RFC for PEP 663: Improving and Standardizing Enum str(), repr(), and format() behaviors
Ethan Furman
ethan at stoneleaf.us
Wed Jul 21 17:09:30 EDT 2021
PEP: 663
Title: Improving and Standardizing Enum str(), repr(), and format() behaviors
Version: $Revision$
Last-Modified: $Date$
Author: Ethan Furman <ethan at stoneleaf.us>
Discussions-To: python-dev at 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; e.g. if ``Color`` is an
``IntEnum``::
>>> Color.RED
<Color.RED: 1>
>>> str(Color.RED)
'1'
>>> format(Color.RED)
'1'
Note that format() already produces the correct output in 3.10, 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 | 3.10 | | | | <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 | 3.10 | | | 1 | <Grey.WHITE: 1> | | 1
|
|
+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+
| | new | | | Grey.WHITE | <Grey(1): WHITE> | |
Grey.WHITE |
+-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+
+-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+
| int drop-in | 3.10 | | Hue.LIGHT | | <Color.RED|GREEN: 3> |
Color.RED|GREEN | |
|
+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+
| | new | | -1 | | <Color(3): RED|GREEN> | 3 |
|
+-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+
+-------------+----------+-----------------+------------+-----------------------+-----------------------+------------------------+-----------------------+
| global | 3.10 | <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 | 3.10 | <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 | 3.10 | <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.
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