On Mon, Dec 6, 2021 at 10:04 PM Paul Moore <p.f.mo...@gmail.com> wrote:
> Here's a prototype implementation, and a demonstration of how it would
> be used to implement late bound arguments. Please note, I understand
> that the syntax here is horrible. That's exactly the point, this needs
> language support to be non-horrible. That's what a "deferred
> expression" proposal would provide.
>
> # Explicitly creating Deferred objects is horrible, this is the bit
> that *really* needs language support
> class Deferred:
> def __init__(self, callable):
> self.callable = callable
>
> # This could easily be a builtin function (or an operator if people
> prefer syntax) once we have deferred objects.
> def undefer(expr):
> if isinstance(expr, Deferred):
> return expr.callable()
> return expr
>
>
> x = 12
> # def f(a=defer x):
> def f(a=Deferred(lambda: x)):
> a = undefer(a)
> return a
>
> assert f(9) == 9
> assert f() == 12
> x = 8
> assert f() == 8
> assert f(9) == 9
>
> If anyone wants to take this and make a *proper* deferred object
> proposal out of it, then please do so. If not, then at a minimum I
> think this offers something vaguely concrete to discuss regarding the
> "why deferred objects are a more general solution to the late bound
> argument" question.
>
The reason I consider this to be an independent proposal, and NOT a
mechanism for late-bound defaults, is this problem:
def f(lst, n=>len(lst)):
lst.append(1)
print(n)
f([10, 20, 30])
A late-bound default should print 3. A deferred expression should
print 4. They're not a more general solution to the same question;
they're a solution to a different question that has some overlap in
what it can achieve. A None-coalescing operator would also have some
overlap with each of the above, but it is, again, not the same thing.
ChrisA
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