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).

I don't know from SQL, so "group by" doesn't mean anything to me, but this:

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)

seems to me that the issue here is that there is not way to have a "defaultdict comprehension"

I can't think of syntactically clean way to make that possible, though.

Could itertools.groupby help here? It seems to work, but boy! it's ugly:

In [45]: student_school_list

Out[45]:

[('Fred', 'SchoolA'),

('Bob', 'SchoolB'),

('Mary', 'SchoolA'),

('Jane', 'SchoolB'),

('Nancy', 'SchoolC')]

In [46]: {a:[t[0] for t in b] for a,b in groupby(sorted(student_school_list, key=lambda t: t[1]), key=lambda t: t[

...: 1])}

...:

Out[46]: {'SchoolA': ['Fred', 'Mary'], 'SchoolB': ['Bob', 'Jane'], 'SchoolC': ['Nancy']}

-CHB

Christopher Barker, Ph.D.
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