bpo-28146: Fix a confusing error message in str.format() (pythonGH-24213)

Automerge-Triggered-By: GH:pitrou

Author: iritkatriel

Lib/test/test_unicode.py

@@ -1231,8 +1231,11 @@ def __repr__(self):
                           0, 1, 2, 3, 4, 5, 6, 7)
 
         # string format spec errors
-        self.assertRaises(ValueError, "{0:-s}".format, '')
-        self.assertRaises(ValueError, format, "", "-")
+        sign_msg = "Sign not allowed in string format specifier"
+        self.assertRaisesRegex(ValueError, sign_msg, "{0:-s}".format, '')
+        self.assertRaisesRegex(ValueError, sign_msg, format, "", "-")
+        space_msg = "Space not allowed in string format specifier"
+        self.assertRaisesRegex(ValueError, space_msg, "{: }".format, '')
         self.assertRaises(ValueError, "{0:=s}".format, '')
 
         # Alternate formatting is not supported

Misc/NEWS.d/next/Core and Builtins/2021-01-13-19-34-41.bpo-28146.AZBBkH.rst

@@ -0,0 +1 @@
+Fix a confusing error message in :func:`str.format`.

Python/formatter_unicode.c

@@ -773,8 +773,14 @@ format_string_internal(PyObject *value, const InternalFormatSpec *format,
 
     /* sign is not allowed on strings */
     if (format->sign != '\0') {
-        PyErr_SetString(PyExc_ValueError,
-                        "Sign not allowed in string format specifier");
+        if (format->sign == ' ') {
+            PyErr_SetString(PyExc_ValueError,
+                "Space not allowed in string format specifier");
+        }
+        else {
+            PyErr_SetString(PyExc_ValueError,
+                "Sign not allowed in string format specifier");
+        }
         goto done;
     }