Add func_is_stateless_with_exact_type feature in pybind11/functional.h

Author: rwgk

include/pybind11/functional.h

@@ -9,6 +9,7 @@
 
 #pragma once
 
+#define PYBIND11_HAS_TYPE_CASTER_STD_FUNCTION_FUNC_IS_STATELESS_WITH_EXACT_TYPE
 #define PYBIND11_HAS_TYPE_CASTER_STD_FUNCTION_SPECIALIZATIONS
 
 #include "pybind11.h"
@@ -65,8 +66,15 @@ struct type_caster<std::function<Return(Args...)>> {
     using retval_type = conditional_t<std::is_same<Return, void>::value, void_type, Return>;
     using function_type = Return (*)(Args...);
 
+private:
+    bool func_is_stateless_with_exact_type_ = false;
+
 public:
+    bool func_is_stateless_with_exact_type() const { return func_is_stateless_with_exact_type_; }
+
     bool load(handle src, bool convert) {
+        func_is_stateless_with_exact_type_ = false;
+
         if (src.is_none()) {
             // Defer accepting None to other overloads (if we aren't in convert mode):
             if (!convert) {
@@ -110,6 +118,7 @@ struct type_caster<std::function<Return(Args...)>> {
                             function_type f;
                         };
                         value = ((capture *) &rec->data)->f;
+                        func_is_stateless_with_exact_type_ = true;
                         return true;
                     }
                     rec = rec->next;

tests/test_callbacks.cpp

@@ -29,6 +29,9 @@ double apply_custom_transform(const py::object &src, double value) {
     if (!func_caster.load(src, /*convert*/ false)) {
         return -100;
     }
+    if (!func_caster.func_is_stateless_with_exact_type()) {
+        return -200;
+    }
     return static_cast<std::function<raw_t> &>(func_caster)(value);
 }
 

tests/test_callbacks.py

@@ -241,9 +241,8 @@ def test_boost_histogram_apply_custom_transform():
     cti = m.boost_histogram_custom_transform_int
     apply = m.boost_histogram_apply_custom_transform
     assert apply(ctd, 5) == 15
-    with pytest.raises(TypeError):
-        assert apply(cti, 0)
+    assert apply(cti, 0) == -200
     assert apply(None, 0) == -100
-    assert apply(lambda value: value * 10, 4) == 40
+    assert apply(lambda value: value, 9) == -200
     assert apply({}, 0) == -100
     assert apply("", 0) == -100