bpo-41816: add `StrEnum` (GH-22337)

`StrEnum` ensures that its members were already strings, or intended to
be strings.

Doc/library/enum.rst

@@ -44,6 +44,11 @@ helper, :class:`auto`.
     Base class for creating enumerated constants that are also
     subclasses of :class:`int`.
 
+.. class:: StrEnum
+
+    Base class for creating enumerated constants that are also
+    subclasses of :class:`str`.
+
 .. class:: IntFlag
 
     Base class for creating enumerated constants that can be combined using
@@ -601,6 +606,25 @@ However, they still can't be compared to standard :class:`Enum` enumerations::
     [0, 1]
 
 
+StrEnum
+^^^^^^^
+
+The second variation of :class:`Enum` that is provided is also a subclass of
+:class:`str`.  Members of a :class:`StrEnum` can be compared to strings;
+by extension, string enumerations of different types can also be compared
+to each other.  :class:`StrEnum` exists to help avoid the problem of getting
+an incorrect member::
+
+    >>> class Directions(StrEnum):
+    ...     NORTH = 'north',    # notice the trailing comma
+    ...     SOUTH = 'south'
+
+Before :class:`StrEnum`, ``Directions.NORTH`` would have been the :class:`tuple`
+``('north',)``.
+
+.. versionadded:: 3.10
+
+
 IntFlag
 ^^^^^^^
 
@@ -1132,6 +1156,20 @@ all-uppercase names for members)::
 .. versionchanged:: 3.5
 
 
+Creating members that are mixed with other data types
+"""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+When subclassing other data types, such as :class:`int` or :class:`str`, with
+an :class:`Enum`, all values after the `=` are passed to that data type's
+constructor.  For example::
+
+    >>> class MyEnum(IntEnum):
+    ...     example = '11', 16      # '11' will be interpreted as a hexadecimal
+    ...                             # number
+    >>> MyEnum.example
+    <MyEnum.example: 17>
+
+
 Boolean value of ``Enum`` classes and members
 """""""""""""""""""""""""""""""""""""""""""""
 

Lib/enum.py

@@ -4,7 +4,7 @@ from types import MappingProxyType, DynamicClassAttribute
 
 __all__ = [
         'EnumMeta',
-        'Enum', 'IntEnum', 'Flag', 'IntFlag',
+        'Enum', 'IntEnum', 'StrEnum', 'Flag', 'IntFlag',
         'auto', 'unique',
         ]
 
@@ -688,7 +688,35 @@ class Enum(metaclass=EnumMeta):
 
 
 class IntEnum(int, Enum):
-    """Enum where members are also (and must be) ints"""
+    """
+    Enum where members are also (and must be) ints
+    """
+
+
+class StrEnum(str, Enum):
+    """
+    Enum where members are also (and must be) strings
+    """
+
+    def __new__(cls, *values):
+        if len(values) > 3:
+            raise TypeError('too many arguments for str(): %r' % (values, ))
+        if len(values) == 1:
+            # it must be a string
+            if not isinstance(values[0], str):
+                raise TypeError('%r is not a string' % (values[0], ))
+        if len(values) > 1:
+            # check that encoding argument is a string
+            if not isinstance(values[1], str):
+                raise TypeError('encoding must be a string, not %r' % (values[1], ))
+            if len(values) > 2:
+                # check that errors argument is a string
+                if not isinstance(values[2], str):
+                    raise TypeError('errors must be a string, not %r' % (values[2], ))
+        value = str(*values)
+        member = str.__new__(cls, value)
+        member._value_ = value
+        return member
 
 
 def _reduce_ex_by_name(self, proto):

Lib/test/test_enum.py

@@ -5,7 +5,7 @@ import sys
 import unittest
 import threading
 from collections import OrderedDict
-from enum import Enum, IntEnum, EnumMeta, Flag, IntFlag, unique, auto
+from enum import Enum, IntEnum, StrEnum, EnumMeta, Flag, IntFlag, unique, auto
 from io import StringIO
 from pickle import dumps, loads, PicklingError, HIGHEST_PROTOCOL
 from test import support
@@ -48,14 +48,9 @@ except Exception as exc:
     FlagStooges = exc
 
 # for pickle test and subclass tests
-try:
-    class StrEnum(str, Enum):
-        'accepts only string values'
-    class Name(StrEnum):
-        BDFL = 'Guido van Rossum'
-        FLUFL = 'Barry Warsaw'
-except Exception as exc:
-    Name = exc
+class Name(StrEnum):
+    BDFL = 'Guido van Rossum'
+    FLUFL = 'Barry Warsaw'
 
 try:
     Question = Enum('Question', 'who what when where why', module=__name__)
@@ -665,14 +660,13 @@ class TestEnum(unittest.TestCase):
             tau = 'Tau'
         self.assertTrue(phy.pi < phy.tau)
 
-    def test_strenum_inherited(self):
-        class StrEnum(str, Enum):
-            pass
+    def test_strenum_inherited_methods(self):
         class phy(StrEnum):
             pi = 'Pi'
             tau = 'Tau'
         self.assertTrue(phy.pi < phy.tau)
-
+        self.assertEqual(phy.pi.upper(), 'PI')
+        self.assertEqual(phy.tau.count('a'), 1)
 
     def test_intenum(self):
         class WeekDay(IntEnum):
@@ -2014,13 +2008,6 @@ class TestEnum(unittest.TestCase):
         self.assertTrue(issubclass(ReformedColor, int))
 
     def test_multiple_inherited_mixin(self):
-        class StrEnum(str, Enum):
-            def __new__(cls, *args, **kwargs):
-                for a in args:
-                    if not isinstance(a, str):
-                        raise TypeError("Enumeration '%s' (%s) is not"
-                                        " a string" % (a, type(a).__name__))
-                return str.__new__(cls, *args, **kwargs)
         @unique
         class Decision1(StrEnum):
             REVERT = "REVERT"
@@ -2043,6 +2030,33 @@ class TestEnum(unittest.TestCase):
         local_ls = {}
         exec(code, global_ns, local_ls)
 
+    def test_strenum(self):
+        class GoodStrEnum(StrEnum):
+            one = '1'
+            two = '2'
+            three = b'3', 'ascii'
+            four = b'4', 'latin1', 'strict'
+        with self.assertRaisesRegex(TypeError, '1 is not a string'):
+            class FirstFailedStrEnum(StrEnum):
+                one = 1
+                two = '2'
+        with self.assertRaisesRegex(TypeError, "2 is not a string"):
+            class SecondFailedStrEnum(StrEnum):
+                one = '1'
+                two = 2,
+                three = '3'
+        with self.assertRaisesRegex(TypeError, '2 is not a string'):
+            class ThirdFailedStrEnum(StrEnum):
+                one = '1'
+                two = 2
+        with self.assertRaisesRegex(TypeError, 'encoding must be a string, not %r' % (sys.getdefaultencoding, )):
+            class ThirdFailedStrEnum(StrEnum):
+                one = '1'
+                two = b'2', sys.getdefaultencoding
+        with self.assertRaisesRegex(TypeError, 'errors must be a string, not 9'):
+            class ThirdFailedStrEnum(StrEnum):
+                one = '1'
+                two = b'2', 'ascii', 9
 
 class TestOrder(unittest.TestCase):
 

Misc/NEWS.d/next/Library/2020-09-19-12-22-08.bpo-41816.ynynXJ.rst

@@ -0,0 +1,2 @@
+StrEnum added: it ensures that all members are already strings or string
+candidates