[Python] Deprecate C++ nullptr comparison with None

This comparison might give confusing results because they are
contradicting with None being a singleton type, so it's better to
deprecate this type of comparison.

Closes #20283.

Author: guitargeek

README/ReleaseNotes/v638/index.md

@@ -49,6 +49,7 @@ The following people have contributed to this new version:
   Relative RPATHs to the main ROOT libraries are unconditionally appended to all ROOT executables and libraries if the operating system supports it.
   If you want a ROOT build without RPATHs, use the canonical CMake variable `CMAKE_SKIP_INSTALL_RPATH=TRUE`.
 * The `TH1K` class is deprecated and will be removed in 6.40. It did not implement the `TH1` interface consistently, and limited the usability of the k-neighbors method it implemented by closely coupling the algorithm with the histogram class. Please use the new `TMath::KNNDensity` function that implements the same mathematical logic.
+* Comparing C++ `nullptr` objects with `None` in Python now results in a `FutureWarning`, and will result in a `TypeError` starting from ROOT 6.40. This is to prevent confusing results where `x == None` and `x is None` are not identical. Use truth-value checks like `if not x` or `x is None` instead.
 
 ## Build System
 * Improve building ROOT when ROOT is already installed in the system. ROOT now correctly handles system-include folders that both contain a package that ROOT depends on and a ROOT installation. Dependent packages are included with `-isystem` instead of `-I`, so installed ROOT headers will not interfere with a ROOT build. See [#8708](https://github.com/root-project/root/issues/8708) for details.

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

@@ -573,6 +573,19 @@ static PyObject* op_richcompare(CPPInstance* self, PyObject* other, int op)
     if (op == Py_EQ || op == Py_NE) {
     // special case for None to compare True to a null-pointer
         if ((PyObject*)other == Py_None && !self->fObject) {
+            const char *msg =
+                "\nComparison of C++ nullptr objects with `None` is deprecated and will raise a TypeError starting from ROOT 6.40."
+                "\n\nThis currently treats `None` as equivalent to a null C++ pointer, which may lead to confusing results."
+                "\nFor example, `x == None` may return True even though `x is None` is False."
+                "\n\nTo test whether a C++ object is null or not, check its truth value instead:"
+                "\n    if not x: ..."
+                "\nor use `x is None` to explicitly check for Python None."
+                "\n";
+
+            // Equivalent to: warnings.warn(msg, FutureWarning, stacklevel=0)
+            if (PyErr_WarnEx(PyExc_FutureWarning, msg, 0) < 0) {
+                return NULL;  // Propagate the error if warning turned into an exception
+            }
             if (op == Py_EQ) { Py_RETURN_TRUE; }
             Py_RETURN_FALSE;
         }