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Date: Sun, 10 May 2015 13:33:00 +1000
Subject: Re: functions, optional parameters
From: Chris Angelico <rosuav@gmail.com>
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Xref: csiph.com comp.lang.python:90255

On Sun, May 10, 2015 at 12:45 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> This is the point where some people try to suggest some sort of complicated,
> fragile, DWIM heuristic where the compiler tries to guess whether the user
> actually wants the default to use early or late binding, based on what the
> expression looks like. "0 is an immutable int, use early binding; [] is a
> mutable list, use late binding." sort of thing. Such a thing might work
> well for the obvious cases, but it would be a bugger to debug and
> work-around for the non-obvious cases when it guesses wrong -- and it will.

What you could have is "late-binding semantics, optional early binding
as an optimization but only in cases where the result is
indistinguishable". That would allow common cases (int/bool/str/None
literals) to be optimized, since there's absolutely no way for them to
evaluate differently.

I personally don't think it'd be that good an idea, but it's a simple
enough rule that it wouldn't break anything. As far as anyone's code
is concerned, the rule is "late binding, always". In fact, that would
be the language definition; the rest is an optimization. (It's like
how "x.y()" technically first looks up attribute "y" on object x, then
calls the result; but it's perfectly reasonable for a Python
implementation to notice this extremely common case and do an
"optimized method call" that doesn't actually create a function
object.) The simpler the rule, the easier to grok, and therefore the
less chance of introducing bugs.

ChrisA