On 12/9/2021 10:32 AM, Eric V. Smith wrote:
On 12/6/2021 1:24 AM, Christopher Barker wrote:
But I honestly don’t get it. My idea of a deferred object would be
quite different that this, would not be a great replacement for this,
and could quite happily co-exist with this idea. Clearly I’m missing
something.
I've used this example before.
Let's assume backticks create a "deferred object" (a name I hate, but
I'll continue to use in this discussion). I realize backticks won't
fly, I'm using them deliberately so as to not argue about the syntax here.
If I want to specify a late bound dataclasses default parameter,
currently I have to say:
@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 = `[]`
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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