Explanation of this Python language feature? [x for x in x for x in x] (to flatten a nested list)

[Steven D'Aprano]

On Fri, 04 Apr 2014 02:13:13 -0600, Ian Kelly wrote:

> On Fri, Apr 4, 2014 at 1:52 AM, Steven D'Aprano
> <steve+comp.lang.python at pearwood.info> wrote:
>> py> from decimal import *
>> py> getcontext().prec = 16
>> py> x = Decimal("0.7777777777787516") py> y =
>> Decimal("0.7777777777787518") py> (x + y) / 2
>> Decimal('0.7777777777787515')
>>
>> "Guido, why can't Python do maths???"
> 
> Well, you need to work within the system:
> 
>>>> (5*x + 5*y) / 10
> Decimal('0.7777777777787517')
> 
> Actually, I have no idea whether that formula can be relied upon or the
> correctness of the above was just luck.


And what happens when x+y would have been calculated correctly, but one, 
or both, of 5*x or 5*y loses catastrophically loses accuracy due to 
overflow?

py> x = 3.1e307
py> y = 3.3e307
py> (x+y)/2
3.2e+307
py> (5*x+5*y)/10
inf

(I've used regular floats here out of laziness, the same principle 
applies to Decimals -- there will be *some* number x which is finite, but 
5*x overflows to infinity.)



-- 
Steven D'Aprano
http://import-that.dreamwidth.org/