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Elide to-python conversion of setter return values (#4621)
* Reproducer for property setter with return type that is not wrapped. * Use `py::class_<OptionsBase>()` to work around the return value conversion issue. * WIP drop_return_value * Remove struct drop_return_value * Introduce `return_value_policy::return_none` for use by setters. * Add `is_setter` to attr.h and use from `.def_property()` * Merge the new test into test_methods_and_attributes * Remove return_none return_value_policy again. * Fix oversight (NOLINTNEXTLINE placement). * Simplification (for the better) found while searching for a way to resolve GCC build failures. Example of failure resolved by this change: g++ (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0 ``` cd /build/tests && /usr/bin/c++ -DPYBIND11_TEST_EIGEN -Dpybind11_tests_EXPORTS -I/mounted_pybind11/include -isystem /usr/include/python3.8 -isystem /build/_deps/eigen-src -g -std=c++17 -fPIC -fvisibility=hidden -Wall -Wextra -Wconversion -Wcast-qual -Wdeprecated -Wundef -Wnon-virtual-dtor -MD -MT tests/CMakeFiles/pybind11_tests.dir/test_buffers.cpp.o -MF CMakeFiles/pybind11_tests.dir/test_buffers.cpp.o.d -o CMakeFiles/pybind11_tests.dir/test_buffers.cpp.o -c /mounted_pybind11/tests/test_buffers.cpp In file included from /mounted_pybind11/include/pybind11/stl.h:12, from /mounted_pybind11/tests/test_buffers.cpp:10: /mounted_pybind11/include/pybind11/pybind11.h: In instantiation of ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_property(const char*, const Getter&, const Setter&, const Extra& ...) [with Getter = pybind11::cpp_function; Setter = std::nullptr_t; Extra = {pybind11::return_value_policy}; type_ = pybind11::buffer_info; options = {}]’: /mounted_pybind11/include/pybind11/pybind11.h:1716:58: required from ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_property_readonly(const char*, const pybind11::cpp_function&, const Extra& ...) [with Extra = {pybind11::return_value_policy}; type_ = pybind11::buffer_info; options = {}]’ /mounted_pybind11/include/pybind11/pybind11.h:1684:9: required from ‘pybind11::class_<type_, options>& pybind11::class_<type_, options>::def_readonly(const char*, const D C::*, const Extra& ...) [with C = pybind11::buffer_info; D = long int; Extra = {}; type_ = pybind11::buffer_info; options = {}]’ /mounted_pybind11/tests/test_buffers.cpp:209:61: required from here /mounted_pybind11/include/pybind11/pybind11.h:1740:25: error: call of overloaded ‘cpp_function(std::nullptr_t&, pybind11::is_setter)’ is ambiguous 1740 | name, fget, cpp_function(method_adaptor<type>(fset), is_setter()), extra...); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /mounted_pybind11/include/pybind11/pybind11.h:101:5: note: candidate: ‘pybind11::cpp_function::cpp_function(Func&&, const Extra& ...) [with Func = std::nullptr_t&; Extra = {pybind11::is_setter}; <template-parameter-1-3> = void]’ 101 | cpp_function(Func &&f, const Extra &...extra) { | ^~~~~~~~~~~~ In file included from /mounted_pybind11/include/pybind11/stl.h:12, from /mounted_pybind11/tests/test_buffers.cpp:10: /mounted_pybind11/include/pybind11/pybind11.h:87:5: note: candidate: ‘pybind11::cpp_function::cpp_function(std::nullptr_t, const Extra& ...) [with Extra = {pybind11::is_setter}; std::nullptr_t = std::nullptr_t]’ 87 | cpp_function(std::nullptr_t, const Extra &...) {} | ^~~~~~~~~~~~ ``` * Bug fix: obvious in hindsight. I thought the original version was incrementing the reference count for None, but no. Discovered via many failing tests in the wild (10s of thousands). It is very tricky to construct a meaningful unit test for this bug specifically. It's unlikely to come back, because 10s of thousands of tests will fail again.
