[Python-ideas] grouping / dict of lists

[Guido van Rossum]

On a quick skim I see nothing particularly objectionable or controversial
in your PEP, except I'm unclear why it needs to be a class method on
`dict`. Adding something to a builtin like this is rather heavy-handed. Is
there a really good reason why it can't be a function in `itertools`? (I
don't think that it's relevant that it doesn't return an iterator -- it
takes in an iterator.)

Also, your pure-Python implementation appears to be O(N log N) if key is
None but O(N) otherwise; and the version for key is None uses an extra
temporary array of size N. Is that intentional?

Finally, the first example under "Group and Aggregate" is described as a
dict of sets but it actually returns a dict of (sorted) lists.

On Fri, Jun 29, 2018 at 10:54 AM Michael Selik <mike at selik.org> wrote:

> Hello,
>
> I've drafted a PEP for an easier way to construct groups of elements from
> a sequence. https://github.com/selik/peps/blob/master/pep-9999.rst
>
> As a teacher, I've found that grouping is one of the most awkward tasks
> for beginners to learn in Python. While this proposal requires
> understanding a key-function, in my experience that's easier to teach than
> the nuances of setdefault or defaultdict. Defaultdict requires passing a
> factory function or class, similar to a key-function. Setdefault is
> awkwardly named and requires a discussion of references and mutability.
> Those topics are important and should be covered, but I'd like to let them
> sink in gradually. Grouping often comes up as a question on the first or
> second day, especially for folks transitioning from Excel.
>
> I've tested this proposal on actual students (no students were harmed
> during experimentation) and found that the majority appreciate it. Some are
> even able to guess what it does (would do) without any priming.
>
> Thanks for your time,
> -- Michael
>
>
>
>
>
>
> On Thu, Jun 28, 2018 at 8:38 AM Michael Selik <mike at selik.org> wrote:
>
>> On Thu, Jun 28, 2018 at 8:25 AM Nicolas Rolin <nicolas.rolin at tiime.fr>
>> wrote:
>>
>>> I use list and dict comprehension a lot, and a problem I often have is
>>> to do the equivalent of a group_by operation (to use sql terminology).
>>>
>>> For example if I have a list of tuples (student, school) and I want to
>>> have the list of students by school the only option I'm left with is to
>>> write
>>>
>>>     student_by_school = defaultdict(list)
>>>     for student, school in student_school_list:
>>>         student_by_school[school].append(student)
>>>
>>
>> Thank you for bringing this up. I've been drafting a proposal for a
>> better grouping / group-by operation for a little while. I'm not quite
>> ready to share it, as I'm still researching use cases.
>>
>> I'm +1 that this task needs improvement, but -1 on this particular
>> solution.
>>
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-- 
--Guido van Rossum (python.org/~guido)
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