Attempting to use itertools.groupby

Question

So basically I have this list of tuples,like this

list1 = [(1, 0.3), (2, 0.0), (3, 0.5), (4, 0.1), (5, 0.7), (6, 0.5), (7, 0.0), (8, 0.3)]

And I was trying to organise it into sublists where every sublist has every tuple with the same second value, like this

list1 = [[0.0, (2, 0.0), (7, 0.0)], [0.1, (4, 0.1)], [0.3, (1, 0.3), (8, 0.3)], [0.5, (3, 0.5), (6, 0.5)], [0.7, (5, 0.7)]]

With the value they're organized around being the first value in each sublist.

I figured how to do this with basic built-in functions, but it's kinda ugly and once I found out about itertools.groupby I immediately thought I could use it and make the function simpler. The problem is I can't really get it to work. Can this method even be used for this type of problem or am I just not understanding how to use it?

Answer 1

You could use something like this:

list1 = [(1, 0.3), (2, 0.0), (3, 0.5), (4, 0.1), (5, 0.7), (6, 0.5), (7, 0.0), (8, 0.3)]
d = {k[1] : [] for k in list1}
for item in list1:
    d[item[1]].append(item)
output = sorted([[k] + v for k, v in d.items()])
print(output)

Output:

[[0.0, (2, 0.0), (7, 0.0)], 
 [0.1, (4, 0.1)], 
 [0.3, (1, 0.3), (8, 0.3)], 
 [0.5, (3, 0.5), (6, 0.5)], 
 [0.7, (5, 0.7)]]

Answer 2

Try:

from itertools import groupby
from operator import itemgetter

list1 = [(1, 0.3), (2, 0.0), (3, 0.5), (4, 0.1), (5, 0.7), (6, 0.5), (7, 0.0), (8, 0.3)]

output = [[k, *g] for k, g in groupby(sorted(list1, key=itemgetter(1)), key=itemgetter(1))]
print(output)
# [[0.0, (2, 0.0), (7, 0.0)], [0.1, (4, 0.1)], [0.3, (1, 0.3), (8, 0.3)], [0.5, (3, 0.5), (6, 0.5)], [0.7, (5, 0.7)]]

Note that you first need to sort the list, so that the groupby sees that the items to be grouped are adjacent to each other.