GH-131798: Narrow the return type of _FORMAT_SIMPLE and _FORMAT_WITH_SPEC to str

Author: NekoAsakura

Lib/test/test_capi/test_opt.py

@@ -2263,6 +2263,44 @@ def testfunc(n):
         self.assertNotIn("_GUARD_TOS_UNICODE", uops)
         self.assertIn("_BINARY_OP_ADD_UNICODE", uops)
 
+    def test_format_simple_narrows_to_str(self):
+        def testfunc(n):
+            x = []
+            for _ in range(n):
+                v = 42
+                s = f"{v}"
+                t = "hello" + s
+                x.append(t)
+            return x
+
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(res, ["hello42"] * TIER2_THRESHOLD)
+        self.assertIsNotNone(ex)
+        uops = get_opnames(ex)
+
+        self.assertIn("_FORMAT_SIMPLE", uops)
+        self.assertNotIn("_GUARD_TOS_UNICODE", uops)
+        self.assertIn("_BINARY_OP_ADD_UNICODE", uops)
+
+    def test_format_with_spec_narrows_to_str(self):
+        def testfunc(n):
+            x = []
+            for _ in range(n):
+                v = 3.14
+                s = f"{v:.2f}"
+                t = "pi=" + s
+                x.append(t)
+            return x
+
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(res, ["pi=3.14"] * TIER2_THRESHOLD)
+        self.assertIsNotNone(ex)
+        uops = get_opnames(ex)
+
+        self.assertIn("_FORMAT_WITH_SPEC", uops)
+        self.assertNotIn("_GUARD_TOS_UNICODE", uops)
+        self.assertIn("_BINARY_OP_ADD_UNICODE", uops)
+
     def test_binary_op_subscr_str_int(self):
         def testfunc(n):
             x = 0

Misc/NEWS.d/next/Core_and_Builtins/2026-03-30-17-01-34.gh-issue-131798.WSefcr.rst

@@ -0,0 +1,2 @@
+Allow the JIT to remove unicode guards after ``_FORMAT_SIMPLE`` and
+``_FORMAT_WITH_SPEC`` by setting the return type to string.

Python/optimizer_bytecodes.c

@@ -1551,6 +1551,14 @@ dummy_func(void) {
         set = sym_new_type(ctx, &PySet_Type);
     }
 
+    op(_FORMAT_SIMPLE, (value -- res)) {
+        res = sym_new_type(ctx, &PyUnicode_Type);
+    }
+
+    op(_FORMAT_WITH_SPEC, (value, fmt_spec -- res)) {
+        res = sym_new_type(ctx, &PyUnicode_Type);
+    }
+
     op(_SET_UPDATE, (set, unused[oparg-1], iterable -- set, unused[oparg-1], i)) {
         (void)set;
         i = iterable;