Allow module-local classes to be loaded externally

The main point of `py::module_local` is to make the C++ -> Python cast
unique so that returning/casting a C++ instance is well-defined.
Unfortunately it also makes loading unique, but this isn't particularly
desirable: when an instance contains `Type` instance there's no reason
it shouldn't be possible to pass that instance to a bound function
taking a `Type` parameter, even if that function is in another module.

This commit solves the issue by allowing foreign module (and global)
type loaders have a chance to load the value if the local module loader
fails.  The implementation here does this by storing a module-local
loading function in a capsule in the python type, which we can then call
if the local (and possibly global, if the local type is masking a global
type) version doesn't work.

docs/advanced/classes.rst

@@ -775,27 +775,29 @@ Module-local class bindings
 
 When creating a binding for a class, pybind by default makes that binding
 "global" across modules.  What this means is that a type defined in one module
-can be passed to functions of other modules that expect the same C++ type.  For
+can be returned from any module resulting in the same Python type.  For
 example, this allows the following:
 
 .. code-block:: cpp
 
     // In the module1.cpp binding code for module1:
     py::class_<Pet>(m, "Pet")
-        .def(py::init<std::string>());
+        .def(py::init<std::string>())
+        .def_readonly("name", &Pet::name);
 
 .. code-block:: cpp
 
     // In the module2.cpp binding code for module2:
-    m.def("pet_name", [](Pet &p) { return p.name(); });
+    m.def("create_pet", [](std::string name) { return new Pet(name); });
 
 .. code-block:: pycon
 
     >>> from module1 import Pet
-    >>> from module2 import pet_name
-    >>> mypet = Pet("Kitty")
-    >>> pet_name(mypet)
-    'Kitty'
+    >>> from module2 import create_pet
+    >>> pet1 = Pet("Kitty")
+    >>> pet2 = create_pet("Doggy")
+    >>> pet2.name()
+    'Doggy'
 
 When writing binding code for a library, this is usually desirable: this
 allows, for example, splitting up a complex library into multiple Python
@@ -855,39 +857,45 @@ the ``py::class_`` constructor:
     py::class<pets::Pet>(m, "Pet", py::module_local())
         .def("get_name", &pets::Pet::name);
 
-This makes the Python-side ``dogs.Pet`` and ``cats.Pet`` into distinct classes
-that can only be accepted as ``Pet`` arguments within those classes.  This
-avoids the conflict and allows both modules to be loaded.
+This makes the Python-side ``dogs.Pet`` and ``cats.Pet`` into distinct classes,
+avoiding the conflict and allowing both modules to be loaded.  C++ code in the
+``dogs`` module that casts or returns a ``Pet`` instance will result in a
+``dogs.Pet`` Python instance, while C++ code in the ``cats`` module will result
+in a ``cats.Pet`` Python instance.
 
-One limitation of this approach is that because ``py::module_local`` types are
-distinct on the Python side, it is not possible to pass such a module-local
-type as a C++ ``Pet``-taking function outside that module.  For example, if the
-above ``cats`` and ``dogs`` module are each extended with a function:
+This does come with two caveats, however: First, external modules cannot return
+or cast a ``Pet`` instance to Python (unless they also provide their own local
+bindings).  Second, from the Python point of view they are two distinct classes.
+
+Note that the locality only applies in the C++ -> Python direction.  When
+passing such a ``py::module_local`` type into a C++ function, the module-local
+classes are still considered.  This means that if the following function is
+added to any module (including but not limited to the ``cats`` and ``dogs``
+modules above) it will be callable with either a ``dogs.Pet`` or ``cats.Pet``
+argument:
 
 .. code-block:: cpp
 
-    m.def("petname", [](pets::Pet &p) { return p.name(); });
+    m.def("pet_name", [](const pets::Pet &pet) { return pet.name(); });
 
-you will only be able to call the function with the local module's class:
+For example, suppose the above function is added to each of ``cats.cpp``,
+``dogs.cpp`` and ``frogs.cpp`` (where ``frogs.cpp`` is some other module that
+does *not* bind ``Pets`` at all).
 
