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@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')]}