[Python-checkins] bpo-43945: [Enum] Deprecate non-standard mixin format() behavior (GH-25649)

[ethanfurman]

https://github.com/python/cpython/commit/5987b8c463892e0ab7a63cdae92f34b5eb79732d
commit: 5987b8c463892e0ab7a63cdae92f34b5eb79732d
branch: master
author: Ethan Furman <ethan at stoneleaf.us>
committer: ethanfurman <ethan at stoneleaf.us>
date: 2021-04-26T22:42:57-07:00
summary:

bpo-43945: [Enum] Deprecate non-standard mixin format() behavior (GH-25649)

In 3.12 the enum member, not the member's value, will be used for
format() calls.  Format specifiers can be used to retain the current
display of enum members:

Example enumeration:

    class Color(IntEnum):
        RED = 1
        GREEN = 2
        BLUE = 3

Current behavior:

    f'{Color.RED}'  -->  '1'

Future behavior:

    f'{Color.RED}'  --> 'RED'

Using d specifier:

    f'{Color.RED:d}'  --> '1'

Using specifiers can be done now and is future-compatible.

files:
A Misc/NEWS.d/next/Library/2021-04-26-20-52-16.bpo-43945.NgERXO.rst
M Lib/enum.py
M Lib/test/test_enum.py

diff --git a/Lib/enum.py b/Lib/enum.py
index ca6aff6f82512d..bcf411c6b6550c 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -1005,6 +1005,14 @@ def __format__(self, format_spec):
             val = str(self)
         # mix-in branch
         else:
+            import warnings
+            warnings.warn(
+                    "in 3.12 format() will use the enum member, not the enum member's value;\n"
+                    "use a format specifier, such as :d for an IntEnum member, to maintain"
+                    "the current display",
+                    DeprecationWarning,
+                    stacklevel=2,
+                    )
             cls = self._member_type_
             val = self._value_
         return cls.__format__(val, format_spec)
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index 4f28b80293d352..983c54b8711991 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -528,6 +528,14 @@ def __format__(self, spec):
         self.assertEqual(str(TestFloat.one), 'one')
         self.assertEqual('{}'.format(TestFloat.one), 'TestFloat success!')
 
+    @unittest.skipUnless(
+            sys.version_info[:2] < (3, 12),
+            'mixin-format now uses member instead of member.value',
+            )
+    def test_mixin_format_warning(self):
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(f'{self.Grades.B}', '4')
+
     def assertFormatIsValue(self, spec, member):
         self.assertEqual(spec.format(member), spec.format(member.value))
 
diff --git a/Misc/NEWS.d/next/Library/2021-04-26-20-52-16.bpo-43945.NgERXO.rst b/Misc/NEWS.d/next/Library/2021-04-26-20-52-16.bpo-43945.NgERXO.rst
new file mode 100644
index 00000000000000..c01c200caa7801
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-04-26-20-52-16.bpo-43945.NgERXO.rst
@@ -0,0 +1,2 @@
+[Enum] Deprecate non-standard mixin format() behavior: in 3.12 the enum
+member, not the member's value, will be used for format() calls.