And my response again, although slightly edited. On 12/11/2021 11:28 AM, Christopher Barker wrote:

Sorry, accidentally off-list. Here it is again.

On Fri, Dec 10, 2021 at 4:50 PM Christopher Barker wrote:

Thanks Eric, this is a great example, thanks.

It does raise some questions. Though.

If we could use any expression as a deferred expression, then we still have the two key questions:

When does it get evaluated, and what namespaces does it use?

I agree those are good questions. I think, like PEP 671, it would get evaluated at the start of the function (in this case, __init__). It's easier for dataclasses, because I could just force the evaluation there. But for normal function arguments, maybe we'd have to say that before the function starts executing, any arguments which are deferred objects automatically are evaluated. And I think the namespace would be where it's defined. I'll admit I haven't thought all of this through in enough detail to implement it. I'm just trying to point out that we could use the general concept in other places. One thing in particular I haven't thought through: what if you really want to pass in a "deferred object" to a function? Can you keep it from being evaluated? Would you specify that on the caller side, or the callee side?

...

@dataclasses.dataclass class A:     my_list: list = dataclasses.field(default_factory=list)

What I'd like to be able to say:

@dataclasses.dataclass class A:     my_list: list = `[]`

I think in the data classes case, you could clearly define both of those. But in the general case?

def fun(n):     return `len(n)`

@dataclasses.dataclass class A:   length: n = fun()

What would that put in the signature? What namespace would the express e evaluated in?

I think it should be evaluated in the context of "fun". Clearly it would need to create a closure.

Yes, that is completely contrived, but it does bring up the complications of a “general” solution.

Maybe we could solve the dataclass problem with late bound class attributes:

@dataclasses.dataclass class A:     length: n => len(n)

Though I suppose that would still get evaluated before the dataclass decorator would see it :-(

There are absolutely some issues that need thinking through. It's entirely possible the idea can't be made to work. I'm just saying we should think it through and see if it could be made to work before we just say "we only want late-bound things as function parameters, and we can't use them anywhere else". I'm worried that if we only do it for function parameters, we might need to change the syntax (and I guess possibly the semantics) in the future in order to support other uses for deferred objects.

Side note: I wonder if dataclasses could be a bit smarter for the common. case: if the type is a callable, and the value is a particular Sentinel, the. Call it to get the default:

@dataclasses.dataclass class A:     my_list: list = dataclasses.Empty

That would work for most of the cases where I need to use field explicitly.

That's an interesting idea. Eric

- CHB

...

In the class A, before @dataclass is called, I want A.my_list to be a "deferred object" that I could then use when @dataclass is generating __init__(). Exactly how and when the "deferred object" would get evaluated is debatable, but not so important for the sake of this discussion. Suffice it to say that it would either be explicitly or implicitly evaluated at the start of __init__.

I think you can see that this would benefit from similar functionality to late-bound parameters, and that if we had this more general mechanism that late-bound parameters could use the same underlying mechanism.

And in case I wasn't clear: to get the late-bound parameter functionality using this syntax, you'd use:

def foo(my_list = `[]`):

That's why I think we should have a larger concept that just late-bound parameters: I think there's a general concept here that can be extended beyond parameters. And that's why I thing not restricting it to a function-definition-only syntax is important: we should produce a syntax that can be used in more places than just functions. This is why I think we need to decide on this larger scope before accepting the narrow function-definition-only syntax: if we decide to add "deferred objects" later, we'd have two ways to specify late-bound parameters [0].

Eric

[0]: Or arguments, I can never remember which is which: someone needs to invent a memorable mnemonic device.

...

Had these "deferred objects" existed when I designed dataclasses, I would have used them instead of the clunky default_factory. PEP 671 does not help with this use case, where a late-bound parameter isn't specified in a function definition. I need the late-bound parameter to be stored in an object I can refer to later.

Eric

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-- Christopher Barker, PhD (Chris)

Python Language Consulting   - Teaching   - Scientific Software Development   - Desktop GUI and Web Development   - wxPython, numpy, scipy, Cython

-- Christopher Barker, PhD (Chris)

Python Language Consulting   - Teaching   - Scientific Software Development   - Desktop GUI and Web Development   - wxPython, numpy, scipy, Cython

_______________________________________________ Python-ideas mailing list --python-ideas@python.org To unsubscribe send an email topython-ideas-leave@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived athttps://mail.python.org/archives/list/python-ideas@python.org/message/IDKDVY... Code of Conduct:http://python.org/psf/codeofconduct/