-- nanobind-3.x compatibility patch from https://github.com/project-gemmi/gemmi/commit/97c808222f468f8188f2ed87266e0d7c5a854ce2

diff --git CMakeLists.txt CMakeLists.txt
index 54e1a4fd6..fbb0a27d3 100644
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -31,6 +31,8 @@ option(GENERATE_STUBS "Generate Python type stubs" ON)
 option(EXTRA_WARNINGS "Set extra warning flags" OFF)
 option(USE_WMAIN "(Windows only) take Unicode arguments in gemmi program" ON)
 option(STANDALONE_PYTHON_MODULE "Avoid linking Python module to libgemmi_cpp DLL" ON)
+option(ENABLE_SPLIT_MODE "Use nanobind split mode (BACKEND_MODULE)" OFF)
+option(ENABLE_FREE_THREADED "Enable free-threaded Python support" OFF)
 if (WIN32)
   set(GEMMI_INSTALL_CMAKEDIR "cmake" CACHE STRING
       "Install path for gemmi CMake files")
@@ -472,10 +474,20 @@ if (USE_PYTHON)
       COMMAND "${Python_EXECUTABLE}" -m nanobind --cmake_dir
       OUTPUT_STRIP_TRAILING_WHITESPACE OUTPUT_VARIABLE nb_cmake_dir)
     list(APPEND CMAKE_PREFIX_PATH "${nb_cmake_dir}")
-    find_package(nanobind 2.4.0 CONFIG REQUIRED)
+    find_package(nanobind CONFIG REQUIRED)
+    if (nanobind_VERSION VERSION_LESS "2.4.0" OR nanobind_VERSION VERSION_GREATER_EQUAL "4.0.0")
+      message(FATAL_ERROR "nanobind version must be >= 2.4.0 and < 4.0.0 (found ${nanobind_VERSION})")
+    endif()
     message(STATUS "Found nanobind ${nanobind_VERSION}: ${NB_DIR}")
   endif()
-  nanobind_add_module(gemmi_py NOMINSIZE
+  set(gemmi_py_flags NOMINSIZE)
+  if (ENABLE_SPLIT_MODE)
+    list(APPEND gemmi_py_flags BACKEND_MODULE nanobind_backend)
+  endif()
+  if (ENABLE_FREE_THREADED)
+    list(APPEND gemmi_py_flags FREE_THREADED)
+  endif()
+  nanobind_add_module(gemmi_py ${gemmi_py_flags}
           python/gemmi.cpp python/align.cpp
           python/ccp4.cpp python/chemcomp.cpp python/cif.cpp
           python/elem.cpp python/grid.cpp python/hkl.cpp
diff --git python/hkl.cpp python/hkl.cpp
index f991f90d5..91d725e48 100644
--- python/hkl.cpp
+++ python/hkl.cpp
@@ -318,7 +318,7 @@ void add_hkl(nb::module_& m) {
         self.add_if_valid({h(i, 0), h(i, 1), h(i, 2)}, 0, 0, v(i), s(i));
       self.type = DataType::Unmerged;
       self.switch_to_asu_indices();
-    }, nb::arg("cell"), nb::arg("sg").none(false),
+    }, nb::arg("cell"), nb::arg("sg"),
        nb::arg("miller_array"), nb::arg("value_array"), nb::arg("sigma_array"))
     ;
 
diff --git python/make_iterator.h python/make_iterator.h
index bbb5c4fcf..f800754ad 100644
--- python/make_iterator.h
+++ python/make_iterator.h
@@ -4,7 +4,18 @@
 // This wrapper simplifies the call to nb::make_iterator (by assuming
 // the name "iterator") and changes the default rv_policy to what it was
 // in pybind11 and nanobind<2.0.
-template<nb::rv_policy Policy = nb::rv_policy::reference_internal, typename S, typename T>
+#if defined(NB_VERSION_MAJOR) && NB_VERSION_MAJOR >= 3
+template <typename S, typename T>
+auto usual_iterator(const S&, T& value) {
+  return nb::make_iterator<nb::rv_policy::reference_internal>(nb::type<S>(), "iterator", value);
+}
+template <auto Policy, typename S, typename T>
+auto usual_iterator(const S&, T& value) {
+  return nb::make_iterator<Policy>(nb::type<S>(), "iterator", value);
+}
+#else
+template <nb::rv_policy Policy = nb::rv_policy::reference_internal, typename S, typename T>
 auto usual_iterator(const S&, T& value) {
   return nb::make_iterator<Policy>(nb::type<S>(), "iterator", value);
 }
+#endif
diff --git python/mol.cpp python/mol.cpp
index 56eb7dfec..7108965cc 100644
--- python/mol.cpp
+++ python/mol.cpp
@@ -36,7 +36,11 @@ struct returns_references {
     if (!nb::isinstance<nb::sequence>(ret))
       throw std::runtime_error("return value should be a sequence");
     for (nb::handle nurse : ret)
+#if defined(NB_VERSION_MAJOR) && NB_VERSION_MAJOR >= 3
+      nb::keep_alive_obj(nurse, args[0]);
+#else
       nb::detail::keep_alive(nurse.ptr(), args[0]);
+#endif
   }
 };
 
diff --git python/scaling.cpp python/scaling.cpp
index 5bc3690c7..050351b35 100644
--- python/scaling.cpp
+++ python/scaling.cpp
@@ -22,7 +22,7 @@ void add_scaling(nb::module_& m) {
     .def_prop_rw("parameters", &Scaling::get_parameters,
                   (void (Scaling::*)(const std::vector<double>&)) &Scaling::set_parameters)
     .def("prepare_points", &Scaling::prepare_points,
-         nb::arg("calc"), nb::arg("obs"), nb::arg("mask")=static_cast<FPhiData*>(nullptr))
+         nb::arg("calc"), nb::arg("obs"), nb::arg("mask")=nb::none())
     .def("fit_isotropic_b_approximately", &Scaling::fit_isotropic_b_approximately)
     .def("fit_b_star_approximately", &Scaling::fit_b_star_approximately)
     .def("fit_parameters", &Scaling::fit_parameters)
@@ -36,7 +36,7 @@ void add_scaling(nb::module_& m) {
     })
     .def("get_solvent_scale", &Scaling::get_solvent_scale, nb::arg("stol2"))
     .def("scale_data", &Scaling::scale_data,
-         nb::arg("asu_data"), nb::arg("mask_data")=static_cast<FPhiData*>(nullptr))
+         nb::arg("asu_data"), nb::arg("mask_data")=nb::none())
     .def("scale_value", &Scaling::scale_value,
          nb::arg("hkl"), nb::arg("f_value"), nb::arg("mask_value"))
     ;