On 07/03/2016 15:31, Chris Angelico wrote:
On Tue, Mar 8, 2016 at 12:25 AM, BartC <b...@freeuk.com> wrote:
(The Python version of that program is here:
http://pastebin.com/cHx3UhQb. It should work with any Python.)
Actually, it won't work with any Python - not if it gets a broken
file. Your abortjpeg() function doesn't work :)
What does it do when it doesn't work? I just get 'can't load file' then
it stops if there's no such file. With an extra message if there's a
format error. (I'm not sure what 'raise' does, but it doesn't complain
about it. I think I realised I didn't know what came next and left it.)
But what you have there is a messy mixture of old-style classes used
as if they were C structs, array.array() as if it were a C array, and
utterly uncommented code that looks like it was ported from, again, C.
Benchmarking Py2 vs Py3 on that kind of code is like trying to figure
out whether it's easier to drag a 747 by your teeth or your toes.
Yeah, you might get a result, but it's a useless one.
I disagree. The program does its job perfectly (you will have to take it
further to verify the results, such as writing out the .ppm file and
viewing the contents).
But Py3 is slower doing this task than Py2 by 10 or 20% (or even 30% for
a smaller file). /This is in line with other observations./
> You're not going to prove anything useful about Python - any version
> of Python - by using it badly. Try implementing JPEG decoding in a
> more Pythonic way - or, better still, use pillow and don't write that
> kind of code yourself at all.
Before I wrote this, I looked at three other existing Python decoders,
all more Pythonic (one had been ported from C++ I think and was crammed
full of classes).
One worked so poorly that it was dismissed. The other two were full of
bugs and didn't fully support the 3 main colour formats, but when they
did work, were 3-6 times slower than my version IIRC.
They also only worked with Python 2 (so presumably would have been even
slower on 3!) It also meant I couldn't test PyPy on them. I tried psyco
but it made little difference. And actually the main purpose was to have
something substantial to try PyPy on.
So I don't know who's using Python more badly: me or those other authors.
(I'm quite pleased with my version: smaller, faster, works on all the
Pythons, supports all 3 colour formats and no decoding bugs that I'm
aware of, and it's the first Python program I've written that does
something useful.)
--
Bartc
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