bpo-39829: __len__() called twice in the list() constructor

thatbirdguythatuknownot · web-flow · commit 0a5d672e2a53 · 2022-03-08T13:38:08.000+08:00
diff --git a/Objects/listobject.c b/Objects/listobject.c
@@ -889,9 +889,16 @@ list_extend(PyListObject *self, PyObject *iterable)
         /* It should not be possible to allocate a list large enough to cause
         an overflow on any relevant platform */
         assert(m < PY_SSIZE_T_MAX - n);
-        if (list_resize(self, m + n) < 0) {
-            Py_DECREF(iterable);
-            return NULL;
+        if (self->ob_item == NULL) {
+            if (list_preallocate_exact(self, n) < 0) {
+                Py_DECREF(iterable);
+                return NULL;
+            }
+        else {
+            if (list_resize(self, m + n) < 0) {
+                Py_DECREF(iterable);
+                return NULL;
+            }
         }
         /* note that we may still have self == iterable here for the
          * situation a.extend(a), but the following code works
@@ -929,10 +936,16 @@ list_extend(PyListObject *self, PyObject *iterable)
          */
     }
     else {
-        mn = m + n;
-        /* Make room. */
-        if (list_resize(self, mn) < 0)
-            goto error;
+        if (self->ob_item == NULL) {
+            if (list_preallocate_exact(self, n) < 0)
+                goto error;
+        }
+        else {
+            mn = m + n;
+            /* Make room. */
+            if (list_resize(self, mn) < 0)
+                goto error;
+        }
         /* Make the list sane again. */
         Py_SET_SIZE(self, m);
     }
@@ -2814,19 +2827,6 @@ list___init___impl(PyListObject *self, PyObject *iterable)
         (void)_list_clear(self);
     }
     if (iterable != NULL) {
-        if (_PyObject_HasLen(iterable)) {
-            Py_ssize_t iter_len = PyObject_Size(iterable);
-            if (iter_len == -1) {
-                if (!PyErr_ExceptionMatches(PyExc_TypeError)) {
-                    return -1;
-                }
-                PyErr_Clear();
-            }
-            if (iter_len > 0 && self->ob_item == NULL
-                && list_preallocate_exact(self, iter_len)) {
-                return -1;
-            }
-        }
         PyObject *rv = list_extend(self, iterable);
         if (rv == NULL)
             return -1;