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@ -26,6 +26,9 @@ struct is_method {
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explicit is_method(const handle &c) : class_(c) {}
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};
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/// Annotation for setters
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struct is_setter {};
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/// Annotation for operators
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struct is_operator {};
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@ -188,8 +191,8 @@ struct argument_record {
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struct function_record {
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function_record()
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: is_constructor(false), is_new_style_constructor(false), is_stateless(false),
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is_operator(false), is_method(false), has_args(false), has_kwargs(false),
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prepend(false) {}
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is_operator(false), is_method(false), is_setter(false), has_args(false),
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has_kwargs(false), prepend(false) {}
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/// Function name
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char *name = nullptr; /* why no C++ strings? They generate heavier code.. */
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@ -230,6 +233,9 @@ struct function_record {
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/// True if this is a method
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bool is_method : 1;
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/// True if this is a setter
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bool is_setter : 1;
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/// True if the function has a '*args' argument
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bool has_args : 1;
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@ -426,6 +432,12 @@ struct process_attribute<is_method> : process_attribute_default<is_method> {
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}
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};
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/// Process an attribute which indicates that this function is a setter
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template <>
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struct process_attribute<is_setter> : process_attribute_default<is_setter> {
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static void init(const is_setter &, function_record *r) { r->is_setter = true; }
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};
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/// Process an attribute which indicates the parent scope of a method
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template <>
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struct process_attribute<scope> : process_attribute_default<scope> {
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@ -84,6 +84,7 @@ public:
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cpp_function() = default;
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// NOLINTNEXTLINE(google-explicit-constructor)
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cpp_function(std::nullptr_t) {}
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cpp_function(std::nullptr_t, const is_setter &) {}
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/// Construct a cpp_function from a vanilla function pointer
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template <typename Return, typename... Args, typename... Extra>
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@ -244,10 +245,16 @@ protected:
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using Guard = extract_guard_t<Extra...>;
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/* Perform the function call */
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handle result
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= cast_out::cast(std::move(args_converter).template call<Return, Guard>(cap->f),
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handle result;
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if (call.func.is_setter) {
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(void) std::move(args_converter).template call<Return, Guard>(cap->f);
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result = none().release();
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} else {
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result = cast_out::cast(
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std::move(args_converter).template call<Return, Guard>(cap->f),
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policy,
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call.parent);
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}
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/* Invoke call policy post-call hook */
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process_attributes<Extra...>::postcall(call, result);
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@ -1729,7 +1736,8 @@ public:
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template <typename Getter, typename Setter, typename... Extra>
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class_ &
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def_property(const char *name, const Getter &fget, const Setter &fset, const Extra &...extra) {
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return def_property(name, fget, cpp_function(method_adaptor<type>(fset)), extra...);
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return def_property(
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name, fget, cpp_function(method_adaptor<type>(fset), is_setter()), extra...);
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}
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template <typename Getter, typename... Extra>
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class_ &def_property(const char *name,
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@ -177,6 +177,38 @@ struct RValueRefParam {
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std::size_t func4(std::string &&s) const & { return s.size(); }
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};
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namespace pybind11_tests {
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namespace exercise_is_setter {
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struct FieldBase {
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int int_value() const { return int_value_; }
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FieldBase &SetIntValue(int int_value) {
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int_value_ = int_value;
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return *this;
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}
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private:
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int int_value_ = -99;
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};
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struct Field : FieldBase {};
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void add_bindings(py::module &m) {
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py::module sm = m.def_submodule("exercise_is_setter");
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// NOTE: FieldBase is not wrapped, therefore ...
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py::class_<Field>(sm, "Field")
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.def(py::init<>())
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.def_property(
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"int_value",
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&Field::int_value,
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&Field::SetIntValue // ... the `FieldBase &` return value here cannot be converted.
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);
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}
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} // namespace exercise_is_setter
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} // namespace pybind11_tests
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TEST_SUBMODULE(methods_and_attributes, m) {
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// test_methods_and_attributes
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py::class_<ExampleMandA> emna(m, "ExampleMandA");
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@ -456,4 +488,6 @@ TEST_SUBMODULE(methods_and_attributes, m) {
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.def("func2", &RValueRefParam::func2)
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.def("func3", &RValueRefParam::func3)
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.def("func4", &RValueRefParam::func4);
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pybind11_tests::exercise_is_setter::add_bindings(m);
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}
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@ -522,3 +522,12 @@ def test_rvalue_ref_param():
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assert r.func2("1234") == 4
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assert r.func3("12345") == 5
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assert r.func4("123456") == 6
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def test_is_setter():
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fld = m.exercise_is_setter.Field()
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assert fld.int_value == -99
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setter_return = fld.int_value = 100
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assert isinstance(setter_return, int)
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assert setter_return == 100
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assert fld.int_value == 100
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