On Thu, Jun 28, 2018 at 10:24 AM Rob Cliffe via Python-ideas <python-ideas@python.org> wrote:

def group_by(iterable, groupfunc, itemfunc=lambda x:x, sortfunc=lambda x:x): # Python 2 & 3 compatible!
    D = {}
    for x in iterable:
        group = groupfunc(x)
        D[group] = D.get(group, []) + [itemfunc(x)]
    if sortfunc is not None:
        for group in D:
            D[group] = sorted(D[group], key=sortfunc)
    return D

The fact that you didn't use ``setdefault`` here, opting for repeatedly constructing new lists via concatenation, demonstrates the need for a built-in or standard library tool that is easier to use.

I'll submit a proposal for your review soon.