fix: use simple function pointer to avoid potential PyCapsule change in pybind11

include/bh_python/transform.hpp

@@ -58,39 +58,10 @@ struct func_transform {
             return std::make_tuple(ptr, src);
         }
 
-        // If we made it to this point, we probably have a C++ pybind object or an
-        // invalid object. The following is based on the std::function conversion in
-        // pybind11/functional.hpp
-        if(!py::isinstance<py::function>(src))
-            throw py::type_error("Only ctypes double(double) and C++ functions allowed "
-                                 "(must be function)");
-
-        auto func = py::reinterpret_borrow<py::function>(src);
-
-        if(auto cfunc = func.cpp_function()) {
-            auto c = py::reinterpret_borrow<py::capsule>(
-                PyCFunction_GET_SELF(cfunc.ptr()));
-
-            auto rec = c.get_pointer<py::detail::function_record>();
-
-            if(rec && rec->is_stateless
-               && py::detail::same_type(
-                   typeid(raw_t*),
-                   *reinterpret_cast<const std::type_info*>(rec->data[1]))) {
-                struct capture {
-                    raw_t* f;
-                };
-                return std::make_tuple((reinterpret_cast<capture*>(&rec->data))->f,
-                                       src);
-            }
-
-            // Note that each error is slightly different just to help with debugging
-            throw py::type_error("Only ctypes double(double) and C++ functions allowed "
-                                 "(must be stateless)");
-        }
+        // If we made it to this point, we probably have an invalid object.
 
         throw py::type_error("Only ctypes double(double) and C++ functions allowed "
-                             "(must be cpp function)");
+                             "(must be a stateless cpp function)");
     }
 
     func_transform(py::object f, py::object i, py::object c, py::str n)

src/boost_histogram/axis/transform.py

@@ -1,17 +1,21 @@
 from __future__ import annotations
 
 import copy
+import ctypes
 from typing import Any, ClassVar, TypeVar
 
 import boost_histogram
 
+from .. import _core
 from .._core import axis as ca
 from .._utils import register
 
 T = TypeVar("T", bound="AxisTransform")
 
 __all__ = ["AxisTransform", "Function", "Pow", "log", "sqrt"]
 
+LIB = ctypes.CDLL(_core.__file__)
+
 
 def __dir__() -> list[str]:
     return __all__
@@ -150,7 +154,8 @@ def _produce(self, bins: int, start: float, stop: float) -> Any:
 
 
 def _internal_conversion(name: str) -> Any:
-    return getattr(ca.transform, name)
+    ftype = ctypes.CFUNCTYPE(ctypes.c_double, ctypes.c_double)
+    return ctypes.cast(getattr(LIB, name), ftype)
 
 
 sqrt = Function("_sqrt_fn", "_sq_fn", convert=_internal_conversion, name="sqrt")

src/register_transforms.cpp

@@ -32,10 +32,10 @@ py::class_<T> register_transform(py::module& mod, Args&&... args) {
 }
 
 extern "C" {
-double _log_fn(double v) { return std::log(v); }
-double _exp_fn(double v) { return std::exp(v); }
-double _sqrt_fn(double v) { return std::sqrt(v); }
-double _sq_fn(double v) { return v * v; }
+PYBIND11_EXPORT double _log_fn(double v) { return std::log(v); }
+PYBIND11_EXPORT double _exp_fn(double v) { return std::exp(v); }
+PYBIND11_EXPORT double _sqrt_fn(double v) { return std::sqrt(v); }
+PYBIND11_EXPORT double _sq_fn(double v) { return v * v; }
 }
 
 void register_transforms(py::module& mod) {