Dynamically replacing an objects __class__; is it safe?
python at deborahswanson.net
Thu Mar 16 22:07:03 EDT 2017
Steve D'Aprano wrote,on March 16, 2017 5:07 AM
> On Thu, 16 Mar 2017 09:03 am, Gregory Ewing wrote:
> > Steve D'Aprano wrote:
> >> You probably can't make a whale fly just by changing the class to
> >> bird. It will need wings, and feathers, at the very least.
> > Some things succeed in flying with neither wings nor feathers.
> > Helicopters, for example.
> For some definition of "no wings".
> But regardless of rockets, balloons, helicopters, Boeing 747s
> and Dr Strange's mystical Cloak Of Levitation, *birds*
> require wings and feathers to fly. If you just paint "BIRD"
> on the side of a whale, it won't get off the ground, and if
> you do manage to get it airborne (via a catapult,
> perhaps) it will just come down with a rather large splat.
> The point is that dynamically swapping the class of an
> existing instance at runtime is *not* just a way of doing
> duck-typing. It really does matter if your duck flies by
> flapping feathered wings or by blasting an exhaust of hot
> gasses out of its rear end at high speed.
> With duck-typing, you don't care whether you have a duck or a
> goose, so long as they both can fly: you don't care *how* it
> flies, so long as it does, and even a rocket-propelled
> balloon will be fine.
> But when you dynamically swap out __class__ (I think
> Objective-C "swizzling" is conceptually equivalent, so I'll
> call it by that word) you have to care about the
> implementation. The whole point of swizzling is that it
> allows you to swap out one implementation ("run forward and
> flap your wings") with another implementation ("light the
> blue touch paper"). But if you do that, you have to care
> about implementation details. There's no point in setting
> your swizzled fly() method to that of Rocket if your instance
> doesn't have blue touch paper to light.
> With duck-typing, making the methods work isn't your
> responsibility. But when you swizzle, you are responsible for
> making sure that the instance provides whatever the methods
> need to work.
> A very old but good example of Python swizzling is here:
"Cheer up," they said, "things could be worse." So I cheered up, and
sure enough, things got worse.
Condolences, Steve. Nobody on this thread wants to plumb the depths of
Python swizzling. Or anything else Python, so it appears.
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