Fix panic from test_int__format__locale

fix #4588

Lib/test/test_types.py

@@ -403,7 +403,6 @@ class TypesTests(unittest.TestCase):
             self.assertEqual(locale.format_string('%g', x, grouping=True), format(x, 'n'))
             self.assertEqual(locale.format_string('%.10g', x, grouping=True), format(x, '.10n'))
 
-    @unittest.skip("TODO: RustPython format code n is not integrated with locale")
     @run_with_locale('LC_NUMERIC', 'en_US.UTF8')
     def test_int__format__locale(self):
         # test locale support for __format__ code 'n' for integers
@@ -422,6 +421,9 @@ class TypesTests(unittest.TestCase):
             self.assertEqual(len(format(0, rfmt)), len(format(x, rfmt)))
             self.assertEqual(len(format(0, lfmt)), len(format(x, lfmt)))
             self.assertEqual(len(format(0, cfmt)), len(format(x, cfmt)))
+    
+    if sys.platform != "darwin":
+        test_int__format__locale = unittest.expectedFailure(test_int__format__locale)
 
     def test_float__format__(self):
         def test(f, format_spec, result):

common/src/format.rs

@@ -335,10 +335,11 @@ impl FormatSpec {
         let offset = (disp_digit_cnt % (inter + 1) == 0) as i32;
         let disp_digit_cnt = disp_digit_cnt + offset;
         let pad_cnt = disp_digit_cnt - magnitude_len;
-        if pad_cnt > 0 {
+        let sep_cnt = disp_digit_cnt / (inter + 1);
+        let diff = pad_cnt - sep_cnt;
+        if pad_cnt > 0 && diff > 0 {
             // separate with 0 padding
-            let sep_cnt = disp_digit_cnt / (inter + 1);
-            let padding = "0".repeat((pad_cnt - sep_cnt) as usize);
+            let padding = "0".repeat(diff as usize);
             let padded_num = format!("{padding}{magnitude_str}");
             FormatSpec::insert_separator(padded_num, inter, sep, sep_cnt)
         } else {

extra_tests/snippets/builtin_format.py

@@ -133,3 +133,9 @@ assert f"{3.1415:#.1}" == "3.e+00"
 assert f"{3.1415:#.2}" == "3.1"
 assert f"{3.1415:#.3}" == "3.14"
 assert f"{3.1415:#.4}" == "3.142"
+
+# test issue 4558
+x = 123456789012345678901234567890
+for i in range(0, 30):
+    format(x, ',')
+    x = x // 10