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[Python-ideas] Python3.3 Decimal Library Released

MRAB python at mrabarnett.plus.com
Tue Mar 4 03:10:29 CET 2014 
On 2014-03-04 01:55, Steven D'Aprano wrote:
> On Tue, Mar 04, 2014 at 10:42:57AM +1100, Chris Angelico wrote:
>
>> You could probably make the same performance argument against making
>> Unicode the default string datatype.
>
> I don't think so -- for ASCII strings the performance cost of Unicode is
> significantly less than the performance hit for Decimal:
>
> [steve at ando ~]$ python3.3 -m timeit -s "s = 'abcdef'*1000" "s.upper()"
> 100000 loops, best of 3: 8.76 usec per loop
> [steve at ando ~]$ python3.3 -m timeit -s "s = b'abcdef'*1000" "s.upper()"
> 100000 loops, best of 3: 7.05 usec per loop
>
> [steve at ando ~]$ python3.3 -m timeit -s "x = 123.4567" "x**6"
> 1000000 loops, best of 3: 0.344 usec per loop
> [steve at ando ~]$ python3.3 -m timeit -s "from decimal import Decimal" \
>> -s "x = Decimal('123.4567')" "x**6"
> 1000000 loops, best of 3: 1.41 usec per loop
>
>
> That's a factor of 1.2 times slower for Unicode versus 4.1 for Decimal.
> I think that's *fast enough* for all but the most heavy numeric needs,
> but it's not something we can ignore.
>
>
>> But a stronger argument is that
>> the default string should be the one that does the right thing with
>> text. As of Python 3, that's the case. And the default integer type
>> handles arbitrary sized integers (although Py2 went most of the way
>> there by having automatic promotion). It's reasonable to suggest that
>> the default non-integer numeric type should also simply do the right
>> thing.
>
> Define "the right thing" for numbers.
>
>> It's a trade-off, though, and for most people, float is sufficient.
>
> That's a tricky one. For people doing quote-unquote "serious" numeric
> work, they'll mostly want to stick to binary floats, even if that means
> missing out on all the extra IEEE-754 goodies that the decimal module
> has but floats don't. The momentum of 40+ years of almost entirely
> binary floating point maths does not shift to decimal overnight.
>
[snip]
Won't people doing quote-unquote "serious" numeric work be using numpy?
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