Some C-API functions clear the error indicator?
in [Python]
|
Prev: Killing a Thread
Next: SimpleXMLRPCServer daemon
From: Austin Bingham on 29 Jan 2010 09:37 I've noticed that several (many?) python functions seem to clear the error/exception indicators when they're called from a C/C++ program. For example, both PyImport_ImportModule and traceback.extract_tb() (called via the function call methods) do this: if error indicators are set prior to their call (as indicated by PyErr_Fetch, and including a call to PyErr_Restore), I see that they are unset (using the same method) after the call. This happens even when the functions succeed. The functions that do this don't seem to indicate in their documentation that this will happen. So first, does anyone know why this is happening? Is it because of the context in which I'm making the calls? Is there any pattern or reason behind which functions will do this? Or am I just doing something wrong? If the problem is context-dependent (e.g. I haven't properly established a call stack, or something of that flavor), any pointers on doing things properly would be great. Here's some example code demonstrating the problem: --- #include <Python.h> int main(int argc, char** argv) { Py_Initialize(); // Cause an IndexError PyObject* list = PyList_New(0); PyObject* obj = PyList_GetItem(list, 100); PyObject *t = NULL, *v = NULL, *tb = NULL; // Verify that we see the error PyErr_Fetch(&t, &v, &tb); assert(t); PyErr_Restore(t, v, tb); // Import a module, which seems to be clearing the error indicator PyObject* mod = PyImport_ImportModule("sys"); assert(PyObject_HasAttrString(mod, "path")); // Verify that the error indicator has been cleared PyErr_Fetch(&t, &v, &tb); assert(!t); // <=== The error is gone! PyErr_Restore(t, v, tb); Py_Finalize(); return 0; } --- Thanks in advance. Austin
From: Gabriel Genellina on 29 Jan 2010 15:04 En Fri, 29 Jan 2010 11:37:09 -0300, Austin Bingham <austin.bingham(a)gmail.com> escribi�: > I've noticed that several (many?) python functions seem to clear the > error/exception indicators when they're called from a C/C++ program. > For example, both PyImport_ImportModule and traceback.extract_tb() > (called via the function call methods) do this: if error indicators > are set prior to their call (as indicated by PyErr_Fetch, and > including a call to PyErr_Restore), I see that they are unset (using > the same method) after the call. This happens even when the functions > succeed. It's simple: you have to check *every* function call for failure. Many functions return new object references and you have to properly decrement them in case of failure, so in most cases this means that you have to check each and every call. -- Gabriel Genellina
From: Austin Bingham on 29 Jan 2010 16:25 Maybe I'm not following what you're saying. In my case, I already know that an exception has been thrown. In the course of processing that exception, I call another function which, for whatever reason and even when it succeeds, clears the exception indicators. How can I address this issue by checking function calls for failure? Austin On Fri, Jan 29, 2010 at 9:04 PM, Gabriel Genellina <gagsl-py2(a)yahoo.com.ar> wrote: > En Fri, 29 Jan 2010 11:37:09 -0300, Austin Bingham > <austin.bingham(a)gmail.com> escribió: > >> I've noticed that several (many?) python functions seem to clear the >> error/exception indicators when they're called from a C/C++ program. >> For example, both PyImport_ImportModule and traceback.extract_tb() >> (called via the function call methods) do this: if error indicators >> are set prior to their call (as indicated by PyErr_Fetch, and >> including a call to PyErr_Restore), I see that they are unset (using >> the same method) after the call. This happens even when the functions >> succeed. > > It's simple: you have to check *every* function call for failure. Many > functions return new object references and you have to properly decrement > them in case of failure, so in most cases this means that you have to check > each and every call. > > -- > Gabriel Genellina > > -- > http://mail.python.org/mailman/listinfo/python-list >
From: Gabriel Genellina on 29 Jan 2010 19:11 En Fri, 29 Jan 2010 18:25:14 -0300, Austin Bingham <austin.bingham(a)gmail.com> escribi�: > Maybe I'm not following what you're saying. In my case, I already know > that an exception has been thrown. In the course of processing that > exception, I call another function which, for whatever reason and even > when it succeeds, clears the exception indicators. How can I address > this issue by checking function calls for failure? Maybe if you provide an actual use case we can suggest how to handle it. The code in your original post does not make any sense to me (except by showing that PyImport_ImportModule does clear the error indicator). If you already know there was an error, and you even have retrieved the error details, why do you care if the error indicator gets reset? -- Gabriel Genellina
From: Austin Bingham on 30 Jan 2010 02:10
The original post was, in a nutshell, the "use case"; it's very scaled down the from the real example, and not intended to "make sense". The notion on which I was (apparently incorrectly) basing my exception translation was that I could 1) get and reset references to the error indicators, 2) call other python methods that don't themselves throw, and 3) later retrieve the same "active" exceptions. I was relying on this ability to "re-fetch" exceptions insofar as the functions in my code which dealt with python exceptions all looked up the exception separately. The predicate that "a successful function won't modify the error indicators" appears to be wrong, however, and I've modified my code accordingly. Austin On Sat, Jan 30, 2010 at 1:11 AM, Gabriel Genellina <gagsl-py2(a)yahoo.com.ar> wrote: > En Fri, 29 Jan 2010 18:25:14 -0300, Austin Bingham > <austin.bingham(a)gmail.com> escribió: > >> Maybe I'm not following what you're saying. In my case, I already know >> that an exception has been thrown. In the course of processing that >> exception, I call another function which, for whatever reason and even >> when it succeeds, clears the exception indicators. How can I address >> this issue by checking function calls for failure? > > Maybe if you provide an actual use case we can suggest how to handle it. The > code in your original post does not make any sense to me (except by showing > that PyImport_ImportModule does clear the error indicator). If you already > know there was an error, and you even have retrieved the error details, why > do you care if the error indicator gets reset? > > -- > Gabriel Genellina > > -- > http://mail.python.org/mailman/listinfo/python-list > |