Anti-registration of various ABC methods.

- Issue #25958: Support "anti-registration" of special methods from
  various ABCs, like __hash__, __iter__ or __len__.  All these (and
  several more) can be set to None in an implementation class and the
  behavior will be as if the method is not defined at all.
  (Previously, this mechanism existed only for __hash__, to make
  mutable classes unhashable.)  Code contributed by Andrew Barnert and
  Ivan Levkivskyi.

1 files changed, 23 insertions, 0 deletions

diff --git a/Lib/test/test_unicode.py b/Lib/test/test_unicode.py
index a38e7b1610..78f9668e19 100644
--- a/Lib/test/test_unicode.py
+++ b/Lib/test/test_unicode.py
@@ -986,6 +986,19 @@ class UnicodeTest(string_tests.CommonTest,
             def __format__(self, format_spec):
                 return int.__format__(self * 2, format_spec)
 
+        class M:
+            def __init__(self, x):
+                self.x = x
+            def __repr__(self):
+                return 'M(' + self.x + ')'
+            __str__ = None
+
+        class N:
+            def __init__(self, x):
+                self.x = x
+            def __repr__(self):
+                return 'N(' + self.x + ')'
+            __format__ = None
 
         self.assertEqual(''.format(), '')
         self.assertEqual('abc'.format(), 'abc')
@@ -1200,6 +1213,16 @@ class UnicodeTest(string_tests.CommonTest,
 
         self.assertEqual("0x{:0{:d}X}".format(0x0,16), "0x0000000000000000")
 
+        # Blocking fallback
+        m = M('data')
+        self.assertEqual("{!r}".format(m), 'M(data)')
+        self.assertRaises(TypeError, "{!s}".format, m)
+        self.assertRaises(TypeError, "{}".format, m)
+        n = N('data')
+        self.assertEqual("{!r}".format(n), 'N(data)')
+        self.assertEqual("{!s}".format(n), 'N(data)')
+        self.assertRaises(TypeError, "{}".format, n)
+
     def test_format_map(self):
         self.assertEqual(''.format_map({}), '')
         self.assertEqual('a'.format_map({}), 'a')