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From: Chris Angelico <rosuav@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: x=something, y=somethinelse and z=crud all likely to fail - how do i wrap them up
Date: Sun, 31 Jan 2016 20:40:48 +1100
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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