PEP: 663 Title: Standardizing Enum str(), repr(), and format() behaviors Version: $Revision$ Last-Modified: $Date$ Author: Ethan Furman Discussions-To: python-dev@python.org Status: Rejected Type: Informational Content-Type: text/x-rst Created: 30-Jun-2021 Python-Version: 3.11 Post-History: 20-Jul-2021, 02-Nov-2021 Resolution: https://mail.python.org/archives/list/python-dev@python.org/message/RN3WCRZSTQR55DOHJTZ2KIO6CZPJPCU7/ Abstract ======== Update the ``repr()``, ``str()``, and ``format()`` of the various Enum types to better match their intended purpose. For example, ``IntEnum`` will have its ``str()`` change to match its ``format()``, while a user-mixed int-enum will have its ``format()`` match its ``str()``. In all cases, an enum's ``str()`` and ``format()`` will be the same (unless the user overrides ``format()``). Add a global enum decorator which changes the ``str()`` and ``repr()`` (and ``format()``) of the decorated enum to be a valid global reference: i.e. ``re.IGNORECASE`` instead of ````. Motivation ========== Having the ``str()`` of ``IntEnum`` and ``IntFlag`` not be the value causes bugs and extra work when replacing existing constants. Having the ``str()`` and ``format()`` of an enum member be different can be confusing. The addition of ``StrEnum`` with its requirement to have its ``str()`` be its ``value`` is inconsistent with other provided Enum's ``str``. The iteration of ``Flag`` members, which directly affects their ``repr()``, is inelegant at best, and buggy at worst. Rationale ========= Enums are becoming more common in the standard library; being able to recognize enum members by their ``repr()``, and having that ``repr()`` be easy to parse, is useful and can save time and effort in understanding and debugging code. However, the enums with mixed-in data types (``IntEnum``, ``IntFlag``, and the new ``StrEnum``) need to be more backwards compatible with the constants they are replacing -- specifically, ``str(replacement_enum_member) == str(original_constant)`` should be true (and the same for ``format()``). IntEnum, IntFlag, and StrEnum should be as close to a drop-in replacement of existing integer and string constants as is possible. Towards that goal, the ``str()`` output of each should be its inherent value; e.g. if ``Color`` is an ``IntEnum``:: >>> Color.RED >>> str(Color.RED) '1' >>> format(Color.RED) '1' Note that ``format()`` already produces the correct output, only ``str()`` needs updating. As much as possible, the ``str()``, ``repr()``, and ``format()`` of enum members should be standardized across the standard library. However, up to Python 3.10 several enums in the standard library have a custom ``str()`` and/or ``repr()``. The ``repr()`` of Flag currently includes aliases, which it should not; fixing that will, of course, already change its ``repr()`` in certain cases. Specification ============= There are three broad categories of enum usage: - simple: ``Enum`` or ``Flag`` a new enum class is created with no data type mixins - drop-in replacement: ``IntEnum``, ``IntFlag``, ``StrEnum`` a new enum class is created which also subclasses ``int`` or ``str`` and uses ``int.__str__`` or ``str.__str__`` - user-mixed enums and flags the user creates their own integer-, float-, str-, whatever-enums instead of using enum.IntEnum, etc. There are also two styles: - normal: the enumeration members remain in their classes and are accessed as ``classname.membername``, and the class name shows in their ``repr()`` and ``str()`` (where appropriate) - global: the enumeration members are copied into their module's global namespace, and their module name shows in their ``repr()`` and ``str()`` (where appropriate) Some sample enums:: # module: tools.py class Hue(Enum): # or IntEnum LIGHT = -1 NORMAL = 0 DARK = +1 class Color(Flag): # or IntFlag RED = 1 GREEN = 2 BLUE = 4 class Grey(int, Enum): # or (int, Flag) BLACK = 0 WHITE = 1 Using the above enumerations, the following two tables show the old and new output (blank cells indicate no change): +--------+------------------------+-----------------+------------+-----------------------+ | style | category | enum repr() | enum str() | enum format() | +--------+-------------+----------+-----------------+------------+-----------------------+ | normal | simple | 3.10 | | | | | | +----------+-----------------+------------+-----------------------+ | | | new | | | | | +-------------+----------+-----------------+------------+-----------------------+ | | user mixed | 3.10 | | | 1 | | | +----------+-----------------+------------+-----------------------+ | | | new | | | Grey.WHITE | | +-------------+----------+-----------------+------------+-----------------------+ | | int drop-in | 3.10 | | Hue.LIGHT | | | | +----------+-----------------+------------+-----------------------+ | | | new | | -1 | | +--------+-------------+----------+-----------------+------------+-----------------------+ | global | simple | 3.10 | | Hue.LIGHT | Hue.LIGHT | | | +----------+-----------------+------------+-----------------------+ | | | new | tools.LIGHT | LIGHT | LIGHT | | +-------------+----------+-----------------+------------+-----------------------+ | | user mixed | 3.10 | | Hue.LIGHT | | | | +----------+-----------------+------------+-----------------------+ | | | new | tools.LIGHT | -1 | | +--------+-------------+----------+-----------------+------------+-----------------------+ +--------+------------------------+-----------------------+------------------------+-----------------------+ | style | category | flag repr() | flag str() | flag format() | +--------+-------------+----------+-----------------------+------------------------+-----------------------+ | normal | simple | 3.10 | | Color.RED|GREEN | Color.RED|GREEN | | | +----------+-----------------------+------------------------+-----------------------+ | | | new | | Color.RED|Color.GREEN | Color.RED|Color.GREEN | | +-------------+----------+-----------------------+------------------------+-----------------------+ | | user mixed | 3.10 | | | 1 | | | +----------+-----------------------+------------------------+-----------------------+ | | | new | | | Grey.WHITE | | +-------------+----------+-----------------------+------------------------+-----------------------+ | | int drop-in | 3.10 | | Color.RED|GREEN | | | | +----------+-----------------------+------------------------+-----------------------+ | | | new | | 3 | | +--------+-------------+----------+-----------------------+------------------------+-----------------------+ | global | simple | 3.10 | | Color.RED|GREEN | Color.RED|GREEN | | | +----------+-----------------------+------------------------+-----------------------+ | | | new | tools.RED|tools.GREEN | RED|GREEN | RED|GREEN | | +-------------+----------+-----------------------+------------------------+-----------------------+ | | user mixed | 3.10 | | Grey.WHITE | 1 | | | +----------+-----------------------+------------------------+-----------------------+ | | | new | tools.WHITE | WHITE | WHITE | | +-------------+----------+-----------------------+------------------------+-----------------------+ | | int drop-in | 3.10 | | Color.RED|GREEN | | | | +----------+-----------------------+------------------------+-----------------------+ | | | new | tools.RED|tools.GREEN | 3 | | +--------+-------------+----------+-----------------------+------------------------+-----------------------+ These two tables show the final result: +--------+-------------+-----------------+------------+-----------------------+ | style | category | enum repr() | enum str() | enum format() | +--------+-------------+-----------------+------------+-----------------------+ | normal | simple | | Hue.LIGHT | Hue.LIGHT | | +-------------+-----------------+------------+-----------------------+ | | user mixed | | Grey.WHITE | Grey.WHITE | | +-------------+-----------------+------------+-----------------------+ | | int drop-in | | -1 | -1 | +--------+-------------+-----------------+------------+-----------------------+ | global | simple | tools.LIGHT | LIGHT | LIGHT | | +-------------+-----------------+------------+-----------------------+ | | user mixed | tools.WHITE | WHITE | WHITE | | +-------------+-----------------+------------+-----------------------+ | | int drop-in | tools.LIGHT | -1 | -1 | +--------+-------------+-----------------+------------+-----------------------+ +--------+-------------+-----------------------+------------------------+-----------------------+ | style | category | flag repr() | flag str() | flag format() | +--------+-------------+-----------------------+------------------------+-----------------------+ | normal | simple | | Color.RED|Color.GREEN | Color.RED|Color.GREEN | | +-------------+-----------------------+------------------------+-----------------------+ | | user mixed | | Grey.WHITE | Grey.WHITE | | +-------------+-----------------------+------------------------+-----------------------+ | | int drop-in | | 3 | 3 | +--------+-------------+-----------------------+------------------------+-----------------------+ | global | simple | tools.RED|tools.GREEN | RED|GREEN | RED|GREEN | | +-------------+-----------------------+------------------------+-----------------------+ | | user mixed | tools.WHITE | WHITE | WHITE | | +-------------+-----------------------+------------------------+-----------------------+ | | int drop-in | tools.RED|tools.GREEN | 3 | 3 | +--------+-------------+-----------------------+------------------------+-----------------------+ As can be seen, ``repr()`` is primarily affected by whether the members are global, while ``str()`` is affected by being global or by being a drop-in replacement, with the drop-in replacement status having a higher priority. Also, the basic ``repr()`` and ``str()`` have changed for flags as the old style was flawed. Backwards Compatibility ======================= Backwards compatibility of stringified objects is not guaranteed across major Python versions, and there will be backwards compatibility breaks where software uses the ``repr()``, ``str()``, and ``format()`` output of enums in tests, documentation, data structures, and/or code generation. Normal usage of enum members will not change: ``re.ASCII`` can still be used as ``re.ASCII`` and will still compare equal to ``256``. If the previous output needs to be maintained, for example to ensure compatibility between different Python versions, software projects will need to create their own enum base class with the appropriate methods overridden. Note that by changing the ``str()`` of the drop-in category, we will actually prevent future breakage when ``IntEnum``, et al, are used to replace existing constants. Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.