 .. code-block:: pycon
 
-    >>> import cats, dogs  # No error because of the added py::module_local()
+    >>> import cats, dogs, frogs  # No error because of the added py::module_local()
     >>> mycat, mydog = cats.Cat("Fluffy"), dogs.Dog("Rover")
-    >>> (cats.petname(mycat), dogs.petname(mydog))
+    >>> (cats.pet_name(mycat), dogs.pet_name(mydog))
     ('Fluffy', 'Rover')
-    >>> cats.petname(mydog)
-    Traceback (most recent call last):
-      File "<stdin>", line 1, in <module>
-    TypeError: petname(): incompatible function arguments. The following argument types are supported:
-        1. (arg0: cats.Pet) -> str
+    >>> (cats.pet_name(mydog), dogs.pet_name(mycat), frogs.pet_name(mycat))
+    ('Rover', 'Fluffy', 'Fluffy')
 
-    Invoked with: <dogs.Dog object at 0x123>
-
-It is possible to use ``py::module_local()`` registrations in one module even if another module
-registers the same type globally: within the module with the module-local definition, all C++
-instances will be cast to the associated bound Python type.  Outside the module, any such values
-are converted to the global Python type created elsewhere.
+It is possible to use ``py::module_local()`` registrations in one module even
+if another module registers the same type globally: within the module with the
+module-local definition, all C++ instances will be cast to the associated bound
+Python type.  In other modules any such values are converted to the global
+Python type created elsewhere.
 
 .. note::
 

include/pybind11/cast.h

@@ -51,6 +51,7 @@ struct type_info {
     std::vector<bool (*)(PyObject *, void *&)> *direct_conversions;
     buffer_info *(*get_buffer)(PyObject *, void *) = nullptr;
     void *get_buffer_data = nullptr;
+    void *(*module_local_load)(PyObject *, const type_info *) = nullptr;
     /* A simple type never occurs as a (direct or indirect) parent
      * of a class that makes use of multiple inheritance */
     bool simple_type : 1;
@@ -265,23 +266,30 @@ PYBIND11_NOINLINE inline detail::type_info* get_type_info(PyTypeObject *type) {
     return bases.front();
 }
 
-/// Return the type info for a given C++ type; on lookup failure can either throw or return nullptr.
-/// `check_global_types` can be specified as `false` to only check types registered locally to the
-/// current module.
-PYBIND11_NOINLINE inline detail::type_info *get_type_info(const std::type_index &tp,
-                                                          bool throw_if_missing = false,
-                                                          bool check_global_types = true) {
-    std::type_index type_idx(tp);
+inline detail::type_info *get_local_type_info(const std::type_index &tp) {
     auto &locals = registered_local_types_cpp();
-    auto it = locals.find(type_idx);
+    auto it = locals.find(tp);
     if (it != locals.end())
         return (detail::type_info *) it->second;
-    if (check_global_types) {
-        auto &types = get_internals().registered_types_cpp;
-        it = types.find(type_idx);
-        if (it != types.end())
-            return (detail::type_info *) it->second;
-    }
+    return nullptr;
+}
+
+inline detail::type_info *get_global_type_info(const std::type_index &tp) {
+    auto &types = get_internals().registered_types_cpp;
+    auto it = types.find(tp);
+    if (it != types.end())
+        return (detail::type_info *) it->second;
+    return nullptr;
+}
+
+/// Return the type info for a given C++ type; on lookup failure can either throw or return nullptr.
+PYBIND11_NOINLINE inline detail::type_info *get_type_info(const std::type_index &tp,
+                                                          bool throw_if_missing = false) {
+    if (auto ltype = get_local_type_info(tp))
+        return ltype;
+    if (auto gtype = get_global_type_info(tp))
+        return gtype;
+
     if (throw_if_missing) {
         std::string tname = tp.name();
         detail::clean_type_id(tname);
@@ -578,6 +586,8 @@ public:
     PYBIND11_NOINLINE type_caster_generic(const std::type_info &type_info)
      : typeinfo(get_type_info(type_info)) { }
 
+    type_caster_generic(const type_info *typeinfo) : typeinfo(typeinfo) { }
+
     bool load(handle src, bool convert) {
         return load_impl<type_caster_generic>(src, convert);
     }
@@ -597,7 +607,7 @@ public:
         auto it_instances = get_internals().registered_instances.equal_range(src);
         for (auto it_i = it_instances.first; it_i != it_instances.second; ++it_i) {
             for (auto instance_type : detail::all_type_info(Py_TYPE(it_i->second))) {
-                if (instance_type && instance_type == tinfo)
+                if (instance_type && same_type(*instance_type->cpptype, *tinfo->cpptype))
                     return handle((PyObject *) it_i->second).inc_ref();
             }
         }
@@ -655,8 +665,6 @@ public:
         return inst.release();
     }
 
