[Python-ideas] grouping / dict of lists

[Nicolas Rolin]

My question would be : does it have to be a key function ? Can't we just
remove the "key" argument ?
Because for pretty much all the given examples, I would find my default as
readable and nearly as short as the "key" syntax :

> grouping(words, key=len)
grouping((len(word), word for word in words))

>grouping(names, key=itemgetter(0))
grouping((name_initial, name_initial+_name for name_initial, *_name in
names)

>grouping(contacts, key=itemgetter('city')
grouping((contact['city'], contact for contact in contacts)

>grouping(employees, key=itemgetter('department'))
grouping((employee['department'], employee for employee in employees)

>grouping(os.listdir('.'), key=lambda filepath:
os.path.splitext(filepath)[1])
grouping((os.path.splitext(filepath)[1]), filepath for filepath in
os.listdir('.'))

>grouping(transactions, key=lambda v: 'debit' if v > 0 else 'credit')
grouping(('debit' if v > 0 else 'credit', transaction_amount for
transaction_amount in transactions))

The code is slightly more verbose, but it is akin to filter(iterable,
function) vs (i for i in iterable if function(i)).

-- 
Nicolas Rolin

2018-07-02 11:52 GMT+02:00 Michael Selik <mike at selik.org>:

> On Mon, Jul 2, 2018 at 2:32 AM Nicolas Rolin <nicolas.rolin at tiime.fr>
> wrote:
>
>> I think the current default quite weird, as it pretty much account to a
>> count() of each key (which can be useful, but not really what I except from
>> a grouping). I would prefer a default that might return an error to a
>> default that says ok and output something that is not what I might want.
>> For example the default could be such that grouping unpack tuples (key,
>> value) from the iterator and do what's expected with it (group value by
>> key). It is quite reasonable, and you have one example with (key, value) in
>> your example, and no example with the current default. It also allows to
>> use syntax of the kind
>>
>> >grouping((food_type, food_name for food_type, food_name in foods))
>>
>> which is pretty nice to have.
>>
>
> I'm of two minds on this point. First, I agree that it'd be nice to handle
> the (key, value) pair case more elegantly. It comes to mind often when
> writing examples, even if proportionally less in practice.
>
> Second, I'll paraphrase "Jakob's Law of the Internet User Experience" --
> users spend most of their time using *other* functions. Because
> itertools.groupby and other functions in Python established a standard for
> the behavior of key-functions, I want to keep that standard.
>
> Third, some classes might have a rich equality method that allows many
> interesting values to all wind up in the same group even if using the
> default "identity" key-function.
>
> Thanks for the suggestion. I'll include it in the PEP, at least for
> documenting all reasonable options.
>



-- 

--
*Nicolas Rolin* | Data Scientist
+ 33 631992617 - nicolas.rolin at tiime.fr <prenom.nom at tiime.fr>


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