Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!goblin3!goblin2!goblin.stu.neva.ru!newsfeed.xs4all.nl!newsfeed2a.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
MIME-Version: 1.0
Sender: zachary.ware@gmail.com
In-Reply-To: <53206e6a$0$2886$e4fe514c@news2.news.xs4all.nl>
References: <53206e6a$0$2886$e4fe514c@news2.news.xs4all.nl>
From: Zachary Ware <zachary.ware+pylist@gmail.com>
Date: Wed, 12 Mar 2014 10:00:09 -0500
Subject: Re: Deep vs. shallow copy?
To: "python-list@python.org" <python-list@python.org>
Content-Type: text/plain; charset=UTF-8
Cc: Alex van der Spek <zdoor@xs4all.nl>
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.8093.1394636439.18130.python-list@python.org>
Lines: 56
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:68285

On Wed, Mar 12, 2014 at 9:25 AM, Alex van der Spek <zdoor@xs4all.nl> wrote:
> I think I understand the difference between deep vs. shallow copies but
> I was bitten by this:
>
> with open(os.path.join('path', 'foo.txt', 'rb') as txt:
>      reader = csv.reader(txt)
>      data = [row.append(year) for row in reader]
>
> This does not work although the append does complete. The below works:
>
> with open(os.path.join('path', 'foo.txt', 'rb') as txt:
>      reader = csv.reader(txt)
>      data = [row + [year] for row in reader]
>
> However in this context I am baffled. If someone can explain what is
> going on here, I would be most grateful.

Deep/shallow copying doesn't really come into this.  row.append()
mutates the list (row), it doesn't return a new list.  Like most
in-place/mutating methods in Python, it returns None instead of self
to show that mutation was done, so your listcomp fills `data` with
Nones; there is no copying done at all.  The second example works as
you expected because `row + [year]` results in a new list, which the
listcomp is happy to append to `data`--which does mean that `row` is
copied.

To avoid the copy that the second listcomp is doing (which really
shouldn't be necessary anyway, unless your rows are astronomically
huge), you have a couple of options.  First, you can expand your
listcomp and use append:

   with open(os.path.join('path', 'foo.txt'), 'rb') as txt: # with
your typo fixed ;)
       reader = csv.reader(txt)
       data = []
       for row in reader:
           row.append(year)
           data.append(row)

To me, that's pretty readable and pretty clear about what it's doing.
Then there's this option, which I don't recommend:

   import operator
   with open(os.path.join('path', 'foo.txt'), 'rb') as txt:
       reader = csv.reader(txt)
       data = [operator.iadd(row, [year]) for row in reader]

This works because operator.iadd is basically shorthand for
row.__iadd__([year]), which does return self (otherwise, the
assignment part of `row += [year]` couldn't work).  But, it's not as
clear about what's happening, and only saves a whole two lines (maybe
3 if you already have operator imported).

Hope this helps,
-- 
Zach