-protected:
-
     // Base methods for generic caster; there are overridden in copyable_holder_caster
     void load_value(value_and_holder &&v_h) {
         auto *&vptr = v_h.value_ptr();
@@ -686,13 +694,41 @@ protected:
     }
     void check_holder_compat() {}
 
+    PYBIND11_NOINLINE static void *local_load(PyObject *src, const type_info *ti) {
+        auto caster = type_caster_generic(ti);
+        if (caster.load(src, false))
+            return caster.value;
+        return nullptr;
+    }
+
+    /// Try to load with foreign typeinfo, if available. Used when there is no
+    /// native typeinfo, or when the native one wasn't able to produce a value.
+    PYBIND11_NOINLINE bool try_load_foreign_module_local(handle src) {
+        constexpr auto *local_key = "_pybind11_module_local_typeinfo";
+        const auto pytype = src.get_type();
+        if (!hasattr(pytype, local_key))
+            return false;
+
+        type_info *foreign_typeinfo = reinterpret_borrow<capsule>(getattr(pytype, local_key));
+        // Only consider this foreign loader if actually foreign and is a loader of the correct cpp type
+        if (foreign_typeinfo->module_local_load == &local_load
+                || !same_type(*typeinfo->cpptype, *foreign_typeinfo->cpptype))
+            return false;
+
+        if (auto result = foreign_typeinfo->module_local_load(src.ptr(), foreign_typeinfo)) {
+            value = result;
+            return true;
+        }
+        return false;
+    }
+
     // Implementation of `load`; this takes the type of `this` so that it can dispatch the relevant
     // bits of code between here and copyable_holder_caster where the two classes need different
     // logic (without having to resort to virtual inheritance).
     template <typename ThisT>
     PYBIND11_NOINLINE bool load_impl(handle src, bool convert) {
-        if (!src || !typeinfo)
-            return false;
+        if (!src) return false;
+        if (!typeinfo) return try_load_foreign_module_local(src);
         if (src.is_none()) {
             // Defer accepting None to other overloads (if we aren't in convert mode):
             if (!convert) return false;
@@ -757,7 +793,17 @@ protected:
             if (this_.try_direct_conversions(src))
                 return true;
         }
-        return false;
+
+        // Failed to match local typeinfo. Try again with global.
+        if (typeinfo->module_local) {
+            if (auto gtype = get_global_type_info(*typeinfo->cpptype)) {
+                typeinfo = gtype;
+                return load(src, false);
+            }
+        }
+
+        // Global typeinfo has precedence over foreign module_local
+        return try_load_foreign_module_local(src);
     }
 
 

include/pybind11/pybind11.h

@@ -829,7 +829,7 @@ protected:
             pybind11_fail("generic_type: cannot initialize type \"" + std::string(rec.name) +
                           "\": an object with that name is already defined");
 
-        if (get_type_info(*rec.type, false /* don't throw */, !rec.module_local))
+        if (rec.module_local ? get_local_type_info(*rec.type) : get_global_type_info(*rec.type))
             pybind11_fail("generic_type: type \"" + std::string(rec.name) +
                           "\" is already registered!");
 
@@ -866,6 +866,12 @@ protected:
             auto parent_tinfo = get_type_info((PyTypeObject *) rec.bases[0].ptr());
             tinfo->simple_ancestors = parent_tinfo->simple_ancestors;
         }
+
+        if (rec.module_local) {
+            // Stash the local typeinfo and loader so that external modules can access it.
+            tinfo->module_local_load = &type_caster_generic::local_load;
+            setattr(m_ptr, "_pybind11_module_local_typeinfo", capsule(tinfo));
+        }
     }
 
     /// Helper function which tags all parents of a type using mult. inheritance

tests/local_bindings.h

@@ -18,7 +18,7 @@ using NonLocal2 = LocalBase<2>;
 using LocalExternal = LocalBase<3>;
 /// Mixed: registered local first, then global
 using MixedLocalGlobal = LocalBase<4>;
-/// Mixed: global first, then local (which fails)
+/// Mixed: global first, then local
 using MixedGlobalLocal = LocalBase<5>;
 
 using LocalVec = std::vector<LocalType>;
@@ -29,6 +29,15 @@ using NonLocalVec2 = std::vector<NonLocal2>;
 using NonLocalMap = std::unordered_map<std::string, NonLocalType>;
 using NonLocalMap2 = std::unordered_map<std::string, uint8_t>;
 
