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Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up

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On Sun, Jan 31, 2016 at 8:21 PM, Steven D'Aprano <steve@pearwood.info> wrote:
> Hmmm. Well, I've never used lxml, but the first obvious problem I see is
> that your lines:
>
> description = li_item.find_class('vip')[0].text_content()
>
> link = li_item.find_class('vip')[0].get('href')
>
> price_dollar = li_item.find_class('lvprice prc')[0].xpath('span')[0].text
>
> bids = li_item.find_class('lvformat')[0].xpath('span')[0].text
>
>
> look suspiciously like a violation of the Liskov Substitution Principle.
> ("Talk to your dog, not to the dog's legs!") A long series of chained dot
> accesses (or equivalent getitem, call, getitem, dot, etc) is a code-smell
> suggesting that you are trying to control your dog's individual legs,
> instead of just calling the dog.

(Isn't that the Law of Demeter, not LSP?)

The principle of "one dot maximum" is fine when dots represent a form
of ownership. The dog owns his legs; you own (or, have a relationship
with) the dog. But in this case, the depth of subscripting is more
about the inherent depth of the document, and it's more of a data
thing than a code one. Imagine taking a large and complex JSON blob
and loading it into a Python structure with nested lists and dicts -
it wouldn't violate software design principles to call up
info["records"][3]["name"], even though that's three indirections in a
row. Parsing HTML is even worse, as there's generally going to be
numerous levels of structure that have no semantic meaning (they're
there for layout) - so instead of three levels, you might easily have
a dozen.

ChrisA

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x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up "Veek. M" <vek.m1234@gmail.com> - 2016-01-31 10:28 +0530
  Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Chris Angelico <rosuav@gmail.com> - 2016-01-31 16:23 +1100
    Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up "Veek. M" <vek.m1234@gmail.com> - 2016-01-31 11:59 +0530
      Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up "Veek. M" <vek.m1234@gmail.com> - 2016-01-31 12:01 +0530
      Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Steven D'Aprano <steve@pearwood.info> - 2016-01-31 20:21 +1100
        Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Chris Angelico <rosuav@gmail.com> - 2016-01-31 20:40 +1100
          Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Steven D'Aprano <steve@pearwood.info> - 2016-01-31 21:14 +1100
            Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Chris Angelico <rosuav@gmail.com> - 2016-02-01 00:27 +1100
      Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Peter Otten <__peter__@web.de> - 2016-01-31 11:40 +0100
      Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Larry Hudson <orgnut@yahoo.com> - 2016-01-31 13:27 -0800
  Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Steven D'Aprano <steve@pearwood.info> - 2016-01-31 18:22 +1100
  Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up "Veek. M" <vek.m1234@gmail.com> - 2016-01-31 20:55 +0530
  Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up Vincent Davis <vincent@vincentdavis.net> - 2016-02-01 11:40 -0700

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