Python itertools groupby multiple use in list comprehension

Question

I'm trying to do something like that:

groups = groupby(all_data, key=itemgetter(1))
result = []
for k,g in groups:
    gg = list(g)
    count = len(gg)
    v = gg[0][3:] #No empty groups, right?
    hosts = ";".join([x[0] for x in gg])
    result.append(v + (count,) + (hosts,))

I hate loops. ;) Is there any chance to do it with comprehension? The problem is g is iterator, not list and I have no idea how can I convert it to list in comprehension.

Maybe you look for something like that [list(g)[0][3:] + (len(list(g)),) + (';'.join(x[0] for x in list(g)[0][3:])) for k, g in itertools.groupby(all_data, itemgetter(1))] however it would be better if you provide your input and your expected output. — What
– What, Commented May 4, 2018 at 10:20
This is way too much code to put into a list comprehension. Why would you ever try to convert 8(!) lines of code into a single one? If you want a better way to write this loop, ask for a better way to write this loop. Don't ask for a list comprehension, because that's a worse way to write this loop. — Aran-Fey
– Aran-Fey, Commented May 4, 2018 at 10:21

jpp · Accepted Answer · 2018-05-04 10:51:35Z

3

There is no need or significant benefit in converting this into a list comprehension. You may be left with unreadable code that only you can understand.

One stylistic point I do support is not to create lists prematurely or unnecessarily. This leaves the option open to iterate results rather than build a list containing all the results.

For example:

from itertools import groupby
from operator import itemgetter

all_data = [('A', 'B', 'C', 'S', 'T'),
            ('E', 'B', 'C', 'U', 'V'),
            ('H', 'I', 'J', 'W', 'X')]

groups = groupby(all_data, key=itemgetter(1))

def fun(groups):
    for _, g in groups:
        gg = list(g)
        hosts = ';'.join(list(map(itemgetter(0), gg)))
        yield gg[0][3:] + (len(gg),) + (hosts,)

res = list(fun(groups))

[('S', 'T', 2, 'A;E'),
 ('W', 'X', 1, 'H')]

answered May 4, 2018 at 10:51

jpp

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