help with list comprehension

[George Sakkis]

On May 2, 2:17 am, Matimus <mccre... at gmail.com> wrote:

> On May 1, 10:50 pm, George Sakkis <george.sak... at gmail.com> wrote:
>
>
>
> > On May 1, 11:46 pm, Carsten Haese <carsten.ha... at gmail.com> wrote:
>
> > > Yves Dorfsman wrote:
>
> > > > In the following script, m1() and m2() work fine. I am assuming m2() is
> > > > faster although I haven't checked that (loops through the list twice
> > > > instead of once).
>
> > > Well, let's check it:
>
> > > $ python -m timeit -s "import x" "x.m1()"
> > > 100000 loops, best of 3: 6.43 usec per loop
>
> > > $ python -m timeit -s "import x" "x.m2()"
> > > 100000 loops, best of 3: 8.34 usec per loop
>
> > > As it turns out, m1 is faster than m2. The reason is that list
> > > comprehensions do the loop in C, whereas the for-append pattern does the
> > > loop on the Python level.
>
> > > > Now what I am trying to do is something like m3(). As currently written
> > > > it does not work, and I have tried different ways, but I haven't managed
> > > > to make it work.
>
> > > > Is there a possibility ? Or is m2() the optimum ?
>
> > > > [...]
> > > > def m1():
> > > >   colours = [ e['colour'] for e in l ]
> > > >   nums    = [ e['num']    for e in l ]
>
> > > > def m2():
> > > >   colours = []
> > > >   nums    = []
> > > >   for e in l:
> > > >     colours.append(e['colour'])
> > > >     nums.append(e['num'])
>
> > > > #def m3():
> > > > #  colours, nums = [ e['colour'], e['num'] for e in l ]
>
> > > m3 doesn't work because you're building a list of 10 color/number pairs
> > > that you're trying to unpack that into just two names. The working
> > > "derivative" of m3 is m1, which is the most natural, fastest and
> > > clearest solution to your problem.
>
> > Another alternative is:
>
> > from operator import itemgetter
>
> > def m3():
> >     colours, nums = zip(*map(itemgetter('colour','num'), l))
>
> > It's slower than m1() but faster than m2(); it's also the most
> > concise, especially if you extract more than two keys.
>
> > George
>
> Why deal with zip and unpacking?

DRY [1].

This seems more obvious to me:
>
> colours, nums = map(itemgetter('colour'), l), map(itemgetter('num'),
> l)

Maybe, but now extend it to three keys. And then four. And then... you
see my point.

George

[1] http://en.wikipedia.org/wiki/Don%27t_repeat_yourself