On Wed, Mar 21, 2018 at 09:46:06AM -0700, Nathaniel Smith wrote: [...] > For me this is an argument against is_integer() rather than for it :-). > is_prime(float) should *obviously*[1] be a TypeError. Primality is only > meaningfully defined over the domain of integers
And 3.0 is an integer. Just because it is float *object* does not mean it is not an integer *value*. Do not mistake the leaky abstraction of multiple numeric types for the mathematical number three. Primality-related functions are not limited to integers. For example, the prime counting function is defined on the reals: https://en.wikipedia.org/wiki/Prime-counting_function and there's no reason not to extend the domain of is_prime to any real. "Practicality beats purity" -- why should the result be different just because the input has a ".0" at the end? Mathematically it doesn't: the answer to something like "Is 3.0 a prime?" is a clear Yes, not "I'm sorry, I don't understand the question!" which an exception would imply. As programmers, there is always a tension between the leaky abstraction of our numeric types, and the mathematical equality of: 3 == 3.0 == 9/3 == 3+0j etc. The decision on whether to be more or less restrictive on the *types* a function accepts is up to the individual developer. Having decided to be *less* restrictive, an is_integer method would be useful. For what it's worth, Wolfram|Alpha gives inconsistant results. It allows testing of rationals for primality: "Is 9/3 a prime?" evaluates as true, but: "Is 3.0 a prime?" gets parsed as "Is 3 a prime number?" and yet evaluates as false. A clear bug for software using a natural-language interface and intended to be used by students and non-mathematicans. > and this is a case where > operator.index is exactly what you want. It is exactly not what I want. > Of course it's just an example, and perhaps there are other, better > examples. But it makes me nervous that this is the best example you could > quickly come up with. I actually had to work hard to come up with an example as simple and understandable as primality testing. The first example I thought of was Bessel functions of the 1st and 2nd kind with arbitrary real-valued orders, where you *absolutely* do want order 3.0 (float) and order 3 (int) to be precisely the same. But I avoided giving it because I thought it would be too technical and it would intimidate people. I thought that the prime number example would be easier to understand. Next time I want to make a point, I'll go for argument by intimidation. *wink* -- Steve _______________________________________________ Python-Dev mailing list [email protected] https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com