[Python-Dev] Re: Future division detection (original) (raw)

Michael Hudson mwh@python.net
05 Nov 2001 11:48:16 -0500

com-nospam@ccraig.org (Christopher A. Craig) writes:

"Tim Peters" <tim.one@home.com> writes:

> [Christopher A. Craig, playing with modifying the meaning of division] > > ... > > While doing this I was thinking that I would change truedivision on > > ints and floats to return a rational and change the rational code to > > return a long if the denominator is 1. This works great, except that > > if future division is off then rationals can suddenly become longs and > > do not automatically cast back. This makes it virtually impossible to > > guarantee a correct result to nearly any rational computation that > > involves a division. > > > > So I wanted to know if there is some way to detect, at the object > > level, if the COFUTUREDIVISION feature is active. > > I'm unclear on what you're asking. In case it helps, note this section in > future.py: > Hmm, I thought I was being too verbose, I guess I was being too unclear. I have a rational module (in C) which I am trying to patch to be an object satisfying PEP-239. The problem is that I would like to have an rational cast back to a integer iff (1) the denominator is 1 and (2) future division is active. If I don't check (2) then I get the situation that if future division is inactive then (rational('1/3')*3)/5 would yield 0 instead of 1/5. This makes rationals pretty much useless unless future division is active. What I would like to do is have a function that can check to see if the COFUTUREDIVISION flag is set and if it is and the denominator is 1 then return the numerator, else return the rational. I have a strong suspicion that this can't be done, in which case I just won't do the cast back automatically.

Does this:

int is_future_div(void) { PyCompilerFlags cf; PyEval_MergeCompilerFlags(&cf); return cf.cf_flags & CO_FUTURE_DIVISION; }

work?

You'll need to change this when future division becomes the default, but I think it'll work today. This is a murky dark corner of the interpreter, though -- so don't blame me when it breaks!

Cheers, M.

-- >> REVIEW OF THE YEAR, 2000 << It was shit. Give us another one. -- NTK Know, 2000-12-29, http://www.ntk.net/