Enum `auto()` functionally fails following alias creation

[oscar-LT]

What is the issue?

The base _generate_next_value_(name, start, count, last_values) in class Enum is not sufficiently rigorous as a default implementation for assigning values with auto().

Why is it problematic?

For a user employing auto() to assign values for their enums, the expectation is that the function will automatically assign unique values to new enums without the user needing to specify those values manually. However, when used following the creation of an alias to a pre-existing enum, the value generated by auto() is not guaranteed to be unique.

Given the following code:

from enum import Enum, auto

class Example(Enum):
    A = auto()
    B = auto()
    C = A
    D = auto()

the expectation is that A, B, D have been assigned unique values whereas C is an alias of A. A printout of each of the Example enum's values proves that not to be the case though:

>>> print(Example.A)
Example.A
>>> print(Example.B)
Example.B
>>> print(Example.C)
Example.A
>>> print(Example.D)     ### UNEXPECTED!
Example.B

effectively rendering D as an alias to B rather than its own separate value.

Suggested Cause

Upon closer inspection, the reason seems to be that the base _generate_next_value_ which auto() relies on is incrementing the "last value that was assigned" rather than the "last NEW value that was assigned"

cpython/Lib/enum.py

Lines 1171 to 1186 in a8abb76

	def _generate_next_value_(name, start, count, last_values):
	"""
	Generate the next value when not given.
	name: the name of the member
	start: the initial start value or None
	count: the number of existing members
	last_value: the last value assigned or None
	"""
	for last_value in reversed(last_values):
	try:
	return last_value + 1
	except TypeError:
	pass
	else:
	return start

Current Workaround

At the moment, to workaround this issue, the user can:

• Ensure that all aliases are only declared after any and all enums assigned with auto()
• Implement a replacement _generate_next_value_(name, start, count, last_values) function as described in the docs

---

This issue only affects code that combines the use of auto() with aliases. Although straightforward workarounds do exist (overloading _generate_next_value_ is well described in the docs), it seems unintuitive and unfriendly as the default behavior of auto(). Even simply sorting last_values before incrementing may be a sufficient solution to this issue, e.g.

    def _generate_next_value_(name, start, count, last_values):
        # for last_value in reversed(last_values):
        for last_value in sorted(last_values, reverse=True):
            try:
                return last_value + 1
            except TypeError:
                pass
        else:
            return start

[oscar-LT]

Published a tentative PR with the suggested solution

[ethanfurman]

Okay, let's change the default _generate_next_value_ to do the sort before picking the highest number, and add a warning to the docs that auto() will fail if used in an enum with non-numeric data. If the user wants to mix numeric and non-numeric data and still use auto() they'll have to create their own _generate_next_value_ to handle it.

[ethanfurman]

Change of plans: the current behavior is documented and tested, so a deprecation warning for 3.11 and 3.12 is appropriate. #91457 has been updated.