Why Python 3?

[Ian Kelly]

On Apr 19, 2014 2:54 PM, "Chris Angelico" <rosuav at gmail.com> wrote:
>
> On Sun, Apr 20, 2014 at 6:38 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> >> Or you just cast one of them to float. That way you're sure you're
> >> working with floats.
> >
> > Which is inappropriate if the type passed in was a Decimal or a complex.
>
> In that case, you already have a special case in your code, so whether
> that special case is handled by the language or by your code makes
> little difference. Is your function so generic that it has to be able
> to handle float, Decimal, or complex, and not care about the
> difference, and yet has to ensure that int divided by int doesn't
> yield int? Then say so; put in that special check. Personally, I've
> yet to meet any non-toy example of a function that needs that exact
> handling; most code doesn't ever think about complex numbers, and a
> lot of things look for one specific type:

When I'm writing a generic average function, I probably don't know whether
it will ever be used to average complex numbers. That shouldn't matter,
because I should be able to rely on this code working for whatever numeric
type I pass in:

def average(values):
    return sum(values) / len(values)

This works for decimals, it works for fractions, it works for complex
numbers, it works for numpy types, and in Python 3 it works for ints.

> Maybe it's not your code that should be caring about what happens when
> you divide two integers, but the calling code. If you're asking for
> the average of a list of numbers, and they're all integers, and the
> avg() function truncates to integer, then the solution is to use sum()
> and explicitly cast to floating point before dividing.

First, that's not equivalent.  Try the following in Python 3:

values = [int(sys.float_info.max / 10)] * 20
print(average(values))

Now try this:

print(average(map(float, values)))

I don't have an interpreter handy to test, but I expect the former to
produce the correct result and the latter to raise OverflowError on the
call to sum.

Second, why should the calling code have to worry about this implementation
detail anyway? The point of a generic function is that it's generic.
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