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Elide to-python conversion of setter return values (#4621)
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* 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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Ralf W. Grosse-Kunstleve authored May 8, 2023
1 parent 90312a6 commit e9b961d
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Showing 4 changed files with 70 additions and 7 deletions.
16 changes: 14 additions & 2 deletions include/pybind11/attr.h
Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,9 @@ struct is_method {
explicit is_method(const handle &c) : class_(c) {}
};

/// Annotation for setters
struct is_setter {};

/// Annotation for operators
struct is_operator {};

Expand Down Expand Up @@ -188,8 +191,8 @@ struct argument_record {
struct function_record {
function_record()
: is_constructor(false), is_new_style_constructor(false), is_stateless(false),
is_operator(false), is_method(false), has_args(false), has_kwargs(false),
prepend(false) {}
is_operator(false), is_method(false), is_setter(false), has_args(false),
has_kwargs(false), prepend(false) {}

/// Function name
char *name = nullptr; /* why no C++ strings? They generate heavier code.. */
Expand Down Expand Up @@ -230,6 +233,9 @@ struct function_record {
/// True if this is a method
bool is_method : 1;

/// True if this is a setter
bool is_setter : 1;

/// True if the function has a '*args' argument
bool has_args : 1;

Expand Down Expand Up @@ -426,6 +432,12 @@ struct process_attribute<is_method> : process_attribute_default<is_method> {
}
};

/// Process an attribute which indicates that this function is a setter
template <>
struct process_attribute<is_setter> : process_attribute_default<is_setter> {
static void init(const is_setter &, function_record *r) { r->is_setter = true; }
};

/// Process an attribute which indicates the parent scope of a method
template <>
struct process_attribute<scope> : process_attribute_default<scope> {
Expand Down
18 changes: 13 additions & 5 deletions include/pybind11/pybind11.h
Original file line number Diff line number Diff line change
Expand Up @@ -84,6 +84,7 @@ class cpp_function : public function {
cpp_function() = default;
// NOLINTNEXTLINE(google-explicit-constructor)
cpp_function(std::nullptr_t) {}
cpp_function(std::nullptr_t, const is_setter &) {}

/// Construct a cpp_function from a vanilla function pointer
template <typename Return, typename... Args, typename... Extra>
Expand Down Expand Up @@ -244,10 +245,16 @@ class cpp_function : public function {
using Guard = extract_guard_t<Extra...>;

/* Perform the function call */
handle result
= cast_out::cast(std::move(args_converter).template call<Return, Guard>(cap->f),
policy,
call.parent);
handle result;
if (call.func.is_setter) {
(void) std::move(args_converter).template call<Return, Guard>(cap->f);
result = none().release();
} else {
result = cast_out::cast(
std::move(args_converter).template call<Return, Guard>(cap->f),
policy,
call.parent);
}

/* Invoke call policy post-call hook */
process_attributes<Extra...>::postcall(call, result);
Expand Down Expand Up @@ -1729,7 +1736,8 @@ class class_ : public detail::generic_type {
template <typename Getter, typename Setter, typename... Extra>
class_ &
def_property(const char *name, const Getter &fget, const Setter &fset, const Extra &...extra) {
return def_property(name, fget, cpp_function(method_adaptor<type>(fset)), extra...);
return def_property(
name, fget, cpp_function(method_adaptor<type>(fset), is_setter()), extra...);
}
template <typename Getter, typename... Extra>
class_ &def_property(const char *name,
Expand Down
34 changes: 34 additions & 0 deletions tests/test_methods_and_attributes.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -177,6 +177,38 @@ struct RValueRefParam {
std::size_t func4(std::string &&s) const & { return s.size(); }
};

namespace pybind11_tests {
namespace exercise_is_setter {

struct FieldBase {
int int_value() const { return int_value_; }

FieldBase &SetIntValue(int int_value) {
int_value_ = int_value;
return *this;
}

private:
int int_value_ = -99;
};

struct Field : FieldBase {};

void add_bindings(py::module &m) {
py::module sm = m.def_submodule("exercise_is_setter");
// NOTE: FieldBase is not wrapped, therefore ...
py::class_<Field>(sm, "Field")
.def(py::init<>())
.def_property(
"int_value",
&Field::int_value,
&Field::SetIntValue // ... the `FieldBase &` return value here cannot be converted.
);
}

} // namespace exercise_is_setter
} // namespace pybind11_tests

TEST_SUBMODULE(methods_and_attributes, m) {
// test_methods_and_attributes
py::class_<ExampleMandA> emna(m, "ExampleMandA");
Expand Down Expand Up @@ -456,4 +488,6 @@ TEST_SUBMODULE(methods_and_attributes, m) {
.def("func2", &RValueRefParam::func2)
.def("func3", &RValueRefParam::func3)
.def("func4", &RValueRefParam::func4);

pybind11_tests::exercise_is_setter::add_bindings(m);
}
9 changes: 9 additions & 0 deletions tests/test_methods_and_attributes.py
Original file line number Diff line number Diff line change
Expand Up @@ -522,3 +522,12 @@ def test_rvalue_ref_param():
assert r.func2("1234") == 4
assert r.func3("12345") == 5
assert r.func4("123456") == 6


def test_is_setter():
fld = m.exercise_is_setter.Field()
assert fld.int_value == -99
setter_return = fld.int_value = 100
assert isinstance(setter_return, int)
assert setter_return == 100
assert fld.int_value == 100

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