Enum: auto() is not activated if combined with other values on assignment line

[huwcbjones]

Bug report

Creating a Enum that uses automatic values and tuple assignment doesn't work!

from enum import Enum, auto
from typing import Any

class Day(Enum):
    def __new__(cls, value, abbr=None):
        obj = Enum.__new__(cls, value)
        obj.abbr = abbr
        return obj

    @staticmethod
    def _generate_next_value_(name: str, start: int, count: int, last_values: list[Any]) -> Any:
        return name

    MONDAY = auto(), "Mon"
    # etc

print(Day.MONDAY, Day.MONDAY.value, Day.MONDAY.abbr)

Run that snippet and you get

Traceback (most recent call last):
  File "mwe.py", line 5, in 
    class Day(Enum):
  File "lib/python3.9/enum.py", line 288, in __new__
    enum_member = __new__(enum_class, *args)
  File "mwe.py", line 8, in __new__
    obj = Enum.__new__(cls, value)
  File "lib/python3.9/enum.py", line 702, in __new__
    raise ve_exc
ValueError: <enum.auto object at 0x10b673fa0> is not a valid Day

Your environment

• CPython versions tested on: 3.9.2/3.9.7/3.10.2
• Operating system and architecture: Debian 11 (amd64)/macOS 11.4 (amd64)

• PR: gh-93464: [Enum] fix auto() failure during multiple assignment #99148

• PR: [3.11] gh-93464: [Enum] fix auto() failure during multiple assignment (GH-99148) #99149

• PR: gh-93464: [Enum] Add versionchanged tag #99997

• PR: [3.11] gh-93464: [Enum] Add versionchanged tag (GH-99997) #100004

[huwcbjones]

You can bodge around this by replacing the tuple definition + __new__ with a map+property but that feels digusting

from enum import Enum, auto
from typing import Any

class Day(Enum):

    def _generate_next_value_(name: str, start: int, count: int, last_values: list[Any]) -> Any:
        return name

    MONDAY = auto()
    # etc

    @property
    def abbr(self):
        return {
            self.MONDAY: "Mon",
            # etc
        }[self]

print(Day.MONDAY, Day.MONDAY.value, Day.MONDAY.abbr)

prints

Day.MONDAY MONDAY Mon

as expected

[ethanfurman]

"auto" is short for automatic, the idea being that you don't need to type anything beyound auto() on the member assignment line.

If you are adding more on the assignment line, such as , "Mon", then auto() is not the right tool.

[huwcbjones]

auto() is not the right tool.

As the current docs say

If the exact value is unimportant you can use auto:

I didn't care about the exact value in the "complicated" tuple assignment, hence attempting to using auto.
Would it be possible to update the docs to reflect that then as this is clearly not the case?

If it had been documented that doing auto is for "simple" things, then I wouldn't have opened this issue and also wouldn't have sunk half a day trying to monkey patch the ever living shit out of Enum to try to make it work 🙃

[ethanfurman]

Fair point. You may also want to check out my aenum library. Using it would look like:

class Day(Enum):
    def __new__(cls, value, abbr=None):
        obj = Enum.__new__(cls, value)
        obj.abbr = abbr
        return obj

    @staticmethod
    def _generate_next_value_(name: str, start: int, count: int, last_values: list[Any]) -> Any:
        return name

    MONDAY = auto(abbr="Mon")

[huwcbjones]

You may also want to check out my aenum library

Ta, shall have a look :)

[d0sboots]

Just ran into the same issue. There's nothing whatsoever in the docs that would lead you to believe that auto() isn't a simple function that calls _generate_next_value_. And the docs say "If the exact value is unimportant you can use auto," which was precisely my use case: I don't care about the value, but I want to also specify additional data which has specifics that I do care about.

In my case, it was easier to just manually specify the values (even though I don't care about them), rather than try to work around the issues of auto().

An example of such "simple" use of auto that ideally would work:

from enum import Enum, auto

class FoodEnum(Enum):
    KETCHUP = auto(), "red"
    MAYO = auto(), "white"
    PICKLES = auto(), "green"
    POPCORN = auto(), "white"

    def __init__(self, value, color):
        self._value_ = value
        self.color = color

print(repr(FoodEnum.MAYO))

[ethanfurman]

I am convinced that auto() should always call _generate_next_value_ -- reclassifying as a bug.