gh-159739: Reject the minimum dynamic formatting width

Backport to 3.15 and 3.14 (adapt the str formatting fix to Objects/unicodeobject.c on 3.14).

Author: JelleZijlstra

Lib/test/test_format.py

@@ -324,6 +324,8 @@ def test_common_format(self):
                         "format argument 1: too big for width")
         test_exc_common('%*r', (-2**1000, 1), OverflowError,
                         "format argument 1: too big for width")
+        test_exc_common('%*r', (-maxsize - 1, 1), OverflowError,
+                        "format argument 1: too big for width")
         test_exc_common('%.*r', (2**1000, 1), OverflowError,
                         "format argument 1: too big for precision")
         test_exc_common('%.*r', (-2**1000, 1), OverflowError,

Misc/NEWS.d/next/Core_and_Builtins/2026-10-19-20-38-17.gh-issue-159739.78e530.rst

@@ -0,0 +1,2 @@
+Raise :exc:`OverflowError` when the absolute value of a dynamic printf-style
+formatting width is too large.

Objects/bytesobject.c

@@ -852,6 +852,10 @@ _PyBytes_FormatEx(const char *format, Py_ssize_t format_len,
                     goto error;
                 }
                 if (width < 0) {
+                    if (width == PY_SSIZE_T_MIN) {
+                        FORMAT_ERROR(PyExc_OverflowError, "too big for width%s", "");
+                        goto error;
+                    }
                     flags |= F_LJUST;
                     width = -width;
                 }

Objects/unicode_format.c

@@ -564,6 +564,10 @@ unicode_format_arg_parse(struct unicode_formatter_t *ctx,
             return -1;
         }
         if (arg->width < 0) {
+            if (arg->width == PY_SSIZE_T_MIN) {
+                FORMAT_ERROR(PyExc_OverflowError, "too big for width%s", "");
+                return -1;
+            }
             arg->flags |= F_LJUST;
             arg->width = -arg->width;
         }