+PYBIND11_MAKE_OPAQUE(LocalVec);
+PYBIND11_MAKE_OPAQUE(LocalVec2);
+PYBIND11_MAKE_OPAQUE(LocalMap);
+PYBIND11_MAKE_OPAQUE(NonLocalVec);
+//PYBIND11_MAKE_OPAQUE(NonLocalVec2); // same type as LocalVec2
+PYBIND11_MAKE_OPAQUE(NonLocalMap);
+PYBIND11_MAKE_OPAQUE(NonLocalMap2);
+
+
 // Simple bindings (used with the above):
 template <typename T, int Adjust, typename... Args>
 py::class_<T> bind_local(Args && ...args) {
@@ -36,3 +45,16 @@ py::class_<T> bind_local(Args && ...args) {
         .def(py::init<int>())
         .def("get", [](T &i) { return i.i + Adjust; });
 };
+
+// Simulate a foreign library base class (to match the example in the docs):
+namespace pets {
+class Pet {
+public:
+    Pet(std::string name) : name_(name) {}
+    std::string name_;
+    const std::string &name() { return name_; }
+};
+}
+
+struct MixGL { int i; MixGL(int i) : i{i} {} };
+struct MixGL2 { int i; MixGL2(int i) : i{i} {} };

tests/pybind11_cross_module_tests.cpp

@@ -87,4 +87,21 @@ PYBIND11_MODULE(pybind11_cross_module_tests, m) {
     m.def("load_vector_via_binding", [](std::vector<int> &v) {
         return std::accumulate(v.begin(), v.end(), 0);
     });
+
+    // test_cross_module_calls
+    m.def("return_self", [](LocalVec *v) { return v; });
+    m.def("return_copy", [](const LocalVec &v) { return LocalVec(v); });
+
+    class Dog : public pets::Pet { public: Dog(std::string name) : Pet(name) {}; };
+    py::class_<pets::Pet>(m, "Pet", py::module_local())
+        .def("name", &pets::Pet::name);
+    // Binding for local extending class:
+    py::class_<Dog, pets::Pet>(m, "Dog")
+        .def(py::init<std::string>());
+    m.def("pet_name", [](pets::Pet &p) { return p.name(); });
+
+    py::class_<MixGL>(m, "MixGL", py::module_local()).def(py::init<int>());
+    m.def("get_gl_value", [](MixGL &o) { return o.i + 100; });
+
+    py::class_<MixGL2>(m, "MixGL2", py::module_local()).def(py::init<int>());
 }

tests/test_local_bindings.cpp

@@ -14,13 +14,6 @@
 #include <pybind11/stl_bind.h>
 #include <numeric>
 
-PYBIND11_MAKE_OPAQUE(LocalVec);
-PYBIND11_MAKE_OPAQUE(LocalVec2);
-PYBIND11_MAKE_OPAQUE(LocalMap);
-PYBIND11_MAKE_OPAQUE(NonLocalVec);
-PYBIND11_MAKE_OPAQUE(NonLocalMap);
-PYBIND11_MAKE_OPAQUE(NonLocalMap2);
-
 TEST_SUBMODULE(local_bindings, m) {
     // test_local_bindings
     // Register a class with py::module_local:
@@ -84,4 +77,21 @@ TEST_SUBMODULE(local_bindings, m) {
     m.def("load_vector_via_caster", [](std::vector<int> v) {
         return std::accumulate(v.begin(), v.end(), 0);
     });
+
+    // test_cross_module_calls
+    m.def("return_self", [](LocalVec *v) { return v; });
+    m.def("return_copy", [](const LocalVec &v) { return LocalVec(v); });
+
+    class Cat : public pets::Pet { public: Cat(std::string name) : Pet(name) {}; };
+    py::class_<pets::Pet>(m, "Pet", py::module_local())
+        .def("get_name", &pets::Pet::name);
+    // Binding for local extending class:
+    py::class_<Cat, pets::Pet>(m, "Cat")
+        .def(py::init<std::string>());
+    m.def("pet_name", [](pets::Pet &p) { return p.name(); });
+
+    py::class_<MixGL>(m, "MixGL").def(py::init<int>());
+    m.def("get_gl_value", [](MixGL &o) { return o.i + 10; });
+
+    py::class_<MixGL2>(m, "MixGL2").def(py::init<int>());
 }

tests/test_local_bindings.py

@@ -20,16 +20,14 @@ def test_local_bindings():
     assert not hasattr(i1, 'get2')
     assert not hasattr(i2, 'get3')
 
