Adding tp_reachable by mjp41 · Pull Request #65 · mjp41/cpython

mjp41 · 2026-01-16T22:10:48Z

📚 Documentation preview 📚: https://cpython-previews--65.org.readthedocs.build/

xFrednet

The addition itself looks good, but it doesn't set the tp_reachable for most built-in types. Once that is done it should be good to go.

mjp41 · 2026-01-19T09:47:11Z

The addition itself looks good, but it doesn't set the tp_reachable for most built-in types. Once that is done it should be good to go.

Sounds good. I mostly wanted to get someone else's eyes on it, and then we can decide how broad a blast radius we want.

mjp41 · 2026-02-26T16:40:25Z

Modules/_immutablemodule.c

+static PyObject *
+immutable_clear_tp_reachable(PyObject *module, PyObject *obj)
+{
+    if (!PyType_Check(obj)) {
+        PyErr_SetString(PyExc_TypeError, "Expected a type");
+        return NULL;
+    }
+    PyTypeObject *tp = (PyTypeObject *)obj;
+    tp->tp_reachable = NULL;
+    Py_RETURN_NONE;
+}
+


Not sure I am happy exposing this for a test. We should find a better way to test this. I think we really need to build a C-Module that doesn't have tp_reachable, and never will.

There are c-modules in CPython which are only used for testing. They expose C Types for testing. Would that be sufficient?

mjp41 · 2026-02-26T16:46:50Z

Objects/codeobject.c

+    if (co->_co_cached != NULL) {
+        Py_VISIT(co->_co_cached->_co_code);
+        Py_VISIT(co->_co_cached->_co_varnames);
+        Py_VISIT(co->_co_cached->_co_cellvars);
+        Py_VISIT(co->_co_cached->_co_freevars);
+    }


This makes me worry about write barriers on the cache. There is a lot of stuff around modifying this for NoGIL

mjp41 · 2026-02-27T09:40:53Z

Objects/typeobject.c

+/*
+ * TODO: We should discuss this.  I am a little confused about the
+ * correct way to do this.  This gives a good level of compatibility,
+ * but is perhaps too much.
+ *


Can we discuss this?

xFrednet · 2026-03-02T10:54:55Z

Objects/dictobject.c

+static int
+dict_reachable(PyObject *op, visitproc visit, void *arg)


NIT: When I implemented this, I used a dict_visit function with a flag to indicate it the keys should be used or not:

cpython/Objects/dictobject.c

Lines 4975 to 5025 in e6fe674

static int

dict_visit(PyObject *op, visitproc visit, void *arg, bool visit_unicode_keys)

{

PyDictObject *mp = (PyDictObject *)op;

PyDictKeysObject *keys = mp->ma_keys;

Py_ssize_t i, n = keys->dk_nentries;

if (DK_IS_UNICODE(keys)) {

if (_PyDict_HasSplitTable(mp)) {

if (!mp->ma_values->embedded) {

for (i = 0; i < n; i++) {

if (visit_unicode_keys) {

Py_VISIT(DK_UNICODE_ENTRIES(mp->ma_keys)[i].me_key);

}

Py_VISIT(mp->ma_values->values[i]);

}

}

}

else {

PyDictUnicodeEntry *entries = DK_UNICODE_ENTRIES(keys);

for (i = 0; i < n; i++) {

if (visit_unicode_keys) {

Py_VISIT(entries[i].me_key);

}

Py_VISIT(entries[i].me_value);

}

}

}

else {

PyDictKeyEntry *entries = DK_ENTRIES(keys);

for (i = 0; i < n; i++) {

if (entries[i].me_value != NULL) {

Py_VISIT(entries[i].me_value);

Py_VISIT(entries[i].me_key);

}

}

}

return 0;

}

static int

dict_traverse(PyObject *op, visitproc visit, void *arg)

{

return dict_visit(op, visit, arg, false);

}

int

_PyDict_Reachable(PyObject *op, visitproc visit, void *arg)

