[Jaime Wyant]
> I've found that the code below will crash if I don't have the
> PyErr_Clear() function call. Should I always call PyErr_Clear()?
That's not the right approach. Nearly all Python C API calls can
fail. They return a special value if they do, primarily NULL for a
call that returns a pointer, or -1 for a call that returns an int.
The correct thing to do is to explicitly check every call that *might*
fail to see whether it *did* fail. If it did, then you also have to
explictly decide what to do about it: either pass the exception on to
your caller (by returning your function's error value), or suppress
the exception via PyErr_Clear(), or replace it with another exception.
You have to do one of those before making another Python C API call.
Ignoring these issues always leads to bad problems eventually.
> /* Code that fails if PyErr_Clear() is removed */
> #include "python.h"
>
> int main()
> {
> PyObject *pName, *pModule;
> int i;
>
> Py_Initialize();
> for (i = 0 ; i < 30; i++) {
> // First, import the module
> pName = PyString_FromString("badmodule");
That can fail. You must check whether pName == NULL, and either pass
the exception on (although that doesn't make much sense in a main()),
or clear the error.
> pModule = PyImport_Import(pName);
Ditto.
> Py_DECREF(pName);
That can't fail, if the code preceding it is correct. But it's not in
this case. The PyString_FromString() call from which pName was set
may have failed (for example, it may not have been able to find enough
memory to create a new string object). In that case, the Py_DECREF
would segfault, because pName would be NULL here.
> if (!pModule)
> {
> fprintf(stderr, "couldn't import badmodule\n");
> PyErr_Clear();
> }
> }
That's OK, although it's tempting fate to stuff code between a call
and checking that call's return value for an error.
Note that the Python implementation itself uses the Python C API
heavily, in its C files. You can study those to get many examples of
best practice. It will help to remember that coding in C isn't
supposed to fun <0.9 wink>.
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