[Python-ideas] grouping / dict of lists

[Michael Selik]

On Tue, Jul 3, 2018, 6:32 PM Steven D'Aprano <steve at pearwood.info> wrote:

> On Tue, Jul 03, 2018 at 10:33:55AM -0700, Chris Barker wrote:
> > On Tue, Jul 3, 2018 at 8:33 AM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> >
> > > but why are we using key values by hand when grouping ought to do it
> for
> > >> us, as Michael Selik's version does?
> > >
> > >     grouping(words, key=len)
> >
> >
> > because supplying a key function is sometimes cleaner, and sometimes
> uglier
> > than building up a comprehension -- which I think comes down to:
> >
> > 1) taste (style?)
> >
> > 2) whether the key function is as simple as the expression
> >
> > 3) whether you ned to transform the value in any way.
>
>
> Of course you can prepare the sequence any way you like, but these are
> not equivalent:
>
>     grouping(words, keyfunc=len)
>
>     grouping((len(word), word) for word in words)
>
> The first groups words by their length; the second groups pairs of
> (length, word) tuples by equality.
>
>
> py> grouping("a bb ccc d ee fff".split(), keyfunc=len)
> {1: ['a', 'd'], 2: ['bb', 'ee'], 3: ['ccc', 'fff']}
>
> py> grouping((len(w), w) for w in "a bb ccc d ee fff".split())
> {(3, 'ccc'): [(3, 'ccc')], (1, 'd'): [(1, 'd')], (2, 'ee'): [(2, 'ee')],
> (3, 'fff'): [(3, 'fff')], (1, 'a'): [(1, 'a')], (2, 'bb'): [(2, 'bb')]}
>

This handles the case that someone is passing in n-tuple rows and wants to
keep the rows unchanged.

>
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