bpo-42501: Revise the usage note for Enums with the choices (GH-23563)

restart_subprocess is a method of self, the pyshell.InteractiveInterpreter instance. The latter does not have an interp attribute redundantly referring to itself. (The PyShell instance does have an interp attribute, referring to the InteractiveInterpreter instance.)

Doc/library/argparse.rst

@@ -1133,20 +1133,9 @@ container should match the type_ specified::
 
 Any container can be passed as the *choices* value, so :class:`list` objects,
 :class:`set` objects, and custom containers are all supported.
-This includes :class:`enum.Enum`, which could be used to restrain
-argument's choices; if we reuse previous rock/paper/scissors game example,
-this could be as follows::
 
-   >>> from enum import Enum
-   >>> class GameMove(Enum):
-   ...     ROCK = 'rock'
-   ...     PAPER = 'paper'
-   ...     SCISSORS = 'scissors'
-   ...
-   >>> parser = argparse.ArgumentParser(prog='game.py')
-   >>> parser.add_argument('move', type=GameMove, choices=GameMove)
-   >>> parser.parse_args(['rock'])
-   Namespace(move=<GameMove.ROCK: 'rock'>)
+Use of :class:`enum.Enum` is not recommended because it is difficult to
+control its appearance in usage, help, and error messages.
 
 
 required

Lib/collections/__init__.py

@@ -1001,7 +1001,7 @@ class ChainMap(_collections_abc.MutableMapping):
     def __iter__(self):
         d = {}
         for mapping in reversed(self.maps):
-            d.update(mapping)                   # reuses stored hash values if possible
+            d.update(dict.fromkeys(mapping))    # reuses stored hash values if possible
         return iter(d)
 
     def __contains__(self, key):

Lib/idlelib/pyshell.py

@@ -757,7 +757,7 @@ class ModifiedInterpreter(InteractiveInterpreter):
     def runcode(self, code):
         "Override base class method"
         if self.tkconsole.executing:
-            self.interp.restart_subprocess()
+            self.restart_subprocess()
         self.checklinecache()
         debugger = self.debugger
         try:

Lib/test/test_collections.py

@@ -196,6 +196,22 @@ class TestChainMap(unittest.TestCase):
              ('e', 55), ('f', 666), ('g', 777), ('h', 88888),
              ('i', 9999), ('j', 0)])
 
+    def test_iter_not_calling_getitem_on_maps(self):
+        class DictWithGetItem(UserDict):
+            def __init__(self, *args, **kwds):
+                self.called = False
+                UserDict.__init__(self, *args, **kwds)
+            def __getitem__(self, item):
+                self.called = True
+                UserDict.__getitem__(self, item)
+
+        d = DictWithGetItem(a=1)
+        c = ChainMap(d)
+        d.called = False
+
+        set(c)  # iterate over chain map
+        self.assertFalse(d.called, '__getitem__ was called')
+
     def test_dict_coercion(self):
         d = ChainMap(dict(a=1, b=2), dict(b=20, c=30))
         self.assertEqual(dict(d), dict(a=1, b=2, c=30))

Misc/NEWS.d/next/Library/2020-11-28-04-31-20.bpo-42487.iqtC4L.rst

@@ -0,0 +1 @@
+ChainMap.__iter__ no longer calls __getitem__ on underlying maps