Segfault when indexing nested pybind11::list in C++

[ascended121]

In a test (implemented in GTest), I'm trying to index a nested list, like so:

pybind11::list list;
pybind11::list sublist;
sublist.append(1);
sublist.append(2);
sublist.append(3);
list.append(sublist);

list[0][0];

The indexing operation segfaults with the following backtrace:

#0  __strlen_evex () at ../sysdeps/x86_64/multiarch/strlen-evex.S:77
#1  0x00007ffff7b41f7d in PyUnicode_FromString () from /lib/x86_64-linux-gnu/libpython3.10.so.1.0
#2  0x00005555555b8ef4 in pybind11::str::str (this=0x7fffffffd828, c=0x0) at .venv/include/pybind11/pytypes.h:1542
#3  0x00005555555dd7af in pybind11::detail::object_api<pybind11::detail::accessor<pybind11::detail::accessor_policies::list_item> >::operator[] (this=0x7fffffffd8b0, key=0x0)
    at .venv/include/pybind11/pytypes.h:2456
#4  0x00005555555aecd2 in CommonWrap_TypeToPyListMatrix_Test::TestBody (this=0x5555557fd650) at py_wrap_utils/tests/test_py_wrap_util.cpp:149
#5  0x000055555564bbfb in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) ()
#6  0x0000555555643b23 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) ()
#7  0x000055555561d794 in testing::Test::Run() ()
#8  0x000055555561e2af in testing::TestInfo::Run() ()
#9  0x000055555561ecb5 in testing::TestSuite::Run() ()
#10 0x000055555562f1d8 in testing::internal::UnitTestImpl::RunAllTests() ()

Am I doing something wrong here? Or is this a bug in pybind11?

[evguran]

Operator [] return list_accessor which has no [] operator.

[ascended121]

If the [] operator doesn't exist on a list_accessor, then shouldn't that have been a compiler error?

How do I retrieve the elements from a list_accessor? I don't need all the elements from the list, so I don't want to iterate over it.

[evguran]

Actually, it has.. I was wrong
The code below prints 2:

py::list lst;
py::list sublst;
sublst.append(1);
sublst.append(2);
sublst.append(3);
lst.append(sublst);
auto aa = lst[(py::int_)0][(py::int_)1];
std::cout << aa.attr("__str__")().cast<std::string>() << std::endl;

See this for more details

[ascended121]

Ah, so the key is you have to use python ints for the indices. Thanks!