bpo-43945: [Enum] reduce scope of new format() behavior (GH-26752)

* [Enum] reduce scope of new format behavior

Instead of treating all Enums the same for format(), only user mixed-in
enums will be affected.  In other words, IntEnum and IntFlag will not be
changing the format() behavior, due to the requirement that they be
drop-in replacements of existing integer constants.

If a user creates their own integer-based enum, then the new behavior
will apply:

    class Grades(int, Enum):
        A = 5
        B = 4
        C = 3
        D = 2
        F = 0

Now:  format(Grades.B)  -> DeprecationWarning and '4'
3.12:                   -> no warning, and 'B'

Author: ethanfurman

Doc/library/enum.rst

@@ -22,7 +22,7 @@
    * :ref:`Advanced Tutorial <enum-advanced-tutorial>`
    * :ref:`Enum Cookbook <enum-cookbook>`
 
-----------------
+---------------
 
 An enumeration:
 
@@ -58,6 +58,7 @@ are not normal Python classes.  See
      :attr:`Color.RED` is ``RED``, the value of :attr:`Color.BLUE` is
      ``3``, etc.)
 
+---------------
 
 Module Contents
 ---------------
@@ -73,12 +74,12 @@ Module Contents
    :class:`IntEnum`
 
       Base class for creating enumerated constants that are also
-      subclasses of :class:`int`.
+      subclasses of :class:`int`. (`Notes`_)
 
    :class:`StrEnum`
 
       Base class for creating enumerated constants that are also
-      subclasses of :class:`str`.
+      subclasses of :class:`str`. (`Notes`_)
 
    :class:`Flag`
 
@@ -89,7 +90,7 @@ Module Contents
 
       Base class for creating enumerated constants that can be combined using
       the bitwise operators without losing their :class:`IntFlag` membership.
-      :class:`IntFlag` members are also subclasses of :class:`int`.
+      :class:`IntFlag` members are also subclasses of :class:`int`. (`Notes`_)
 
    :class:`EnumCheck`
 
@@ -132,6 +133,7 @@ Module Contents
 .. versionadded:: 3.6  ``Flag``, ``IntFlag``, ``auto``
 .. versionadded:: 3.10  ``StrEnum``, ``EnumCheck``, ``FlagBoundary``
 
+---------------
 
 Data Types
 ----------
@@ -647,6 +649,7 @@ Data Types
 
 .. versionadded:: 3.10
 
+---------------
 
 Utilites and Decorators
 -----------------------
@@ -710,3 +713,25 @@ Utilites and Decorators
    on the decorated enumeration.
 
 .. versionadded:: 3.10
+
+---------------
+
+Notes
+-----
+
+:class:`IntEnum`, :class:`StrEnum`, and :class:`IntFlag`
+
+    These three enum types are designed to be drop-in replacements for existing
+    integer- and string-based values; as such, they have extra limitations:
+
+    - ``format()`` will use the value of the enum member, unless ``__str__``
+      has been overridden
+
+    - ``StrEnum.__str__`` uses the value and not the name of the enum member
+
+    If you do not need/want those limitations, you can create your own base
+    class by mixing in the ``int`` or ``str`` type yourself::
+
+        >>> from enum import Enum
+        >>> class MyIntEnum(int, Enum):
+        ...     pass

Lib/asyncio/unix_events.py

@@ -126,7 +126,7 @@ def add_signal_handler(self, sig, callback, *args):
                     logger.info('set_wakeup_fd(-1) failed: %s', nexc)
 
             if exc.errno == errno.EINVAL:
-                raise RuntimeError(f'sig {sig:d} cannot be caught')
+                raise RuntimeError(f'sig {sig} cannot be caught')
             else:
                 raise
 
@@ -160,7 +160,7 @@ def remove_signal_handler(self, sig):
             signal.signal(sig, handler)
         except OSError as exc:
             if exc.errno == errno.EINVAL:
-                raise RuntimeError(f'sig {sig:d} cannot be caught')
+                raise RuntimeError(f'sig {sig} cannot be caught')
             else:
                 raise
 

Lib/enum.py

@@ -993,9 +993,9 @@ def __format__(self, format_spec):
         # mixed-in Enums should use the mixed-in type's __format__, otherwise
         # we can get strange results with the Enum name showing up instead of
         # the value
-
+        #
         # pure Enum branch, or branch with __str__ explicitly overridden
-        str_overridden = type(self).__str__ not in (Enum.__str__, Flag.__str__)
+        str_overridden = type(self).__str__ not in (Enum.__str__, IntEnum.__str__, Flag.__str__)
         if self._member_type_ is object or str_overridden:
             cls = str
             val = str(self)
@@ -1005,7 +1005,7 @@ def __format__(self, format_spec):
                 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 "
+                        "use a format specifier, such as :d for an integer-based Enum, to maintain "
                         "the current display",
                         DeprecationWarning,
                         stacklevel=2,
@@ -1044,6 +1044,22 @@ class IntEnum(int, Enum):
     Enum where members are also (and must be) ints
     """
 
+    def __str__(self):
+        return "%s" % (self._name_, )
+
+    def __format__(self, format_spec):
+        """
+        Returns format using actual value unless __str__ has been overridden.
+        """
+        str_overridden = type(self).__str__ != IntEnum.__str__
+        if str_overridden:
+            cls = str
+            val = str(self)
+        else:
+            cls = self._member_type_
+            val = self._value_
+        return cls.__format__(val, format_spec)
+
 
 class StrEnum(str, Enum):
     """
@@ -1072,6 +1088,8 @@ def __new__(cls, *values):
 
     __str__ = str.__str__
 
+    __format__ = str.__format__
+
     def _generate_next_value_(name, start, count, last_values):
         """
         Return the lower-cased version of the member name.
@@ -1300,6 +1318,16 @@ class IntFlag(int, Flag, boundary=EJECT):
     Support for integer-based Flags
     """
 
+    def __format__(self, format_spec):
+        """
+        Returns format using actual value unless __str__ has been overridden.
+        """
+        str_overridden = type(self).__str__ != Flag.__str__
+        value = self
+        if not str_overridden:
+            value = self._value_
+        return int.__format__(value, format_spec)
+
     def __or__(self, other):
         if isinstance(other, self.__class__):
             other = other._value_