gh-110222: Add support of PyStructSequence in copy.replace() (GH-110223)

Objects/structseq.c

@@ -8,6 +8,7 @@
 */
 
 #include "Python.h"
+#include "pycore_dict.h"          // _PyDict_Pop()
 #include "pycore_tuple.h"         // _PyTuple_FromArray()
 #include "pycore_object.h"        // _PyObject_GC_TRACK()
 
@@ -380,9 +381,82 @@ error:
     return NULL;
 }
 
+
+static PyObject *
+structseq_replace(PyStructSequence *self, PyObject *args, PyObject *kwargs)
+{
+    PyStructSequence *result = NULL;
+    Py_ssize_t n_fields, n_unnamed_fields, i;
+
+    if (!_PyArg_NoPositional("__replace__", args)) {
+        return NULL;
+    }
+
+    n_fields = REAL_SIZE(self);
+    if (n_fields < 0) {
+        return NULL;
+    }
+    n_unnamed_fields = UNNAMED_FIELDS(self);
+    if (n_unnamed_fields < 0) {
+        return NULL;
+    }
+    if (n_unnamed_fields > 0) {
+        PyErr_Format(PyExc_TypeError,
+                     "__replace__() is not supported for %.500s "
+                     "because it has unnamed field(s)",
+                     Py_TYPE(self)->tp_name);
+        return NULL;
+    }
+
+    result = (PyStructSequence *) PyStructSequence_New(Py_TYPE(self));
+    if (!result) {
+        return NULL;
+    }
+
+    if (kwargs != NULL) {
+        // We do not support types with unnamed fields, so we can iterate over
+        // i >= n_visible_fields case without slicing with (i - n_unnamed_fields).
+        for (i = 0; i < n_fields; ++i) {
+            PyObject *key = PyUnicode_FromString(Py_TYPE(self)->tp_members[i].name);
+            if (!key) {
+                goto error;
+            }
+            PyObject *ob = _PyDict_Pop(kwargs, key, self->ob_item[i]);
+            Py_DECREF(key);
+            if (!ob) {
+                goto error;
+            }
+            result->ob_item[i] = ob;
+        }
+        // Check if there are any unexpected fields.
+        if (PyDict_GET_SIZE(kwargs) > 0) {
+            PyObject *names = PyDict_Keys(kwargs);
+            if (names) {
+                PyErr_Format(PyExc_TypeError, "Got unexpected field name(s): %R", names);
+                Py_DECREF(names);
+            }
+            goto error;
+        }
+    }
+    else
+    {
+        // Just create a copy of the original.
+        for (i = 0; i < n_fields; ++i) {
+            result->ob_item[i] = Py_NewRef(self->ob_item[i]);
+        }
+    }
+
+    return (PyObject *)result;
+
+error:
+    Py_DECREF(result);
+    return NULL;
+}
+
 static PyMethodDef structseq_methods[] = {
     {"__reduce__", (PyCFunction)structseq_reduce, METH_NOARGS, NULL},
-    {NULL, NULL}
+    {"__replace__", _PyCFunction_CAST(structseq_replace), METH_VARARGS | METH_KEYWORDS, NULL},
+    {NULL, NULL}  // sentinel
 };
 
 static Py_ssize_t