On Fri, Jun 29, 2018 at 2:43 PM Guido van Rossum <guido@python.org> wrote:
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`.

Since it constructs a basic dict, I thought it belongs best as a dict constructor like dict.fromkeys. It seemed to match other classmethods like datetime.now.
Adding something to a builtin like this is rather heavy-handed.

I included an alternate solution of a new class, collections.Grouping, which has some advantages. In addition to having less of that "heavy-handed" feel to it, the class can have a few utility methods that help handle more use cases.
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.)

I considered placing it in the itertools module, but decided against because it doesn't return an iterator. I'm open to that if that's the consensus.
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?

Unintentional. I've been drafting pieces of this over the last year and wasn't careful enough with proofreading. I'll fix that momentarily...
Finally, the first example under "Group and Aggregate" is described as a dict of sets but it actually returns a dict of (sorted) lists.

Doctest complained at the set ordering, so I sorted for printing. You're not the only one to make that point, so I'll use sets for the example and ignore doctest.

Thanks for reading!
-- Michael

PS. I just pushed an update to the GitHub repo, as per these comments.

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

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@selik.org> wrote:
On Thu, Jun 28, 2018 at 8:25 AM Nicolas Rolin <nicolas.rolin@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:

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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