[Python-Dev] Scoping vs augmented assignment vs sets (Re: 'fast locals' in Python 2.5) (original) (raw)

Boris Borcic bborcic at gmail.com
Wed Jun 14 12:05:26 CEST 2006

Terry Reedy wrote:

"Boris Borcic" <bborcic at gmail.com> wrote in message news:e6mcfg$iao$1 at sea.gmane.org...

being transformed to profit from simplifications I expected sets to allow. There, itemwise augmented assigments in loops very naturally transform to wholesale augmented assignments without loops. Except for this wart. Your transformation amounted to switching from collection mutation to object rebinding. In Python, that is a crucial difference.

Ok, that is a crucial difference. The question becomes : is that difference in the case of augmented assignment maintained for practical or for purity aka ideological reasons ?

That the mutation and rebinding were both done with augmented assignments is not terribly important except as this masks the difference. When you read your code, you know that you will only call the inner function with a mutable collection object, so you know that the name will be rebound to the same object after mutation, so you can think of the augmented assignment as being the same as collection mutation.

Not quite. When I read my code (and while I transform it) I know (as an invariant) that nowhere in my function do I initially bind a value to my variable, and I know that it can't be done with augmented assignments, and I know that no working code can /ever/ result from letting a function-local scope capture a name /just/ because it is the target of an augmented assignment.

So, when despite this obvious certainty the compiler translates my code to something that can't possibly run while it happily translates x+=1 and x=x+1 to different bytecodes with no qualms about x possibly being bound to an immutable, well, I do feel victim of something like a hidden agenda.

But the compiler does not know any such thing about the target of the augmented assignment and must therefore treat the statement as an assigment. It was a bug for a2 to do otherwise, even though the bug was locally optimal for this particular usage.

I must thank you for the effort of elaborating a polite and consistent explanation that was almost to the point and believable.

Regards, Boris Borcic

"On naît tous les mètres du même monde"

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