Close issue #8931: Make alternate formatting for 'c' raise an exception. Patch by Torsten Landschoff.

Lib/test/test_types.py

@@ -343,6 +343,8 @@ class TypesTests(unittest.TestCase):
         self.assertRaises(ValueError, 3 .__format__, ",n")
         # can't have ',' with 'c'
         self.assertRaises(ValueError, 3 .__format__, ",c")
+        # can't have '#' with 'c'
+        self.assertRaises(ValueError, 3 .__format__, "#c")
 
         # ensure that only int and float type specifiers work
         for format_spec in ([chr(x) for x in range(ord('a'), ord('z')+1)] +

Misc/NEWS

@@ -43,6 +43,10 @@ Core and Builtins
   replacement fields. It now matches the behavior of str.format() in
   this regard. Patches by Phil Elson and Ramchandra Apte.
 
+- Issue #8931: Make alternate formatting ('#') for type 'c' raise an
+  exception. In versions prior to 3.5, '#' with 'c' had no effect. Now
+  specifying it is an error.  Patch by Torsten Landschoff.
+
 Library
 -------
 

Python/formatter_unicode.c

@@ -846,6 +846,13 @@ format_long_internal(PyObject *value, const InternalFormatSpec *format,
                             " format specifier 'c'");
             goto done;
         }
+        /* error to request alternate format */
+        if (format->alternate) {
+            PyErr_SetString(PyExc_ValueError,
+                            "Alternate form (#) not allowed with integer"
+                            " format specifier 'c'");
+            goto done;
+        }
 
         /* taken from unicodeobject.c formatchar() */
         /* Integer input truncated to a character */