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[3.11] gh-99304: [Enum] clarify what constitutes a flag alias (GH-99395) #99415

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[3.11] gh-99304: [Enum] clarify what constitutes a flag alias (GH-99395) #99415

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49 changes: 46 additions & 3 deletions Doc/howto/enum.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -173,6 +173,7 @@ yourself some work and use :func:`auto()` for the values::
... FRIDAY = auto()
... SATURDAY = auto()
... SUNDAY = auto()
... WEEKEND = SATURDAY | SUNDAY


.. _enum-advanced-tutorial:
Expand Down Expand Up @@ -305,6 +306,10 @@ Iterating over the members of an enum does not provide the aliases::

>>> list(Shape)
[<Shape.SQUARE: 2>, <Shape.DIAMOND: 1>, <Shape.CIRCLE: 3>]
>>> list(Weekday)
[<Weekday.MONDAY: 1>, <Weekday.TUESDAY: 2>, <Weekday.WEDNESDAY: 4>, <Weekday.THURSDAY: 8>, <Weekday.FRIDAY: 16>, <Weekday.SATURDAY: 32>, <Weekday.SUNDAY: 64>]

Note that the aliases ``Shape.ALIAS_FOR_SQUARE`` and ``Weekday.WEEKEND`` aren't shown.

The special attribute ``__members__`` is a read-only ordered mapping of names
to members. It includes all names defined in the enumeration, including the
Expand All @@ -324,6 +329,11 @@ the enumeration members. For example, finding all the aliases::
>>> [name for name, member in Shape.__members__.items() if member.name != name]
['ALIAS_FOR_SQUARE']

.. note::

Aliases for flags include values with multiple flags set, such as ``3``,
and no flags set, i.e. ``0``.


Comparisons
-----------
Expand Down Expand Up @@ -751,7 +761,7 @@ flags being set, the boolean evaluation is :data:`False`::
False

Individual flags should have values that are powers of two (1, 2, 4, 8, ...),
while combinations of flags won't::
while combinations of flags will not::

>>> class Color(Flag):
... RED = auto()
Expand Down Expand Up @@ -1107,8 +1117,8 @@ example of when ``KEEP`` is needed).

.. _enum-class-differences:

How are Enums different?
------------------------
How are Enums and Flags different?
----------------------------------

Enums have a custom metaclass that affects many aspects of both derived :class:`Enum`
classes and their instances (members).
Expand All @@ -1125,6 +1135,13 @@ responsible for ensuring that various other methods on the final :class:`Enum`
class are correct (such as :meth:`__new__`, :meth:`__getnewargs__`,
:meth:`__str__` and :meth:`__repr__`).

Flag Classes
^^^^^^^^^^^^

Flags have an expanded view of aliasing: to be canonical, the value of a flag
needs to be a power-of-two value, and not a duplicate name. So, in addition to the
:class:`Enum` definition of alias, a flag with no value (a.k.a. ``0``) or with more than one
power-of-two value (e.g. ``3``) is considered an alias.

Enum Members (aka instances)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Expand All @@ -1134,9 +1151,35 @@ The most interesting thing about enum members is that they are singletons.
and then puts a custom :meth:`__new__` in place to ensure that no new ones are
ever instantiated by returning only the existing member instances.

Flag Members
^^^^^^^^^^^^

Flag members can be iterated over just like the :class:`Flag` class, and only the
canonical members will be returned. For example::

>>> list(Color)
[<Color.RED: 1>, <Color.GREEN: 2>, <Color.BLUE: 4>]

(Note that ``BLACK``, ``PURPLE``, and ``WHITE`` do not show up.)

Inverting a flag member returns the corresponding positive value,
rather than a negative value --- for example::

>>> ~Color.RED
<Color.GREEN|BLUE: 6>

Flag members have a length corresponding to the number of power-of-two values
they contain. For example::

>>> len(Color.PURPLE)
2


.. _enum-cookbook:

Enum Cookbook
-------------


While :class:`Enum`, :class:`IntEnum`, :class:`StrEnum`, :class:`Flag`, and
:class:`IntFlag` are expected to cover the majority of use-cases, they cannot
Expand Down
39 changes: 27 additions & 12 deletions Doc/library/enum.rst
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Expand Up @@ -27,7 +27,8 @@
An enumeration:

* is a set of symbolic names (members) bound to unique values
* can be iterated over to return its members in definition order
* can be iterated over to return its canonical (i.e. non-alias) members in
definition order
* uses *call* syntax to return members by value
* uses *index* syntax to return members by name

Expand Down Expand Up @@ -432,19 +433,23 @@ Data Types
in most of the same places that a string can be used. The result of any string
operation performed on or with a *StrEnum* member is not part of the enumeration.

.. note:: There are places in the stdlib that check for an exact :class:`str`
instead of a :class:`str` subclass (i.e. ``type(unknown) == str``
instead of ``isinstance(unknown, str)``), and in those locations you
will need to use ``str(StrEnum.member)``.
.. note::

There are places in the stdlib that check for an exact :class:`str`
instead of a :class:`str` subclass (i.e. ``type(unknown) == str``
instead of ``isinstance(unknown, str)``), and in those locations you
will need to use ``str(StrEnum.member)``.

.. note::

Using :class:`auto` with :class:`StrEnum` results in the lower-cased member
name as the value.

.. note:: :meth:`__str__` is :func:`str.__str__` to better support the
*replacement of existing constants* use-case. :meth:`__format__` is likewise
:func:`str.__format__` for that same reason.
.. note::

:meth:`~object.__str__` is :meth:`!str.__str__` to better support the
*replacement of existing constants* use-case. :meth:`~object.__format__` is likewise
:meth:`!str.__format__` for that same reason.

.. versionadded:: 3.11

Expand Down Expand Up @@ -476,13 +481,17 @@ Data Types

.. method:: __iter__(self):

Returns all contained members::
Returns all contained non-alias members::

>>> list(Color.RED)
[<Color.RED: 1>]
>>> list(purple)
[<Color.RED: 1>, <Color.BLUE: 4>]

.. versionchanged:: 3.11

Aliases are no longer returned during iteration.

.. method:: __len__(self):

Returns number of members in flag::
Expand Down Expand Up @@ -592,9 +601,15 @@ Data Types
Using :class:`auto` with :class:`IntFlag` results in integers that are powers
of two, starting with ``1``.

.. versionchanged:: 3.11 :meth:`__str__` is now :func:`int.__str__` to
better support the *replacement of existing constants* use-case.
:meth:`__format__` was already :func:`int.__format__` for that same reason.
.. versionchanged:: 3.11

:meth:`~object.__str__` is now :meth:`!int.__str__` to better support the
*replacement of existing constants* use-case. :meth:`~object.__format__` was
already :meth:`!int.__format__` for that same reason.

Inversion of a :class:`!IntFlag` now returns a positive value that is the
union of all flags not in the given flag, rather than a negative value.
This matches the existing :class:`Flag` behavior.

.. class:: ReprEnum

Expand Down