+    # Loading within the local module
     assert m.local_value(i1) == 5
     assert cm.local_value(i2) == 10
 
-    with pytest.raises(TypeError) as excinfo:
-        m.local_value(i2)
-    assert "incompatible function arguments" in str(excinfo.value)
-
-    with pytest.raises(TypeError) as excinfo:
-        cm.local_value(i1)
-    assert "incompatible function arguments" in str(excinfo.value)
+    # Cross-module loading works as well (on failure, the type loader looks for
+    # external module-local converters):
+    assert m.local_value(i2) == 10
+    assert cm.local_value(i1) == 5
 
 
 def test_nonlocal_failure():
@@ -60,13 +58,12 @@ def test_stl_bind_local():
     v2.append(cm.LocalType(1))
     v2.append(cm.LocalType(2))
 
-    with pytest.raises(TypeError):
-        v1.append(cm.LocalType(3))
-    with pytest.raises(TypeError):
-        v2.append(m.LocalType(3))
+    # Cross module value loading:
+    v1.append(cm.LocalType(3))
+    v2.append(m.LocalType(3))
 
-    assert [i.get() for i in v1] == [0, 1]
-    assert [i.get() for i in v2] == [2, 3]
+    assert [i.get() for i in v1] == [0, 1, 2]
+    assert [i.get() for i in v2] == [2, 3, 4]
 
     v3, v4 = m.NonLocalVec(), cm.NonLocalVec2()
     v3.append(m.NonLocalType(1))
@@ -158,3 +155,56 @@ def test_stl_caster_vs_stl_bind(msg):
 
     Invoked with: [1, 2, 3]
     """  # noqa: E501 line too long
+
+
+def test_cross_module_calls():
+    import pybind11_cross_module_tests as cm
+
+    v1 = m.LocalVec()
+    v1.append(m.LocalType(1))
+    v2 = cm.LocalVec()
+    v2.append(cm.LocalType(2))
+
+    # Returning the self pointer should get picked up as returning an existing
+    # instance (even when that instance is of a foreign, non-local type).
+    assert m.return_self(v1) is v1
+    assert cm.return_self(v2) is v2
+    assert m.return_self(v2) is v2
+    assert cm.return_self(v1) is v1
+
+    assert m.LocalVec is not cm.LocalVec
+    # Returning a copy, on the other hand, always goes to the local type,
+    # regardless of where the source type came from.
+    assert type(m.return_copy(v1)) is m.LocalVec
+    assert type(m.return_copy(v2)) is m.LocalVec
+    assert type(cm.return_copy(v1)) is cm.LocalVec
+    assert type(cm.return_copy(v2)) is cm.LocalVec
+
+    # Test the example given in the documentation (which also tests inheritance casting):
+    mycat = m.Cat("Fluffy")
+    mydog = cm.Dog("Rover")
+    assert mycat.get_name() == "Fluffy"
+    assert mydog.name() == "Rover"
+    assert m.Cat.__base__.__name__ == "Pet"
+    assert cm.Dog.__base__.__name__ == "Pet"
+    assert m.Cat.__base__ is not cm.Dog.__base__
+    assert m.pet_name(mycat) == "Fluffy"
+    assert m.pet_name(mydog) == "Rover"
+    assert cm.pet_name(mycat) == "Fluffy"
+    assert cm.pet_name(mydog) == "Rover"
+
+    assert m.MixGL is not cm.MixGL
+    a = m.MixGL(1)
+    b = cm.MixGL(2)
+    assert m.get_gl_value(a) == 11
+    assert m.get_gl_value(b) == 12
+    assert cm.get_gl_value(a) == 101
+    assert cm.get_gl_value(b) == 102
+
+    c, d = m.MixGL2(3), cm.MixGL2(4)
+    with pytest.raises(TypeError) as excinfo:
+        m.get_gl_value(c)
+    assert "incompatible function arguments" in str(excinfo)
+    with pytest.raises(TypeError) as excinfo:
+        m.get_gl_value(d)
+    assert "incompatible function arguments" in str(excinfo)