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[cppyy] Preserve C++ docstring when uninformative PyExceptions/no traceback #21989

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dpiparo merged 1 commit into from
Apr 22, 2026
+33 −9
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[cppyy] Preserve C++ docstring when uninformative PyExceptions/no traceback #21989

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42 changes: 33 additions & 9 deletions bindings/pyroot/cppyy/CPyCppyy/src/CPPMethod.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -282,8 +282,10 @@ void CPyCppyy::CPPMethod::SetPyError_(PyObject* msg)
// C++ method to give some context.
// 2. A C++ exception has occurred:
// Augment the exception message with the docstring of this method
// 3. A Python exception has occurred:
// 3. A Python exception has occurred with a traceback:
// Do nothing, Python exceptions are already informative enough
// 4. If the Python exception has no traceback hinting to an internally set error stack,
// extract its message and wrap it with C++ method docstring context.

#if PY_VERSION_HEX >= 0x030c0000
PyObject *evalue = PyErr_Occurred() ? PyErr_GetRaisedException() : nullptr;
Expand All @@ -299,16 +301,32 @@ void CPyCppyy::CPPMethod::SetPyError_(PyObject* msg)
#endif

const bool isCppExc = evalue && PyType_IsSubtype((PyTypeObject*)etype, &CPPExcInstance_Type);
// If the error is not a CPPExcInstance, the error from Python itself is
// already complete and messing with it would only make it less informative.
std::string details;

// If the error is not a CPPExcInstance and has a traceback, the error from
// Python itself is already complete and messing with it would only make it
// less informative.
// Just restore and return.
if (evalue && !isCppExc) {
#if PY_VERSION_HEX >= 0x030c0000
PyErr_SetRaisedException(evalue);
PyObject* tb = PyException_GetTraceback(evalue);
if (tb) {
Py_DECREF(tb);
PyErr_SetRaisedException(evalue);
return;
}
#else
PyErr_Restore(etype, evalue, etrace);
if (etrace) {
PyErr_Restore(etype, evalue, etrace);
return;
}
#endif
return;
// no traceback, extract its message and fall through
PyObject* descr = PyObject_Str(evalue);
if (descr) {
details = CPyCppyy_PyText_AsString(descr);
Py_DECREF(descr);
}
}

PyObject* doc = GetDocString();
Expand All @@ -319,9 +337,15 @@ void CPyCppyy::CPPMethod::SetPyError_(PyObject* msg)
const char* cname = pyname ? CPyCppyy_PyText_AsString(pyname) : "Exception";

if (!isCppExc) {
// this is the case where no Python error has occurred yet, and we set a new
// error with context info
PyErr_Format(errtype, "%s =>\n %s: %s", cdoc, cname, cmsg ? cmsg : "");
// this is the case where no Python error has occured yet, or an internal
// one without traceback set a new error with context
if (details.empty()) {
PyErr_Format(errtype, "%s =>\n %s: %s", cdoc, cname, cmsg ? cmsg : "");
} else if (cmsg) {
PyErr_Format(errtype, "%s =>\n %s: %s (%s)", cdoc, cname, cmsg, details.c_str());
} else {
PyErr_Format(errtype, "%s =>\n %s: %s", cdoc, cname, details.c_str());
}
} else {
// augment the top message with context information
PyObject *&topMessage = ((CPPExcInstance*)evalue)->fTopMessage;
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