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The reason why this fix works is not obvious. Can you add a clarifying comment, alternatively expand on the C API docs fortp_traverse?

Note to self: we also need to document the GC peculiarities of heap-type-inherits-from-static-type (for exampledefdict in the _collections extension module).

🤖 New build scheduled with the buildbot fleet by@kumaraditya303 for commit78c12ff 🤖

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The heap type still has GC but it now inherits traverse and clear slots from its base becausePy_TPFLAGS_HAVE_GC isn't set. Seehttps://docs.python.org/3.12/c-api/gcsupport.html

The doc's a bit unclear here.

When callingPyType_Ready() or some of the APIs that indirectly call it likePyType_FromSpecWithBases() orPyType_FromSpec() the interpreter will automatically populate thetp_flags,tp_traverse andtp_clear fields if the type inherits from a class that implements the garbage collector protocol and the child class does not include thePy_TPFLAGS_HAVE_GC flag.

From the code, we see,

To get thetp_traverse andtp_clear inherited, the child type must have them bothNULL.

This is against the doc. Also, the comparisons in lines 6674 and 6676 are unnecessary.

It was a 22-yr old code13d52f0. I guess the author (@gvanrossum) was trying to write

-      (!type->tp_traverse&& !type->tp_clear)) {+      (!type->tp_traverse|| !type->tp_clear)) {

Don't know what to do. Maybe we should only update the doc.

Don't know what to do. Maybe we should only update the doc.

The best we can do is to fix docs, the behavior here is ancient.

Next time, it would be nice to elaborate the rationale for the fix in the commit, rather than just saying "fix" without any commit message ;-) It's really hard to handle the GC correctly with heap types. Better documentation is required.

The _io module is one of the few C extensions which use class inheritance with heap types implemented in C.

cc@pablogsal: GC issues are hard :-(