{

return dict_visit(op, visit, arg, true);

}

I think that's slightly cleaner than having two mostly identical visit functions, but this is a style thing and this is good to go as is.

xFrednet · 2026-03-02T10:56:22Z

Objects/exceptions.c

+    .tp_reachable = BaseException_reachable,
+    0,                          /* tp_weaklistoffset */
+    0,                          /* tp_iter */
+    0,                          /* tp_iternext */
+    BaseException_methods,      /* tp_methods */
+    BaseException_members,      /* tp_members */
+    BaseException_getset,       /* tp_getset */
+    0,                          /* tp_base */
+    0,                          /* tp_dict */
+    0,                          /* tp_descr_get */
+    0,                          /* tp_descr_set */
+    offsetof(PyBaseExceptionObject, dict), /* tp_dictoffset */
+    BaseException_init,         /* tp_init */
+    0,                          /* tp_alloc */
+    BaseException_new,          /* tp_new */
+    .tp_vectorcall = BaseException_vectorcall,


This seems weird, why does the change add all of these? Shouldn't it just be

.tp_reachable = BaseException_reachable,

xFrednet reviewed Jan 19, 2026

View reviewed changes

WIP

35c470c

mjp41 force-pushed the tp_reaches branch from 40d1fce to 35c470c Compare February 20, 2026 16:59

mjp41 added 3 commits February 23, 2026 16:02

More types tp_reachable

c5e0f1c

More tp_reachable

64ebf81

Message on using tp_traverse

d807b63

mjp41 commented Feb 27, 2026

View reviewed changes

mjp41 requested a review from xFrednet February 27, 2026 09:41

mjp41 marked this pull request as ready for review February 27, 2026 09:41

Pre-Code review comments.

1d8ae54

mjp41 force-pushed the tp_reaches branch from 01fc9f9 to 1d8ae54 Compare February 27, 2026 16:00

Add a few more types.

675a462

xFrednet reviewed Mar 2, 2026

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Adding tp_reachable#65

Adding tp_reachable#65
mjp41 wants to merge 6 commits intoimmutable-mainfrom
tp_reaches

mjp41 commented Jan 16, 2026 •

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		static int
		dict_reachable(PyObject op, visitproc visit, void arg)

	static int
	dict_visit(PyObject op, visitproc visit, void arg, bool visit_unicode_keys)
	{
	PyDictObject mp = (PyDictObject )op;
	PyDictKeysObject *keys = mp->ma_keys;
	Py_ssize_t i, n = keys->dk_nentries;

	if (DK_IS_UNICODE(keys)) {
	if (_PyDict_HasSplitTable(mp)) {
	if (!mp->ma_values->embedded) {
	for (i = 0; i < n; i++) {
	if (visit_unicode_keys) {
	Py_VISIT(DK_UNICODE_ENTRIES(mp->ma_keys)[i].me_key);
	}
	Py_VISIT(mp->ma_values->values[i]);
	}
	}
	}
	else {
	PyDictUnicodeEntry *entries = DK_UNICODE_ENTRIES(keys);
	for (i = 0; i < n; i++) {
	if (visit_unicode_keys) {
	Py_VISIT(entries[i].me_key);
	}
	Py_VISIT(entries[i].me_value);
	}
	}
	}
	else {
	PyDictKeyEntry *entries = DK_ENTRIES(keys);
	for (i = 0; i < n; i++) {
	if (entries[i].me_value != NULL) {
	Py_VISIT(entries[i].me_value);
	Py_VISIT(entries[i].me_key);
	}
	}
	}
	return 0;
	}

	static int
	dict_traverse(PyObject op, visitproc visit, void arg)
	{
	return dict_visit(op, visit, arg, false);
	}

	int
	_PyDict_Reachable(PyObject op, visitproc visit, void arg)
	{
	return dict_visit(op, visit, arg, true);
	}

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mjp41 commented Jan 16, 2026 •

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