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gh-141510: Add frozendict_check_mutable() #144944

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gh-141510: Add frozendict_check_mutable() #144944

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bb2c546
gh-141510: Add frozendict_check_mutable()
vstinner Feb 18, 2026
5ecf0f9
Fix typo
vstinner Feb 18, 2026
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18 changes: 18 additions & 0 deletions Lib/test/test_dict.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1787,6 +1787,24 @@ def test_hash(self):
with self.assertRaisesRegex(TypeError, "unhashable type: 'list'"):
hash(fd)

def test_fromkeys(self):
self.assertEqual(frozendict.fromkeys('abc'),
frozendict(a=None, b=None, c=None))

# special class which keeps a reference to the created dictionary
fd = None
class SpecialDict(frozendict):
def __new__(self):
nonlocal fd
fd = frozendict()
return fd

errmsg = "cannot mutate frozendict already exposed in Python"
with self.assertRaisesRegex(RuntimeError, errmsg):
SpecialDict.fromkeys(frozendict(x=1))
with self.assertRaisesRegex(RuntimeError, errmsg):
SpecialDict.fromkeys("def")


if __name__ == "__main__":
unittest.main()
24 changes: 24 additions & 0 deletions Objects/dictobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -138,6 +138,7 @@ As a consequence of this, split keys have a maximum size of 16.
// Forward declarations
static PyObject* frozendict_new(PyTypeObject *type, PyObject *args,
PyObject *kwds);
static int frozendict_check_mutable(PyObject *self);


/*[clinic input]
Expand Down Expand Up @@ -3317,6 +3318,11 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
}
}
else if (PyFrozenDict_CheckExact(d)) {
// Check if the class constructor kept a reference to the frozendict
if (frozendict_check_mutable(d) < 0) {
return NULL;
}

if (PyDict_CheckExact(iterable)) {
PyDictObject *mp = (PyDictObject *)d;

Expand Down Expand Up @@ -3360,6 +3366,11 @@ dict_iter_exit:;
Py_END_CRITICAL_SECTION();
}
else if (PyFrozenDict_CheckExact(d)) {
// Check if the class constructor kept a reference to the frozendict
if (frozendict_check_mutable(d) < 0) {
goto Fail;
}

while ((key = PyIter_Next(it)) != NULL) {
// anydict_setitem_take2 consumes a reference to key
status = anydict_setitem_take2((PyDictObject *)d,
Expand Down Expand Up @@ -7889,6 +7900,17 @@ _PyObject_InlineValuesConsistencyCheck(PyObject *obj)

// --- frozendict implementation ---------------------------------------------

static int
frozendict_check_mutable(PyObject *self)
{
if (Py_REFCNT(self) > 1) {
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@corona10 corona10 Feb 18, 2026 •
edited
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IIRC, Checking Py_REFCNT at runtime is not thread safe at free-threaded build.

PyErr_SetString(PyExc_RuntimeError,
"cannot mutate frozendict already exposed in Python");
return -1;
}
return 0;
}

static PyNumberMethods frozendict_as_number = {
.nb_or = frozendict_or,
};
Expand Down Expand Up @@ -7994,6 +8016,8 @@ frozendict_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
if (d == NULL) {
return NULL;
}
assert(Py_REFCNT(d) == 1);

PyFrozenDictObject *self = _PyFrozenDictObject_CAST(d);
self->ma_hash = -1;

Expand Down
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