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The PyTypeObject field "tp_version_tag" (new in 2.6) isn't documented.
"tp_del" isn't documented either. I'm not sure when it was added.

I think the reason tp_del isn't documented is that, as we somehow learnt it with the new IO code, it isn't really meant for third-party types.
I would suggest checking with python-dev.
As for tp_version_tag, it's for internal use (it indicated whether the method cache is still fresh), I don't think extension authors should modify it.

tp_cache and tp_weaklist are also for internal use only, but are documented.
One reason for documenting them is that users will run into them when running with a high enough warning level in GCC.

> tp_cache and tp_weaklist are also for internal use only, but are
> documented.
Ok, so I guess tp_version_tag and tp_del should also be documented as "for internal use only".

I've just ran into tp_version_tag, when running the boost python testsuite and wondered what it was... Since upgrading to GCC 4.8, I've started to get a lot more warnings with Python extensions, e.g.:-
boost/python/opaque_pointer_converter.hpp:122:14: warning: missing initializer for member ‘_typeobject::tp_version_tag’ [-Wmissing-field-initializers]
In this instance the testsuite was made to compile with the '-Wextra' flag. The fix was pretty simple; add another zero to the opaque_pointer_convert struct.
I have used the following preprocessor macro to test whether or not to do this. Would this be a good way to test for the addition?
#if PY_VERSION_HEX >= 0x02060000
 0, /* tp_version_tag */
#endif
Cheers,
Alex

tp_del still undocumented.
In my opinion, tp_del corresponds to the __del__() method of classes, because there are no other variable correspondes to it, but methods using for creating(tp_new) and initializing(tp_init) an object are not.
Although __del__ method have some issue according to the offical document, but it is exist, so there must have a pointer to the function corresponding to it.

The reason tp_del has remained undocumented is that it's now obsolete. You should use tp_finalize instead:
https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_finalize
tp_finalize is roughly the C equivalent of __del__ (tp_del was something else, despite the name).

Both tp_del and tp_version_tag are for internal use. Besides, tp_del is obsolete as I mentioned above. So I'm going to close the issue.