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<div class="WordSection1">
<p class="MsoNormal"><span style="color:black">PEP-384 talks about defining a Stable ABI by making PyTypeObject opaque. Thus, banning the use of static PyTypeObjects.<o:p></o:p></span></p>
<p class="MsoNormal" style="caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal" style="caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">Specifically, I’d like to focus on those static PyTypeObjects that are initialized as StructSequences. As of today, there are 14 instances of these types (in timemodule.c, posixmodule.c, etc.) within cpython/Modules. These are all
initialized through PyStructSequence_InitType2. This is very problematic when trying to make these types conform to PEP-384 as they are used through static PyTypeObjects.<br>
<br>
<o:p></o:p></span></p>
<p class="MsoNormal" style="caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">Problems:<o:p></o:p></span></p>
<ul style="margin-top:0in;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px" type="disc">
<li class="MsoListParagraph" style="color:black;margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l3 level1 lfo1">
<u>PyStructSequence_InitType2 overrides the PyTypeObject</u>:<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">This C-API does a direct memcpy from a “prototype†structure. This effectively overrides anything set within the PyTypeObject. For example, if we were to initialize a heap allocated PyTypeObject and pass it on to this function, the
C-API would just get rid of the Py_TPFLAG_HEAPTYPE flag, causing issues with the GC.</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black"> <o:p></o:p></span></p>
<ul style="margin-top:0in;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px" type="disc">
<li class="MsoListParagraph" style="color:black;margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l1 level1 lfo2">
<u>PyStructSequence_InitType2 does not work with heap allocated PyTypeObjects</u>:<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">Even if the function is fixed to preserve the state of the PyTypeObject and only overriding the specific slots (i.e. tp_new, tp_repr, etc.), it is expected that PyStructSequence_InitType2 will call PyType_Ready on the object. That
means that the incoming object shouldn’t be initialized by a function such as PyType_FromSpec, as that would have already called PyType_Ready on it. Therefore, PyStructSequence_InitType2 will now have the responsibility of setting all the slots and properties
of the PyHeapTypeObject, which is not feasible.</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black"> <o:p></o:p></span></p>
<ul style="margin-top:0in;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px" type="disc">
<li class="MsoListParagraph" style="color:black;margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l2 level1 lfo3">
<u>PyStructSequence_NewType is non-functional</u>:<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">This function was meant to be used as a way of creating a heap-allocated PyTypeObject that be passed to PyStructSequence_InitType2, effectively returning a heap allocated PyTypeObject. The current implementation doesn’t work in practice.
Given that this struct is created in the heap, the GC has control over it. Thus, when the GC tries to traverse the type it complains with: “Error: type_traverse() called for non-heap typeâ€, since it doesn’t have the Py_TPFLAG_HEAPTYPE flag. If we add the flag,
we run into bullet point 1, if we are able to preserve the flag then we will still run into the problem of bullet point 2. Extra note: This C-API is not being used anywhere within CPython itself.</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoNormal" style="caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal" style="caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">Solution:<o:p></o:p></span></p>
<ul style="margin-top:0in;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px" type="disc">
<li class="MsoListParagraph" style="color:black;margin-top:0in;margin-bottom:0in;margin-bottom:.0001pt;mso-list:l0 level1 lfo4">
Fix the implementation of PyStructSequence_NewType:<span style="font-size:12.0pt"><o:p></o:p></span></li></ul>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">The best solution would be to fix the implementation of this function. This can easily be done by dynamically creating a PyType_Spec and calling PyType_FromSpec</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">```</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">PyObject*</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">PyStructSequence_NewType(PyStructSequence_Desc *desc)</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">{</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black"> // …</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> PyType_Spec* spec = PyMem_NEW(PyType_Spec, 1);</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> spec->name = desc->name;</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> spec->basicsize = sizeof(PyStructSequence) - sizeof(PyObject *);</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> spec->itemsize = sizeof(PyObject *);</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> spec->flags = Py_TPFLAGS_DEFAULT;</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> spec->slots = PyMem_NEW(PyType_Slot, 6);</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> spec->slots[0].slot = Py_tp_dealloc;</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> spec->slots[0].pfunc = (destructor)structseq_dealloc;</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> // …</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> bases = PyTuple_Pack(1, &PyTuple_Type);</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> type = PyType_FromSpecWithBases(spec, bases);</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> // …</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:0in;margin-right:0in;margin-bottom:0in;margin-left:.5in;margin-bottom:.0001pt;caret-color: rgb(0, 0, 0);font-variant-caps: normal;orphans: auto;text-align:start;widows: auto;-webkit-text-size-adjust: auto;-webkit-text-stroke-width: 0px;word-spacing:0px">
<span style="color:black">```</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> </span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black">This will cleanly create a heap allocated PyStructSequence which can be used just like any stack allocated PyTypeObject initialized through PyStructSequence_InitType2. This means that any C-Extension should be using PyStructSequence_NewType
and only built-in types should be calling PyStructSequence_InitType2. This will enable these types to comply with PEP-384</span><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
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<span style="color:black"> <o:p></o:p></span></p>
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<span style="color:black"> <o:p></o:p></span></p>
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<span style="color:black">As an extra, I already have patches for this proposal. They can be found here:<br>
<br>
Branch:<span class="apple-converted-space"> https://github.com/eduardo-elizondo/cpython/tree/heap-structseq</span>
<br>
Reimplement PyStructSequence_NewType:<span class="apple-converted-space"> </span> https://github.com/eduardo-elizondo/cpython/commit/413f8ca5bc008d84b3397ca1c9565c604d54b661<br>
Patch timemodule with NewType: https://github.com/eduardo-elizondo/cpython/commit/0a35ea263a531cb03c06be9efc9e96d68162b308<br>
<br>
Thoughts?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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