[Python-Dev] PyErr_ Fetch | Restore & friends (original) (raw)

Ethan Furman ethan at stoneleaf.us
Sat Mar 29 18:05:51 CET 2014

I'm working on issue 1615 [1] and came up with this tidbit, which works [2], but not well enough:

slot_tp_getattr_hook(PyObject *self, PyObject *name) { ...

So I tried adding in a second call to PyErr_Fetch and adding the original error as context, like so:

 if (res == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
     PyErr_Fetch(&ctx_error_type, &ctx_error_value, &ctx_error_traceback);
     res = call_attribute(self, getattr, name);
     if (res == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
         PyErr_Fetch(&error_type, &error_value, &error_traceback);
         PyException_SetContext(error_value, ctx_error_value);
         PyErr_Restore(error_type, error_value, error_traceback);
     } else {
         Py_DECREF(ctx_error_value);
     }
     Py_DECREF(ctx_error_type);
     Py_DECREF(ctx_error_traceback);
 }

This bit of code won't even finish compiling. I am not sure if my understanding of references (and how these functions create/consume them) or my understanding of when and where to call PyErr_Fetch|Restore or PyException_SetContext is at fault, but I would greatly appreciate somebody correcting my understanding. :)

-- Ethan

[1] http://http://bugs.python.org/issue1615 [2] works as in everything compiles, but the resulting behavior introduces a new bug (the message of a 'raise AttributeError' in a getattr is ignored).

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