Enum._generate_next_value_ should be static

[RomanSteinberg]

Bug report

Here and in two more places in the same file function _generate_next_value_ supposed to be called for instance. But it will not work properly in that case, because it behaves like static method. Consider staticmethod decorator.

As far as I understand there is no problem right now and all calls are correct. But in future it will be more robust if it would be static.

Linked PRs

• gh-103056: [Enum] ensure final _generate_next_value_ is a staticmethod #103062
• gh-103056: use staticmethod decorator for _gnv_ #103231

[ethanfurman]

_generate_next_value_ is not a static method, and should never be called on an instance. It is a plain function, called by the enum type machinery during value creation. If a user does use the staticmethod decorator, it will be stripped and the decorated function used instead during value creation.

Hmm, I see that the final function stored on the completed class is either a plain function or a static method depending on how it was written in the class -- it should probably always be one or the other; I think staticmethod since that's what I thought it was doing. ;-)

The check and possible staticmethod conversion should be at line 531.

[ethanfurman]

Okay, so it will be a staticmethod, but specifying that when writing a class is optional.

[RomanSteinberg]

@ethanfurman could you, please, explain the following phrase:

If a user does use the staticmethod decorator, it will be stripped and the decorated function used instead during value creation.

[ethanfurman]

If the function was decorated with staticmethod, this line: _gnv = value.__func__ if isinstance(value, staticmethod) else value retrieves the function. When auto() is used, the actual enum doesn't yet exist and the members don't yet exist, so a static method doesn't work -- we need the plain function in order to generate the values.

The change implented by this patch is to wrap the _generate_next_value_ with staticmethod (if not already done) before storing it in the final enum class.

[RomanSteinberg]

@ethanfurman thank you!
As far as I understand, the main reason not to make _generate_next_value_ static is not related to the code I linked, but to the code you linked, i.e. method around this line.

BTW, I do not understand how auto related to my question.

Ok, If I understood you correctly then I have one more question. Why not to decorate the definition of the _generate_next_value_ function with @staticmethod? The line you linked removes decorator for machinery purposes and it will still work fine, but the code becomes more clear for future developers and IDE will not complain.

[ethanfurman]

@RomanSteinberg You make a good point. I have added the staticmethod decorator to the code and the docs.

Thank you for your input!