python · colesbury · Feb 27, 2026 · Feb 27, 2026
diff --git a/Lib/test/test_free_threading/test_str.py b/Lib/test/test_free_threading/test_str.py
@@ -69,6 +69,22 @@ def reader_func():
        for reader in readers:
            reader.join()

+    def test_maketrans_dict_concurrent_modification(self):
+        for _ in range(5):
+            d = {2000: 'a'}
+
+            def work(dct):
+                for i in range(100):
+                    str.maketrans(dct)
+                    dct[2000 + i] = chr(i % 16)
+                    dct.pop(2000 + i, None)
+
+            threading_helper.run_concurrently(
+                work,
+                nthreads=5,
+                args=(d,),
+            )
+

 if __name__ == "__main__":
    unittest.main()
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-02-23-23-18-28.gh-issue-145142.T-XbVe.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-02-23-23-18-28.gh-issue-145142.T-XbVe.rst
@@ -0,0 +1,2 @@
+Fix a crash in the free-threaded build when the dictionary argument to
+:meth:`str.maketrans` is concurrently modified.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
@@ -13366,6 +13366,45 @@ unicode_swapcase_impl(PyObject *self)
    return case_operation(self, do_swapcase);
 }

+static int
+unicode_maketrans_from_dict(PyObject *x, PyObject *newdict)
+{
+    PyObject *key, *value;
+    Py_ssize_t i = 0;
+    int res;
+    while (PyDict_Next(x, &i, &key, &value)) {
+        if (PyUnicode_Check(key)) {
+            PyObject *newkey;
+            int kind;
+            const void *data;
+            if (PyUnicode_GET_LENGTH(key) != 1) {
+                PyErr_SetString(PyExc_ValueError, "string keys in translate"
+                                "table must be of length 1");
+                return -1;
+            }
+            kind = PyUnicode_KIND(key);
+            data = PyUnicode_DATA(key);
+            newkey = PyLong_FromLong(PyUnicode_READ(kind, data, 0));
+            if (!newkey)
+                return -1;
+            res = PyDict_SetItem(newdict, newkey, value);
+            Py_DECREF(newkey);
+            if (res < 0)
+                return -1;
+        }
+        else if (PyLong_Check(key)) {
+            if (PyDict_SetItem(newdict, key, value) < 0)
+                return -1;
+        }
+        else {
+            PyErr_SetString(PyExc_TypeError, "keys in translate table must"
+                            "be strings or integers");
+            return -1;
+        }
+    }
+    return 0;
+}
+
 /*[clinic input]

 @staticmethod
@@ -13451,44 +13490,19 @@ unicode_maketrans_impl(PyObject *x, PyObject *y, PyObject *z)
            }
        }
    } else {
-        int kind;
-        const void *data;
-
        /* x must be a dict */
        if (!PyDict_CheckExact(x)) {
            PyErr_SetString(PyExc_TypeError, "if you give only one argument "
                            "to maketrans it must be a dict");
            goto err;
        }
        /* copy entries into the new dict, converting string keys to int keys */
-        while (PyDict_Next(x, &i, &key, &value)) {
-            if (PyUnicode_Check(key)) {
-                /* convert string keys to integer keys */
-                PyObject *newkey;
-                if (PyUnicode_GET_LENGTH(key) != 1) {
-                    PyErr_SetString(PyExc_ValueError, "string keys in translate "
-                                    "table must be of length 1");
-                    goto err;
-                }
-                kind = PyUnicode_KIND(key);
-                data = PyUnicode_DATA(key);
-                newkey = PyLong_FromLong(PyUnicode_READ(kind, data, 0));
-                if (!newkey)
-                    goto err;
-                res = PyDict_SetItem(new, newkey, value);
-                Py_DECREF(newkey);
-                if (res < 0)
-                    goto err;
-            } else if (PyLong_Check(key)) {
-                /* just keep integer keys */
-                if (PyDict_SetItem(new, key, value) < 0)
-                    goto err;
-            } else {
-                PyErr_SetString(PyExc_TypeError, "keys in translate table must "
-                                "be strings or integers");
-                goto err;
-            }
-        }
+        int errcode;
+        Py_BEGIN_CRITICAL_SECTION(x);
+        errcode = unicode_maketrans_from_dict(x, new);
+        Py_END_CRITICAL_SECTION();
+        if (errcode < 0)
+            goto err;
    }
    return new;
  err:
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Fix a crash in the free-threaded build when the dictionary argument to
		:meth:`str.maketrans` is concurrently modified.