gh-149807: Fix hash(frozendict): compute (key, value) pair hash

Author: vstinner

Lib/test/test_dict.py

@@ -1903,6 +1903,17 @@ def test_hash(self):
         self.assertEqual(hash(frozendict(x=1, y=2)),
                          hash(frozendict(y=2, x=1)))
 
+        # Check that hash() computes the hash of (key, value) pairs
+        cases = [
+            frozendict(a=False, b=True, c=True),
+            frozendict(a=True, b=False, c=True),
+            frozendict(a=True, b=True, c=False),
+            frozendict({False: "a", "b": True, "c": True}),
+            frozendict({"a": "b", False: True, True: "c"}),
+        ]
+        hashes = {hash(fd) for fd in cases}
+        self.assertEqual(len(hashes), 5)
+
         fd = frozendict(x=[1], y=[2])
         with self.assertRaisesRegex(TypeError, "unhashable type: 'list'"):
             hash(fd)

Misc/NEWS.d/next/Core_and_Builtins/2026-05-14-19-41-03.gh-issue-149807.IwGaCo.rst

@@ -0,0 +1,2 @@
+Fix ``hash(frozendict)``: compute the hash of each ``(key, value)`` pair
+correctly. Patch by Victor Stinner.

Objects/dictobject.c

@@ -8228,6 +8228,40 @@ _shuffle_bits(Py_uhash_t h)
     return ((h ^ 89869747UL) ^ (h << 16)) * 3644798167UL;
 }
 
+// Compute hash((key, value)).
+// Code copied from tuple_hash().
+static Py_hash_t
+frozendict_pair_hash(PyObject *key, PyObject *value)
+{
+    Py_ssize_t len = 2;
+    Py_uhash_t acc = _PyTuple_HASH_XXPRIME_5;
+
+    Py_uhash_t lane = PyObject_Hash(key);
+    if (lane == (Py_uhash_t)-1) {
+        return -1;
+    }
+    acc += lane * _PyTuple_HASH_XXPRIME_2;
+    acc = _PyTuple_HASH_XXROTATE(acc);
+    acc *= _PyTuple_HASH_XXPRIME_1;
+
+    lane = PyObject_Hash(value);
+    if (lane == (Py_uhash_t)-1) {
+        return -1;
+    }
+    acc += lane * _PyTuple_HASH_XXPRIME_2;
+    acc = _PyTuple_HASH_XXROTATE(acc);
+    acc *= _PyTuple_HASH_XXPRIME_1;
+
+    /* Add input length, mangled to keep the historical value of hash(()). */
+    acc += len ^ (_PyTuple_HASH_XXPRIME_5 ^ 3527539UL);
+
+    if (acc == (Py_uhash_t)-1) {
+        acc = 1546275796;
+    }
+    return acc;
+}
+
+
 // Code copied from frozenset_hash()
 static Py_hash_t
 frozendict_hash(PyObject *op)
@@ -8244,17 +8278,11 @@ frozendict_hash(PyObject *op)
     PyObject *key, *value;  // borrowed refs
     Py_ssize_t pos = 0;
     while (PyDict_Next(op, &pos, &key, &value)) {
-        Py_hash_t key_hash = PyObject_Hash(key);
-        if (key_hash == -1) {
-            return -1;
-        }
-        hash ^= _shuffle_bits(key_hash);
-
-        Py_hash_t value_hash = PyObject_Hash(value);
-        if (value_hash == -1) {
+        Py_hash_t pair_hash = frozendict_pair_hash(key, value);
+        if (pair_hash == -1) {
             return -1;
         }
-        hash ^= _shuffle_bits(value_hash);
+        hash ^= _shuffle_bits(pair_hash);
     }
 
     /* Factor in the number of active entries */