RFC for PEP 663: Improving and Standardizing Enum str(), repr(), and format() behaviors

PEP: 663 Title: Improving and Standardizing Enum str(), repr(), and format() behaviors Version: $Revision$ Last-Modified: $Date$ Author: Ethan Furman <ethan@stoneleaf.us> Discussions-To: python-dev@python.org Status: Draft Type: Informational Content-Type: text/x-rst Created: 23-Feb-2013 Python-Version: 3.11 Post-History: 20-Jul-2021 Resolution: Abstract ======== Now that we have a few years experience with Enum usage it is time to update the ``repr()``, ``str()``, and ``format()`` of the various enumerations by their intended purpose. Motivation ========== The addition of ``StrEnum`` with its requirement to have its ``str()`` be its ``value`` is inconsistent with other provided Enum's ``str``. 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 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; i.e.:: >>> Color.RED <Color.RED: 1> >>> 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 stardard library. 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 a three broad categories of enum usage: - standard: Enum or Flag a new enum class is created, and the members are used as ``class.member_name`` - 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__``; the ``repr`` can be changed by using the ``global_enum`` decorator - user-mixed enums and flags the user creates their own integer-, float-, str-, whatever-enums instead of using enum.IntEnum, etc. 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 table shows the old and new behavior, while the last shows the final result: +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | type | enum repr() | enum str() | enum format() | flag repr() | flag str() | flag format() | +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | standard | 3.9 | | | | <Color.RED|GREEN: 3> | Color.RED|GREEN | Color.RED|GREEN | | +----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | | new | | | | <Color(3): RED|GREEN> | Color.RED|Color.GREEN | Color.RED|Color.GREEN | +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | user mixed | 3.9 | | | 1 | <Grey.WHITE: 1> | | 1 | | +----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | | new | | | Grey.WHITE | <Grey(1): WHITE> | | Grey.WHITE | +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | int drop-in | 3.9 | | Hue.LIGHT | | <Color.RED|GREEN: 3> | Color.RED|GREEN | | | +----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | | new | | -1 | | <Color(3): RED|GREEN> | 3 | | +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | global | 3.9 | <Hue.LIGHT: -1> | Hue.LIGHT | Hue.LIGHT | <Color.RED|GREEN: 3> | Color.RED|GREEN | Color.RED|GREEN | | +----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | | new | tools.LIGHT | LIGHT | LIGHT | tools.RED|tools.GREEN | RED|GREEN | RED|GREEN | +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | user mixed | 3.9 | <Grey.WHITE: 1 | Grey.WHITE | Grey.WHITE | <Grey.WHITE: 1> | Grey.WHITE | 1 | | +----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | | new | tools.WHITE | WHITE | WHITE | tools.WHITE | WHITE | WHITE | +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | int drop-in | 3.9 | <Hue.LIGHT: -1> | Hue.LIGHT | | <Color.RED|GREEN: 3> | Color.RED|GREEN | | | +----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | | new | tools.LIGHT | -1 | | tools.RED|tools.GREEN | 3 | | +-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ Which will result in: +-------------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | type | enum repr() | enum str() | enum format() | flag repr() | flag str() | flag format() | +-------------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | standard | <Hue.LIGHT: -1> | Hue.LIGHT | Hue.LIGHT | <Color(3): RED|GREEN> | Color.RED|Color.GREEN | Color.RED|Color.GREEN | +-------------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | user mixed | <Grey.WHITE: 1> | Grey.WHITE | Grey.WHITE | <Grey(1): WHITE> | Grey.WHITE | Grey.WHITE | +-------------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | int drop-in | <Hue.LIGHT: -1> | -1 | -1 | <Color(3): RED|GREEN> | 3 | 3 | +-------------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | global | tools.LIGHT | LIGHT | LIGHT | tools.RED|tools.GREEN | RED|GREEN | RED|GREEN | +-------------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | user mixed | tools.WHITE | WHITE | WHITE | tools.WHITE | WHITE | WHITE | +-------------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+ | int drop-in | tools.LIGHT | -1 | -1 | 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 very clunky. The ``repr()`` for Enum vs Flag are different, primarily because the Enum ``repr()`` does not work well for flags. I like being able to tell whether an enum member is a Flag or an Enum based on the ``repr()`` alone, but am open to arguments for changing Enum's ``repr()`` to match Flag's. Backwards Compatibility ======================= My understanding is that ``str()`` and ``repr()`` output has much lower backwards compatibility requirements. Even so, I expect the majority of breakage to be in doc and unit tests. I'm less clear on the policy for ``format()``. 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. Mitigation ========== 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 one wants their own enums in their own code to remain the same they will need to write their own base Enum class and then write the appropriate ``repr``, ``str()``, and ``format()`` methods (or copy them from the 3.10 enum module). Copyright ========= This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.

Thanks for this Ethan. I don't have any substantive comments on what you're proposing (aside from "Yes, that sounds reasonable to me"), so my comments below are just some minor suggested clarifications for the PEP text. On Wed, 21 Jul 2021 at 14:17, Ethan Furman <ethan@stoneleaf.us> wrote:
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; i.e.::
>>> Color.RED <Color.RED: 1> >>> str(Color.RED) 1 >>> format(Color.RED) '1'
The suggested output here looks like the second line was `print(Color.RED)` rather than `str(Color.RED)` (since the quotes on the string are missing). The text should also state explicitly that `Color` in the example is an `IntEnum` subclass.
Note that format() already produces the correct output, only str() needs updating.
This paragraph may be a little clearer if it said "... correct output in Python 3.10, only str() ...".
Using the above enumerations, the following table shows the old and new behavior, while the last shows the final result:
Since the enum output changes were reverted for 3.10, replacing "3.9" with "3.10" in the tables may make it a little clearer that there was no intermediate step between 3.9 and 3.11. Cheers, Nick. -- Nick Coghlan | ncoghlan@gmail.com | Brisbane, Australia
participants (2)
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Ethan Furman
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Nick Coghlan