GH-46145: make PyStructSequence compatible with namedtuple

Lib/test/test_structseq.py

@@ -133,6 +133,28 @@ def test_match_args_with_unnamed_fields(self):
         self.assertEqual(os.stat_result.n_unnamed_fields, 3)
         self.assertEqual(os.stat_result.__match_args__, expected_args)
 
+    def test_tuple_field_keys(self):
+        expected_keys = ('tm_year', 'tm_mon', 'tm_mday', 'tm_hour', 'tm_min',
+                         'tm_sec', 'tm_wday', 'tm_yday', 'tm_isdst')
+        self.assertEqual(time.gmtime()._fields, time.struct_time._fields, expected_keys)
+
+    def test_tuple_field_defaults(self):
+        self.assertEqual(time.gmtime()._field_defaults, time.struct_time._field_defaults, {})
+
+    def test_tuple_asdict(self):
+        t = time.gmtime(0)
+        self.assertEqual(t._asdict(), {
+            'tm_year': 1970,
+            'tm_mon': 1,
+            'tm_mday': 1,
+            'tm_hour': 0,
+            'tm_min': 0,
+            'tm_sec': 0,
+            'tm_wday': 3,
+            'tm_yday': 1,
+            'tm_isdst': 0,
+        })
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/C API/2023-08-29-17-27-01.gh-issue-108647.gyXLmO.rst

@@ -0,0 +1,2 @@
+Add :attr:`_fields` and :attr:`_asdict` helpers to :c:type:`PyStructSequence`,
+improving its compatibility with :class:`collections.namedtuple`.

Objects/structseq.c

@@ -18,6 +18,8 @@ static const char visible_length_key[] = "n_sequence_fields";
 static const char real_length_key[] = "n_fields";
 static const char unnamed_fields_key[] = "n_unnamed_fields";
 static const char match_args_key[] = "__match_args__";
+static const char named_fields_list_key[] = "_fields";
+static const char named_fields_defaults_key[] = "_field_defaults";
 
 /* Fields with this name have only a field index, not a field name.
    They are only allowed for indices < n_visible_fields. */
@@ -365,8 +367,35 @@ structseq_reduce(PyStructSequence* self, PyObject *Py_UNUSED(ignored))
     return NULL;
 }
 
+static PyObject *
+structseq_asdict(PyStructSequence* self, PyObject *Py_UNUSED(ignored))
+{
+    PyObject* dict = NULL;
+    Py_ssize_t n_visible_fields, n_unnamed_fields, i;
+
+    n_visible_fields = VISIBLE_SIZE(self);
+    n_unnamed_fields = UNNAMED_FIELDS(self);
+
+    dict = PyDict_New();
+    if (!dict)
+        return NULL;
+
+    for (i = 0; i < n_visible_fields; i++) {
+        const char *n = Py_TYPE(self)->tp_members[i-n_unnamed_fields].name;
+        if (PyDict_SetItemString(dict, n, self->ob_item[i]) < 0)
+            goto error;
+    }
+
+    return dict;
+
+error:
+    Py_DECREF(dict);
+    return NULL;
+}
+
 static PyMethodDef structseq_methods[] = {
     {"__reduce__", (PyCFunction)structseq_reduce, METH_NOARGS, NULL},
+    {"_asdict", (PyCFunction)structseq_asdict, METH_NOARGS, NULL},
     {NULL, NULL}
 };
 
@@ -386,7 +415,7 @@ count_members(PyStructSequence_Desc *desc, Py_ssize_t *n_unnamed_members) {
 static int
 initialize_structseq_dict(PyStructSequence_Desc *desc, PyObject* dict,
                           Py_ssize_t n_members, Py_ssize_t n_unnamed_members) {
-    PyObject *v;
+    PyObject *v, *defaults = NULL;
 
 #define SET_DICT_FROM_SIZE(key, value)                                         \
     do {                                                                       \
@@ -405,7 +434,7 @@ initialize_structseq_dict(PyStructSequence_Desc *desc, PyObject* dict,
     SET_DICT_FROM_SIZE(real_length_key, n_members);
     SET_DICT_FROM_SIZE(unnamed_fields_key, n_unnamed_members);
 
-    // Prepare and set __match_args__
+    // Prepare and set __match_args__ and _fields
     Py_ssize_t i, k;
     PyObject* keys = PyTuple_New(desc->n_in_sequence);
     if (keys == NULL) {
@@ -432,10 +461,25 @@ initialize_structseq_dict(PyStructSequence_Desc *desc, PyObject* dict,
         goto error;
     }
 
+    if (PyDict_SetItemString(dict, named_fields_list_key, keys) < 0) {
+        goto error;
+    }
+
+    // Set _fields_defaults to an empty dir, as we don't support defaults
+    defaults = PyDict_New();
+    if (!defaults)
+        goto error;
+
+    if (PyDict_SetItemString(dict, named_fields_defaults_key, defaults) < 0) {
+        goto error;
+    }
+
+    Py_DECREF(defaults);
     Py_DECREF(keys);
     return 0;
 
 error:
+    Py_XDECREF(defaults);
     Py_DECREF(keys);
     return -1;
 }