Issue #19646: repr(dict) now uses _PyUnicodeWriter API for better performances

Objects/dictobject.c

@@ -1397,9 +1397,9 @@ static PyObject *
 dict_repr(PyDictObject *mp)
 {
     Py_ssize_t i;
-    PyObject *s, *temp, *colon = NULL;
-    PyObject *pieces = NULL, *result = NULL;
-    PyObject *key, *value;
+    PyObject *key = NULL, *value = NULL;
+    _PyUnicodeWriter writer;
+    int first;
 
     i = Py_ReprEnter((PyObject *)mp);
     if (i != 0) {
@@ -1407,74 +1407,73 @@ dict_repr(PyDictObject *mp)
     }
 
     if (mp->ma_used == 0) {
-        result = PyUnicode_FromString("{}");
-        goto Done;
+        Py_ReprLeave((PyObject *)mp);
+        return PyUnicode_FromString("{}");
     }
 
-    pieces = PyList_New(0);
-    if (pieces == NULL)
-        goto Done;
+    _PyUnicodeWriter_Init(&writer);
+    writer.overallocate = 1;
+    /* "{" + "1: 2" + ", 3: 4" * (len - 1) + "}" */
+    writer.min_length = 1 + 4 + (2 + 4) * (mp->ma_used - 1) + 1;
 
-    colon = PyUnicode_FromString(": ");
-    if (colon == NULL)
-        goto Done;
+    if (_PyUnicodeWriter_WriteChar(&writer, '{') < 0)
+        goto error;
 
     /* Do repr() on each key+value pair, and insert ": " between them.
        Note that repr may mutate the dict. */
     i = 0;
+    first = 1;
     while (PyDict_Next((PyObject *)mp, &i, &key, &value)) {
-        int status;
+        PyObject *s;
+        int res;
+
         /* Prevent repr from deleting key or value during key format. */
         Py_INCREF(key);
         Py_INCREF(value);
+
+        if (!first) {
+            if (_PyUnicodeWriter_WriteASCIIString(&writer, ", ", 2) < 0)
+                goto error;
+        }
+        first = 0;
+
         s = PyObject_Repr(key);
-        PyUnicode_Append(&s, colon);
         if (s == NULL)
-            goto Done;
+            goto error;
+        res = _PyUnicodeWriter_WriteStr(&writer, s);
+        Py_DECREF(s);
+        if (res < 0)
+            goto error;
 
-        PyUnicode_AppendAndDel(&s, PyObject_Repr(value));
-        Py_DECREF(key);
-        Py_DECREF(value);
+        if (_PyUnicodeWriter_WriteASCIIString(&writer, ": ", 2) < 0)
+            goto error;
+
+        s = PyObject_Repr(value);
         if (s == NULL)
-            goto Done;
-        status = PyList_Append(pieces, s);
-        Py_DECREF(s);  /* append created a new ref */
-        if (status < 0)
-            goto Done;
-    }
-
-    /* Add "{}" decorations to the first and last items. */
-    assert(PyList_GET_SIZE(pieces) > 0);
-    s = PyUnicode_FromString("{");
-    if (s == NULL)
-        goto Done;
-    temp = PyList_GET_ITEM(pieces, 0);
-    PyUnicode_AppendAndDel(&s, temp);
-    PyList_SET_ITEM(pieces, 0, s);
-    if (s == NULL)
-        goto Done;
-
-    s = PyUnicode_FromString("}");
-    if (s == NULL)
-        goto Done;
-    temp = PyList_GET_ITEM(pieces, PyList_GET_SIZE(pieces) - 1);
-    PyUnicode_AppendAndDel(&temp, s);
-    PyList_SET_ITEM(pieces, PyList_GET_SIZE(pieces) - 1, temp);
-    if (temp == NULL)
-        goto Done;
-
-    /* Paste them all together with ", " between. */
-    s = PyUnicode_FromString(", ");
-    if (s == NULL)
-        goto Done;
-    result = PyUnicode_Join(s, pieces);
-    Py_DECREF(s);
-
-Done:
-    Py_XDECREF(pieces);
-    Py_XDECREF(colon);
+            goto error;
+        res = _PyUnicodeWriter_WriteStr(&writer, s);
+        Py_DECREF(s);
+        if (res < 0)
+            goto error;
+
+        Py_CLEAR(key);
+        Py_CLEAR(value);
+    }
+
+    writer.overallocate = 0;
+    if (_PyUnicodeWriter_WriteChar(&writer, '}') < 0)
+        goto error;
+
     Py_ReprLeave((PyObject *)mp);
-    return result;
+
+    return _PyUnicodeWriter_Finish(&writer);
+
+error:
+    Py_ReprLeave((PyObject *)mp);
+    _PyUnicodeWriter_Dealloc(&writer);
+    Py_XDECREF(key);
+    Py_XDECREF(value);
+    return NULL;
 }
 
 static Py_ssize_t