(with apologies for the random extra level of quoting in the below...)
On Thu, Mar 13, 2008 at 11:09 AM, Imri Goldberg <lorgandon@gmail.com> wrote:
As I said earlier, I'd like static checkers (like Python-Lint) to catch
this sort of cases, whatever the decision may be.
Hmm. Isn't that tricky? How does the static checker decide whether the objects being compared are floats? I guess one could be content with catching some cases where the operands to == are clearly floats... Wouldn't you have to have run-time warnings to be really sure of catching all the cases?
It's already too late for Python 3.0. Still, I believe it is worth discussing.
Sure. I didn't mean that to come out in quite the dismissive way it did :). Apologies. Maybe a PEP aimed at Python 4.0 is in order. If you're open to the idea of just having some way to enable warnings, it could be much sooner.
While checking against a==0.0 (and other similar conditions) before dividing will indeed protect from outright division by zero, it will enlarge any error you will have in the computation. I guess it would be better to do the same check for 'a is small' for appropriate values of 'small'.
Still, a check for 0.0 is good enough in some cases: if a is tiny, the large intermediate values may appear and then disappear happily before giving a sensible final result. These are usually the sort of cases where just having division by 0.0 return an infinity would have "just worked" too (making the whole "if" redundant), but that's not (currently!) an option in Python. It's a truism that floating-point equality tests should be avoided, but it's just not true that floating-point equality testing is *always* wrong, and I don't think that Python should make it so. Actually, one of the reasons I thought about this subject in the first
place, was dict lookup for floating point numbers. It seems to me that it's something you just shouldn't do.
So your proposal would presumably include making x in dict and x not in dict errors for any float x, regardless of the contents of the dictionary (or list, or set, or frozenset, or...) dict? What would you do about Decimals? A Decimal is just another floating point format (albeit base 10 instead of base 2); so presumably all these warnings/errors should apply equally to Decimal instances? If not, why not? I'm not trying to be negative here---as Aahz says, this is an interesting idea; I'm just trying to understand exactly how things might work. Mark