[Python-Dev] PySet_Next (Was: PySet API) (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Sun Mar 26 19:59:26 CEST 2006

Raymond Hettinger wrote:

The difference is that the PySetNext returns pointers to the table keys and that the mutation occurs AFTER the call to PySetNext, leaving pointers to invalid addresses. IOW, the function cannot detect the mutation.

I'm coming late to the discussion: where did anybody ever suggest that PySet_Next should return a pointer into the set? Looking over the entire discussion, I could not find any mentioning of a specific API.

If it is similar to PyDict_Next, it will have PyObject** /input/ variables, which are really meant as PyObject* /output/ variables. But yes, PyDict_Next returns a borrowed reference, so if the dictionary mutates between calls, your borrowed reference might become stale.

PyIterNext on the other hand returns an object (not a pointer to an object such as those in the hash table).

PyIter_Next behaves identical wrt. to result types to PyDict_Next. The difference is that PyIter_Next always returns a new reference (or NULL in case of an exception).

For the caller, a clear usage strategy follows from this: either discard the references before making a potentially-mutating call, or Py_INCREF the set element before making that mutating call.

Of course, after you made the mutating call, your iteration position might be bogus, as the set might have been reorganized. If the position is represented as a Py_ssize_t (as it is for PyDict_Next), the only consequence of continuing the iteration is that you might see elements twice or not at all - you cannot cause a crash with that.

Regards, Martin

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