Path: csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From: Chris Angelico <rosuav@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: pylint woes
Date: Sun, 8 May 2016 11:36:26 +1000
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On Sun, May 8, 2016 at 11:16 AM, DFS <nospam@dfs.com> wrote:
> On 5/7/2016 1:01 PM, Chris Angelico wrote:
>> The suggestion from a human would be to use zip(), or possibly to
>> change your data structures.
>
>
> Happens like this:
>
> address data is scraped from a website:
>
> names = tree.xpath()
> addr  = tree.xpath()
>
> I want to store the data atomically, so I parse street, city, state, and zip
> into their own lists.
>
> "1250 Peachtree Rd, Atlanta, GA 30303
>
> street = [s.split(',')[0] for s in addr]
> city   = [c.split(',')[1].strip() for c in addr]
> state  = [s[-8:][:2] for s in addr]
> zipcd  = [z[-5:] for z in addr]

So you're iterating over addr lots of times, and building separate
lists. As an alternative, you could iterate over it *once*, and have a
single object representing an address.

> Why is it better to zip() them up and use:
>
> for item1, item2, item3 in zip(list1, list2, list3):
>      do something with the items
>
> than
>
>
> for j in range(len(list1)):
>    do something with list1[j], list2[j], list3[j], etc.

Because 'j' is insignificant here, as is the length of the list. What
you're doing is iterating over three parallel lists - not counting
numbers. Imagine that, instead of lists, you just have *sequences* -
ordered collections of things. You can follow a recipe without knowing
the numbers of the individual lines; you just need to know the
sequence. Here, iterate over this collection:

* Collect ingredients.
* Cream the butter and the sugar.
* Sift the salt into the flour.
* Fold the mixture into an origami crane.

These instructions work whether they're numbered or not.

ChrisA