199 lines
12 KiB
Plaintext
199 lines
12 KiB
Plaintext
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:
|
|
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
|
|
==========
|
|
|
|
Having the ``str()`` of ``IntEnum`` and ``IntFlag`` not be the value causes
|
|
bugs and extra work to get the correct behavior.
|
|
|
|
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 is... not elegant, and can be greatly improved.
|
|
|
|
|
|
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.
|