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gh-155315: Fix marshal round-trip of shared frozendict references #155316

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gh-155315: Fix marshal round-trip of shared frozendict references #155316

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10 changes: 10 additions & 0 deletions Lib/test/test_marshal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -386,6 +386,16 @@ def test_reference_loop_frozendict(self):
for v in range(marshal.version + 1):
self.assertRaises(ValueError, marshal.dumps, a, v)

def test_shared_reference_frozendict(self):
# A frozendict referenced more than once must round-trip with the
# shared identity preserved, like frozenset.
fd = frozendict({'a': 1, 'b': 2})
out = marshal.loads(marshal.dumps([fd, fd]))
self.assertEqual(out[0], fd)
self.assertIs(out[0], out[1])
nested = marshal.loads(marshal.dumps(frozendict({'x': fd, 'y': fd})))
self.assertIs(nested['x'], nested['y'])

def test_loads_reference_loop_list(self):
data = b'\xdb\x01\x00\x00\x00r\x00\x00\x00\x00' # [<R>]
a = marshal.loads(data)
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-07-14-45-02.gh-issue-155315.Mk9Fd2.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix :mod:`marshal` so that a ``frozendict`` referenced more than once in the
serialized data round-trips correctly, instead of failing to load with
:exc:`ValueError`. Patch by tonghuaroot.
5 changes: 5 additions & 0 deletions Python/marshal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1502,6 +1502,11 @@ r_object(RFILE *p)
}
if (type == TYPE_FROZENDICT && v != NULL) {
Py_SETREF(v, PyFrozenDict_New(v));
/* frozendicts use delayed reference registration (like
* frozensets), so fill the slot reserved above now that the
* object exists; otherwise a later TYPE_REF to a shared
* frozendict resolves to an empty slot. */
v = r_ref_insert(v, idx, flag, p);
}
retval = v;
break;
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