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Re: Deep vs. shallow copy?

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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

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Thread

Deep vs. shallow copy? Alex van der Spek <zdoor@xs4all.nl> - 2014-03-12 14:25 +0000
  Re: Deep vs. shallow copy? Skip Montanaro <skip@pobox.com> - 2014-03-12 09:48 -0500
  Re: Deep vs. shallow copy? Zachary Ware <zachary.ware+pylist@gmail.com> - 2014-03-12 10:00 -0500
    Re: Deep vs. shallow copy? Alex van der Spek <zdoor@xs4all.nl> - 2014-03-12 15:29 +0000
      Re: Deep vs. shallow copy? Wayne Brehaut <wbrehaut@mcsnet.ca> - 2014-03-12 13:06 -0600
      Re: Deep vs. shallow copy? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-12 23:07 +0000
        Re: Deep vs. shallow copy? Rustom Mody <rustompmody@gmail.com> - 2014-03-12 20:09 -0700
          Re: Deep vs. shallow copy? Ian <hobson42@gmail.com> - 2014-03-13 11:38 +0000
            Re: Deep vs. shallow copy? Rustom Mody <rustompmody@gmail.com> - 2014-03-13 08:28 -0700
              Re: Deep vs. shallow copy? random832@fastmail.us - 2014-03-13 13:25 -0400
        Re: Deep vs. shallow copy? Roy Smith <roy@panix.com> - 2014-03-13 07:44 -0400
          Re: Deep vs. shallow copy? Marko Rauhamaa <marko@pacujo.net> - 2014-03-13 14:27 +0200
            Re: Deep vs. shallow copy? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-13 23:41 +0000
              Re: Deep vs. shallow copy? Chris Angelico <rosuav@gmail.com> - 2014-03-14 10:55 +1100
                Re: Deep vs. shallow copy? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-14 01:41 +0000
              Re: Deep vs. shallow copy? Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-13 18:08 -0600
              Re: Deep vs. shallow copy? Chris Angelico <rosuav@gmail.com> - 2014-03-14 11:22 +1100
                Re: Deep vs. shallow copy? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-14 01:59 +0000
              Re: Deep vs. shallow copy? Rustom Mody <rustompmody@gmail.com> - 2014-03-13 19:57 -0700
                Re: Deep vs. shallow copy? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-14 04:43 +0000
              Re: Deep vs. shallow copy? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-14 06:17 +0000
          Re: Deep vs. shallow copy? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-13 23:31 +0000

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