <nanobind/nanobind.h>
#include <nanobind/stl/shared_ptr.h>
namespace nb = nanobind;

struct Foo {};
struct Bar {
    std::shared_ptr<Foo> foo;
};

NB_MODULE(nanobind_example_ext, m) {
    nb::class_<Foo>(m, "Foo").def(nb::init<>());
    nb::class_<Bar>(m, "Bar")
        .def(nb::init<>())
        .def_rw("foo", &Bar::foo);
}

and calling them from a Python script like so:

from nanobind_example import Foo, Bar

b = Bar()
b.foo = Foo()

will trigger nanobind's memory leak warnings:

nanobind: leaked 1 instances!
nanobind: leaked 1 types!
 - leaked type "nanobind_example.nanobind_example_ext.Foo"
nanobind: leaked 1 functions!
 - leaked function "__init__"
nanobind: this is likely caused by a reference counting issue in the binding code.

As per @wjakob's reply in that thread, simply moving the Python snippet from the global scope into a function makes the warnings disappear. So does adding an explicit del b statement to the end of the script.

Both that reply and the FAQ section in the docs mention that this is a problem with older python versions. However, the example above produces the same warnings under current versions as well (I tested 3.11, 3.12, and 3.13).

I'm wondering if the FAQ bullet point "Older Python Versions" should be changed to something like "References in the global scope" instead? I could have probably saved some time and frustration if I had tried the workaround of avoiding global scope immediately, but I initially dismissed the "older version" potential reason since I was using 3.13.

Additionally, would it be fair to say that for a library of bindings distributed as a package, disabling nanobind's leak warnings entirely is almost mandatory at the moment? Otherwise everyone who installs the package and uses it in the global scope, or simply in a REPL, would be shown the warnings.

[wjakob]

Hi @dsuedholt,

can you check again with the latest master version including commit fb81576? Please note that you additionally need tp_traverse style callbacks if your objects have self-references (or more complex self-references involving type objects).

[dsuedholt]

This seems to have fixed it, no more leak warnings when using that commit. Thank you so much for the quick response and fix @wjakob!

[wjakob]

Great, thanks for confirming.