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GH-92678: Expose managed dict clear and visit functions #95246

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Jul 25, 2022
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GH-92678: Expose managed dict clear and visit functions #95246

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6946148
Expose functions to clear and visit managed dictionaries.
markshannon Jul 25, 2022
7879a6a
Add a test
markshannon Jul 25, 2022
d322c68
add news
markshannon Jul 25, 2022
da8c7eb
Use faster inline function.
markshannon Jul 25, 2022
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3 changes: 3 additions & 0 deletions Include/cpython/dictobject.h
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Original file line number Diff line number Diff line change
Expand Up @@ -83,3 +83,6 @@ typedef struct {

PyAPI_FUNC(PyObject *) _PyDictView_New(PyObject *, PyTypeObject *);
PyAPI_FUNC(PyObject *) _PyDictView_Intersect(PyObject* self, PyObject *other);

PyAPI_FUNC(int) _PyObject_VisitManagedDict(PyObject *self, visitproc visit, void *arg);
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I think this is quite a bit weird: these functions receive an object and then visit/clear the dict. This is not an API for dictionary objects, it's an API for instances so I think it should not be here but in object.h or some other header.

Looking at these functions, at first sight, it seems that "self" is the dictionary and not the oject.

PyAPI_FUNC(void) _PyObject_ClearManagedDict(PyObject *self);
14 changes: 14 additions & 0 deletions Lib/test/test_capi.py
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Expand Up @@ -722,6 +722,20 @@ def test_export_symbols(self):
with self.subTest(name=name):
self.assertTrue(hasattr(ctypes.pythonapi, name))

def test_clear_managed_dict(self):

class C:
def __init__(self):
self.a = 1

c = C()
_testcapi.clear_managed_dict(c)
self.assertEqual(c.__dict__, {})
c = C()
self.assertEqual(c.__dict__, {'a':1})
_testcapi.clear_managed_dict(c)
self.assertEqual(c.__dict__, {})


class TestPendingCalls(unittest.TestCase):

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/C API/2022-07-25-15-54-27.gh-issue-92678.ziZpxz.rst
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@@ -0,0 +1,3 @@
Adds unstable C-API functions ``_PyObject_VisitManagedDict`` and
``_PyObject_ClearManagedDict`` to allow C extensions to allow the VM to
manage their object's dictionaries.
8 changes: 8 additions & 0 deletions Modules/_testcapimodule.c
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Expand Up @@ -6009,6 +6009,13 @@ settrace_to_record(PyObject *self, PyObject *list)
Py_RETURN_NONE;
}

static PyObject *
clear_managed_dict(PyObject *self, PyObject *obj)
{
_PyObject_ClearManagedDict(obj);
Py_RETURN_NONE;
}


static PyObject *
test_macros(PyObject *self, PyObject *Py_UNUSED(args))
Expand Down Expand Up @@ -6347,6 +6354,7 @@ static PyMethodDef TestMethods[] = {
{"test_code_api", test_code_api, METH_NOARGS, NULL},
{"settrace_to_record", settrace_to_record, METH_O, NULL},
{"test_macros", test_macros, METH_NOARGS, NULL},
{"clear_managed_dict", clear_managed_dict, METH_O, NULL},
{NULL, NULL} /* sentinel */
};

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29 changes: 29 additions & 0 deletions Objects/dictobject.c
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Expand Up @@ -5583,6 +5583,35 @@ _PyObject_FreeInstanceAttributes(PyObject *self)
free_values(*values_ptr);
}

int
_PyObject_VisitManagedDict(PyObject *self, visitproc visit, void *arg)
{
PyTypeObject *tp = Py_TYPE(self);
if((tp->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0) {
return 0;
}
assert(tp->tp_dictoffset);
int err = _PyObject_VisitInstanceAttributes(self, visit, arg);
if (err) {
return err;
}
Py_VISIT(*_PyObject_DictPointer(self));
return 0;
}


void
_PyObject_ClearManagedDict(PyObject *self)
{
PyTypeObject *tp = Py_TYPE(self);
if((tp->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0) {
return;
}
_PyObject_FreeInstanceAttributes(self);
*_PyObject_ValuesPointer(self) = NULL;
Py_CLEAR(*_PyObject_DictPointer(self));
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}

PyObject *
PyObject_GenericGetDict(PyObject *obj, void *context)
{
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