Jack Diederich wrote: > On Wed, Aug 16, 2006 at 03:25:39PM -0700, John Machin wrote:
> > > > > > > > Not the OP's problem, but a bug in the manual: example in the chapter > > > > that the OP was reading acts as though the 2nd arg to PyObject_SetItem > > > > is a C int (as it is for the List and Sequence varieties) -- it is in > > > > fact a (PyObject *), which explains the SystemError that I got. Please don't forget this which is definitely a bug in the docs. > > > > > > The good news is that this is a documentation problem. > > > > Cop-out alert! I disagree. Elsewhere Python goes to great lengths to > > check for nasties instead of just segfaulting. > > > > > When working at the C level you use the concrete API when you can > > > (PyList_*) and at the abstract level (PySequence_*) only when someone > > > passes you something and you don't know what it is, you only know it > > > supports the Sequence API. > > > > > > The exmample should use the PyList API to construct the list. The > > > Sequence API requires valid objects to work. Here the object is only half > > > constructed and list_ass_item tries to DECREF the zeroth member which > > > is NULL because the list isn't constructed yet. PyList_SetItem does > > > an XDECREF (decrement only if the pointer isn't null) because it expects > > > to be used that way. > > > > PyList_SetItem is not sentient. It expects nothing. Now tell us what > > the developer was thinking -- certainly not consistency with the > > documantation. If the Sequence API requires valid objects to work, > > then in this case it should raise an error. > > > > In fact that manual section calls using the PySequence_SetItem the > > recommended approach!!! > > > > Are you suggesting a rework of the manual instead of inserting a X in > > the offending py_DECREF? > > > > Sheesh all round ... > > It takes a big man to admit when he's wrong so I'll blame low blood sugar. :-) > Since this had been the same going all the way back to 2.2 I assumed it was > common. It isn't, simlar functions in other objects guard against the > possibility. I'm a bit curious here: tuples naturally don't fill in the sq_ass_item slot and I couldn't think of another object that would be initialised with NULL(s) in the expectation of being filled in later or maybe not ... enlightenment, please. > > I'll check in the change assuming no one else has already (and assuming > the trunk isn't currently frozen, I'll have to check). For avoidance of doubt: the change is to use Py_XDECREF, yes/no? Cheers, John -- http://mail.python.org/mailman/listinfo/python-list
