Mark Dickinson <dicki...@gmail.com> added the comment: > If Python functions are inconsistent then I think they should either be > made consistent, or if that's not possible they should be clearly > documented as being inconsistent.
I think 'inconsistent' is a bit strong for what happens here. In my view, the expressions log10(x) and log(x, 10) behave consistently: they both give (good) floating-point approximations to the base-10 logarithm of x. However, since they use slightly different methods to get those approximations, the approximations are not identical. Would you also say that atan2(y, x) is inconsistent with atan(y/x) (for positive x, y, say), because they give minutely different results in some cases? > The docs for log10(x) could be improved to say: > "this function gives the correct result for powers of 10, > but breaks consistency with log(a,10) == log10(a)" I find this unnecessarily negative. I propose instead to leave the log documentation as it is, and add something like the following to the log10 documentation, as an explanation of why log10 is still valuable in the presence of the two-argument log function: """Since this function directly wraps the platform log10 function, ``log10(x)`` will usually (depending on the platform) be faster and more accurate than the mathematically equivalent ``log(x, 10)``.""" Note that historically, the two-argument version of math.log appeared long after math.log10 did; this might help explain the apparent redundancy. (I'd actually argue that the two-argument log should not have been added in the first place: it doesn't wrap a standard C math function, and it's really no different from log(x)/log(y), except that having it as a built-in library function gives the illusion that it might be more accurate than it actually is. However, that's academic now.) One possible 'fix' for this situation does occur to me: instead of computing log(x, y) internally as log(x)/log(y), compute it as log10(x)/log10(y). This at least doesn't involve extensive changes or additions to the code. I don't think it really makes any practical difference, except perhaps in reducing the number of bug reports like this one. And it won't help make log2(x) == log(x, 2) if/when the C99 log2 function is added to the math library, either. I'd be -0 on making this change. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue6765> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
