Reject format spec with width above i32::MAX (RustPython#7707)

changjoon-park · web-flow · commit b3d6d2f247b8 · 2026-04-28T21:11:33.000+09:00
CPython rejects format-spec widths that exceed Py_ssize_t::MAX with
ValueError: Too many decimal digits in format string. RustPython's
FormatSpec::_parse only capped precision (via parse_precision); width
was accepted up to usize::MAX, so values like sys.maxsize + 1 silently
produced an effectively-ignored width.

Reject any width above i32::MAX with FormatSpecError::DecimalDigitsTooMany,
matching the existing precision cap and producing the byte-identical
ValueError wording.

Unmasks test_str.StrTest.test_format_huge_width.
diff --git a/Lib/test/test_str.py b/Lib/test/test_str.py
@@ -1463,7 +1463,6 @@ def test_format_huge_precision(self):
         with self.assertRaises(ValueError):
             result = format(2.34, format_string)
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; AssertionError: ValueError not raised
     def test_format_huge_width(self):
         format_string = "{}f".format(sys.maxsize + 1)
         with self.assertRaises(ValueError):
diff --git a/crates/common/src/format.rs b/crates/common/src/format.rs
@@ -315,6 +315,11 @@ impl FormatSpec {
         let (alternate_form, text) = parse_alternate_form(text);
         let (zero, text) = parse_zero(text);
         let (width, text) = parse_number(text)?;
+        if let Some(w) = width
+            && w > i32::MAX as usize
+        {
+            return Err(FormatSpecError::DecimalDigitsTooMany);
+        }
         let (grouping_option, text) = FormatGrouping::parse(text);
         if let Some(grouping) = &grouping_option {
             Self::validate_separator(grouping, text)?;
