Sorry, accidentally off-list. Here it is again. On Fri, Dec 10, 2021 at 4:50 PM Christopher Barker <python...@gmail.com> 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? > > @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? > > 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 :-( > > 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. > > - 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 >> >> _______________________________________________ >> Python-ideas mailing list -- python-ideas@python.org >> To unsubscribe send an email to python-ideas-le...@python.org >> https://mail.python.org/mailman3/lists/python-ideas.python.org/ >> Message archived at >> https://mail.python.org/archives/list/python-ideas@python.org/message/T62V6IMOJDADAFWJ2DIAS4WHJQMI4CHU/ >> Code of Conduct: http://python.org/psf/codeofconduct/ >> > -- > 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
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