On 22 December 2012 01:34, Steven D'Aprano <st...@pearwood.info> wrote:
> On 18/12/12 01:36, Oscar Benjamin wrote:
>
>> I think it's unfortunate that Python's int() function combines two
>> distinct behaviours in this way. In different situations int() is used
>> to:
>> 1) Coerce an object of some type other than int into an int without
>> changing the value of the integer that the object represents.
>
> The second half of the sentence (starting from "without changing") is not
> justified. You can't safely make that assumption. All you know is that
> calling int() on an object is intended to convert the object to an int,
> in whatever way is suitable for that object. In some cases, that will
> be numerically exact (e.g. int("1234") will give 1234), in other cases it
> will not be.If I was to rewrite that sentence would replace the word 'integer' with 'number' but otherwise I'm happy with it. Your reference to "numerically exact" shows that you understood exactly what I meant. >> 2) Round an object with a non-integer value to an integer value. > > > int() does not perform rounding (except in the most generic sense that any > conversion from real-valued number to integer is "rounding"). That is what > the round() function does. int() performs truncating: it returns the > integer part of a numeric value, ignoring any fraction part: I was surprised by your objection to my use of the word "rounding" here. So I looked it up on Wikipedia: http://en.wikipedia.org/wiki/Rounding#Rounding_to_integer That section describes "round toward zero (or truncate..." which is essentially how I would have put it, and also how you put it below: > > * truncate, or round towards zero (drop any fraction part); So I'm not really sure what your objection is to that, though you are free to prefer the word truncate to round in this case (and I am free to disagree). <snip> > So you shouldn't think of int(number) as "convert number to an int", since > that is ambiguous. There are at least six common ways to convert arbitrary > numbers to ints: This is precisely my point. I would prefer if if int(obj) would fail on non-integers leaving me with the option of calling an appropriate rounding function. After catching RoundError (or whatever) you would know that you have a number type object that can be passed to round, ceil, floor etc. > Python provides truncation via the int and math.trunc functions, floor and > ceiling via math.floor and math.ceil, and round to nearest via round. > In Python 2, ties are rounded up, which is biased; in Python 3, the > unbiased banker's rounding is used. I wasn't aware of this change. Thanks for that. > Instead, you should consider int(number) to be one of a pair of functions, > "return integer part", "return fraction part", where unfortunately the > second function isn't provided directly. In general though, you can get > the fractional part of a number with "x % 1". For floats, math.modf also > works. Assuming that you know you have an object that supports algebraic operations in a sensible way then this works, although the complementary function for "x % 1" would be "x // 1" or "math.floor(x)" rather than "int(x)". To get the complementary function for "int(x)" you could do "math.copysign(abs(x) % 1, x)" (maybe there's a simpler way): $ python Python 2.7.3 (default, Sep 26 2012, 21:51:14) [GCC 4.7.2] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> def reconstruct(x): ... return int(x) + x % 1 ... >>> reconstruct(1) 1 >>> reconstruct(1.5) 1.5 >>> reconstruct(-2) -2 >>> reconstruct(-2.5) -1.5 > So, in a sense int() does to double-duty as both a constructor of ints > from non-numbers such as strings, and as a "get integer part" function for > numbers. I'm okay with that. And I am not. Oscar _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
