Simple question about Python lists

[Eric]

On Nov 11, 7:31 pm, Ben Finney <bignose+hates-s... at benfinney.id.au>
wrote:
> Steven D'Aprano <ste... at REMOVE.THIS.cybersource.com.au> writes:
> > On Wed, 12 Nov 2008 10:08:45 +1100, Ben Finney wrote:
>
> > >Eric<eric.sh... at gmail.com> writes:
> > >> In MATLAB, if  I just want the first, fifth  and eighth element I
> > >> might do something like this:
>
> > >> b = a([1 5 8]);
>
> > > Yes: the above code uses magic numbers which convey no semantic
> > > meaning, and should instead use *named* elemets of a container. In
> > > Python, that's done with a mapping type, which has the built-in
> > > type of ‘dict’.
>
> > > In other words: a list is best for sequences where any item is
> > > semantically identical to any other, and you can (for example)
> > > re-order all the elements in the list without changing their
> > > semantic meaning. If, instead, you want semantic meaning, that's
> > > best done via *names*, not magic numbers.
>
> > What the OP is asking for is a generalization of slicing.
>
> I don't think so. (The OP is, of course, welcome to respond to
> this or existing requests for clarification of their intent.)
>
> > Given a slice alist[start:end:stride] the indices are given by the
> > sequence start+i*stride, bounded by end. It seems to me that the OP
> > has asked how to take a slice where the indices are from an
> > arbitrary set, possibly given by an equation, but not necessarily.
>
> I think that's exactly what is *not* being asked for, based on the
> specific example given. The example was “I just want the first,
> fifth, and eighth element”, and the example implementation showed
> them with those numeric literals in the code. On that basis I
> interpreted the need for *those elements specifically*, for some
> semantic reason not yet disclosed.
>
> If, instead, the OP wanted to retrieve “indices … from an arbitrary
> set, possibly given by an equation”, I would recommend that *that
> equation* be abstracted behind an expression, or even a well-named
> function, that would make explicit what the *meaning* of the set was.
>
> I maintain that anything which expresses “retrieve the indices at
> these numbers” without saying *in the code* what the meaning of those
> numbers is, is to be deprecated in favour of an implementation that
> makes the semantic meaning explicit.
>
> And no, before anyone suggests it, an explanatory comment is not
> sufficient for this condition: the names and types used should make
> clear what the meaning of the expression is. A comment saying *why* is
> always appreciated; but if the comment needs to tell me *what* the
> code is doing or *how*, the code instead needs to be re-written to be
> explicit.
>
> --
>  \      “Saying that Java is nice because it works on all OSes is like |
>   `\     saying that anal sex is nice because it works on all genders” |
> _o__)                                                —http://bash.org/|
> Ben Finney

Thanks much for the interesting discussion. In case anyone wonders
what problem I was trying to solve in the first case, I was simply
parsing a large text file. I read in each line, searched for the key
text using line.find('search text') and if I found the text I was
looking for, parsed the line with a=line.split(' ') since there was
only spaces breaking up parts of the line. This provided a list "a"
from which I only wanted certain elements. I wanted to select just
those elements in a particular order and then join them as a comma
delimited string to output to a second file. This is why I wanted to
select only certain items in the list and generally in a non-
consecutive order. I think the original suggestion is probably the
best one:

b = [a[i] for i in [1, 5, 8]]

I understand the basic Python concept that there should be one obvious
way of doing something, so if there is a easier and better approach to
parsing text files (which I end up doing a lot of), feel free to
suggest it.

Thanks,

Eric