[Python] Avoid hack in setting __reduce__ attribute of CPPInstance

bindings/pyroot/cppyy/CPyCppyy/src/CPPInstance.cxx

@@ -400,6 +400,21 @@ static PySequenceMethods op_as_sequence = {
     0,                             // sq_inplace_repeat
 };
 
+std::function<PyObject *(PyObject *)> &CPPInstance::ReduceMethod() {
+   static std::function<PyObject *(PyObject *)> reducer;
+   return reducer;
+}
+
+PyObject *op_reduce(PyObject *self, PyObject * /*args*/)
+{
+   auto &reducer = CPPInstance::ReduceMethod();
+   if (!reducer) {
+      PyErr_SetString(PyExc_NotImplementedError, "");
+      return nullptr;
+   }
+   return reducer(self);
+}
+
 
 //----------------------------------------------------------------------------
 static PyMethodDef op_methods[] = {
@@ -409,6 +424,8 @@ static PyMethodDef op_methods[] = {
       (char*)"dispatch to selected overload"},
     {(char*)"__smartptr__", (PyCFunction)op_get_smartptr, METH_NOARGS,
       (char*)"get associated smart pointer, if any"},
+    {(char*)"__reduce__",  (PyCFunction)op_reduce, METH_NOARGS,
+        (char*)"reduce method for serialization"},
     {(char*)"__reshape__",  (PyCFunction)op_reshape, METH_O,
         (char*)"cast pointer to 1D array type"},
     {(char*)nullptr, nullptr, 0, nullptr}

bindings/pyroot/cppyy/CPyCppyy/src/CPPInstance.h

@@ -15,6 +15,7 @@
 #include "CallContext.h"     // for Parameter
 
 // Standard
+#include <functional>
 #include <utility>
 #include <vector>
 
@@ -83,6 +84,11 @@ class CPPInstance {
     void CastToArray(Py_ssize_t sz);
     Py_ssize_t ArrayLength();
 
+// implementation of the __reduce__ method: doesn't wrap any function by
+// default but can be re-assigned by libraries that add C++ object
+// serialization support, like ROOT
+    static std::function<PyObject *(PyObject *)> &ReduceMethod();
+
 private:
     void  CreateExtension();
     void* GetExtendedObject();

bindings/pyroot/pythonizations/python/ROOT/_pythonization/_cppinstance.py

@@ -8,13 +8,12 @@
 # For the list of contributors see $ROOTSYS/README/CREDITS.                    #
 ################################################################################
 
+
 def pythonize_cppinstance():
-    import cppyy
     from ROOT.libROOTPythonizations import AddCPPInstancePickling
 
-    klass = cppyy._backend.CPPInstance
+    AddCPPInstancePickling()
 
-    AddCPPInstancePickling(klass)
 
 # Instant pythonization (executed at `import ROOT` time), no need of a
 # decorator. CPPInstance is the base for cppyy instance proxies and thus needs

bindings/pyroot/pythonizations/src/CPPInstancePyz.cxx

@@ -62,7 +62,7 @@ PyObject *PyROOT::CPPInstanceExpand(PyObject * /*self*/, PyObject *args)
 /// Turn the object proxy instance into a character stream and return for
 /// pickle, together with the callable object that can restore the stream
 /// into the object proxy instance.
-PyObject *op_reduce(PyObject *self, PyObject * /*args*/)
+PyObject *op_reduce(PyObject *self)
 {
    // keep a borrowed reference around to the callable function for expanding;
    // because it is borrowed, it means that there can be no pickling during the
@@ -120,28 +120,8 @@ PyObject *op_reduce(PyObject *self, PyObject * /*args*/)
 ///
 /// The C++ function op_reduce defined above is wrapped in a Python method
 /// so that it can be injected in CPPInstance
-PyObject *PyROOT::AddCPPInstancePickling(PyObject * /*self*/, PyObject *args)
+PyObject *PyROOT::AddCPPInstancePickling(PyObject * /*self*/, PyObject * /*args*/)
 {
-   PyObject *pyclass = PyTuple_GetItem(args, 0);
-
-   const char *attr = "__reduce__";
-
-   PyMethodDef *pdef = new PyMethodDef();
-   pdef->ml_name = attr;
-   pdef->ml_meth = (PyCFunction)op_reduce;
-   pdef->ml_flags = METH_NOARGS;
-   pdef->ml_doc = nullptr;
-
-   PyObject *func = PyCFunction_New(pdef, nullptr);
-   PyObject *method = CustomInstanceMethod_New(func, nullptr, pyclass);
-
-   // here PyObject_GenericSetAttr is used because CPPInstance does not allow
-   // attribute assignment using PyObject_SetAttr
-   // for more info refer to:
-   // https://bitbucket.org/wlav/cppyy/issues/110/user-defined-classes-in-c-dont-seem-to-be
-   PyObject_GenericSetAttr(pyclass, PyUnicode_FromString(attr), method);
-   Py_DECREF(method);
-   Py_DECREF(func);
-
+   CPPInstance::ReduceMethod() = op_reduce;
    Py_RETURN_NONE;
 }

bindings/pyroot/pythonizations/src/PyROOTModule.cxx

@@ -126,7 +126,7 @@ PyObject *PyObjRefCounterAsStdAny(PyObject * /*self*/, PyObject *args)
 
 // Methods offered by the interface
 static PyMethodDef gPyROOTMethods[] = {
-   {(char *)"AddCPPInstancePickling", (PyCFunction)PyROOT::AddCPPInstancePickling, METH_VARARGS,
+   {(char *)"AddCPPInstancePickling", (PyCFunction)PyROOT::AddCPPInstancePickling, METH_NOARGS,
     (char *)"Add a custom pickling mechanism for Cppyy Python proxy objects"},
    {(char *)"GetBranchAttr", (PyCFunction)PyROOT::GetBranchAttr, METH_VARARGS,
     (char *)"Allow to access branches as tree attributes"},