[Python-Dev] Drop support for ones' complement machines? (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Tue Dec 1 18:45:53 CET 2009

I'd rather prefer to explicitly list what CPython assumes about the outcome of specific operations. If this is just about &, |, ^, and ~, then its fine with me. I'm not even interested in going this far: I still am: with your list of assumptions, it is unclear (to me, at least) what the consequences are. So I'd rather see an explicit list of consequences, instead of buying a pig in a poke. Okay; though I think that my list of assumptions is easier to check directly for any given implementation: it corresponds exactly to items 2 and 4 in C99 J.3.5, and any conforming C implementation is required to explicitly document how it behaves with regard to these items.

I'm in favor stating the assumptions the way you do (*), I just want to have an additional explicit statement what consequences you assume out of these assumptions.

I'm not sure how to decide which particular consequences should be listed, but those for &, |, ^ and ~ could certainly be included.

It should give the CPython contributors an indication what kind of code would be ok, and which would not. Perhaps it should include both a black list and a white list: some may assume that two's complement already provides guarantees on left-shift, when it actually does not (**).

Regards, Martin

(*) I wonder why you are not talking about padding bits (6.2.6.2p1) (**) I also wonder why C fails to make left-shift implementation-defined, perhaps with an even stronger binding to the options for the integer representation.

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