Python: Value disappears from the list when using groupby and converting to a dictionary

Question

I am trying to write a simple function that gives the result of a runoff vote.

I start with a nested list with names of candidates, and I would like to group them by the first element and put that into a dictionary (where the first element is the key and a nested list of all the lists with this first element is the value)

def runoff_rec(xxx):
    print(xxx)


    sortedvotes = groupby(xxx, key=lambda x: x[0])
    votesdict = {}
    for key, value in sortedvotes:
        votesdict[key] = list(value)


    print(votesdict)

at the first print, the nested list looks like this:

[['Johan Liebert', 'Daisuke Aramaki', 'Lex Luthor', 'Gihren Zabi'], 
['Daisuke Aramaki', 'Gihren Zabi', 'Johan Liebert', 'Lex Luthor'], 
['Daisuke Aramaki', 'Lex Luthor', 'Gihren Zabi', 'Johan Liebert'], 
['Johan Liebert', 'Gihren Zabi', 'Lex Luthor', 'Daisuke Aramaki'], 
['Lex Luthor', 'Johan Liebert', 'Daisuke Aramaki', 'Gihren Zabi'], 
['Gihren Zabi', 'Daisuke Aramaki', 'Johan Liebert', 'Lex Luthor']]

but when I print the dictionary it looks like this:

{'Johan Liebert': [['Johan Liebert', 'Gihren Zabi', 'Lex Luthor', 'Daisuke Aramaki']], 
'Daisuke Aramaki': [['Daisuke Aramaki', 'Gihren Zabi', 'Johan Liebert', 'Lex Luthor'], ['Daisuke Aramaki', 'Lex Luthor', 'Gihren Zabi', 'Johan Liebert']],
 'Lex Luthor': [['Lex Luthor', 'Johan Liebert', 'Daisuke Aramaki', 'Gihren Zabi']], 
'Gihren Zabi': [['Gihren Zabi', 'Daisuke Aramaki', 'Johan Liebert', 'Lex Luthor']]}

One of the values from the list (the first one) has disappeared. Any idea why that might happen?

Thank you in advance, have a beautiful day

A dict cannot have duplicate keys. The second assignment for the key `Daisuke Aramaki` has overwritten the previous assignment. The same for `John Liebert`. — j1-lee, Jul 07 '22 at 03:07
I know, but does that apply here? The way I intended it to work is that the `groupby`sorts all the lists into separate elements, each with different key (for example, all the ones starting with `John Liebert` would be in one entry in the groupby object). if they're all in a nested list under a single key, they should not overwrite. `Daisuke Aramaki` seems to work fine, creating a dict value with 2 lists, it's just the `John Liebert` that doesn't work — Janek Pawlak, Jul 07 '22 at 03:54
Oh my bad! I didn't see the second sublist for `Daisuke Aramaki`. But anyway, `Johan Liebert` still might be due to the duplicate key issue; you didn't sort the input list before `groupby`, so that `groupby` treats `Johan Liebert` in two separate cases. Note that `groupby` only groups adjacent entities. This might be an explanation why the answer below works. — j1-lee, Jul 07 '22 at 03:55

495088732qqcom · Accepted Answer · 2022-07-08T08:12:24.123

1

i guess u want this

def runoff_rec(xxx):
    print(xxx)

    xxx.sort(key=lambda x: x[0])
    sortedvotes = groupby(xxx, key=lambda x: x[0])
    votesdict = {}
    for key, value in sortedvotes:
        votesdict[key] = list(value)


    print(votesdict)

groupby comment

""" make an iterator that returns consecutive keys and groups from the iterable iterable Elements to divide into groups according to the key function. key A function for computing the group category for each element. If the key function is not specified or is None, the element itself is used for grouping. """

consecutive is importan

edited Jul 08 '22 at 08:12

answered Jul 07 '22 at 03:21

495088732qqcom

44
5

Not sure how, but it worked. Thank you, I'll still be looking for an explanation but the code you provided works really well. – Janek Pawlak Jul 07 '22 at 03:48
update: the solution explained in the comments, by j1-lee, thank you both for contributing – Janek Pawlak Jul 07 '22 at 05:31
""" make an iterator that returns consecutive keys and groups from the iterable iterable Elements to divide into groups according to the key function. key A function for computing the group category for each element. If the key function is not specified or is None, the element itself is used for grouping. """ consecutive is important – 495088732qqcom Jul 07 '22 at 05:34
@495088732qqcom nice answer! However, remember that others might stumble onto your answer at a later date when they have a similar issue, consider editing your answer to include the explanation rather than leaving it as a comment. Cheers – G. Anderson Jul 07 '22 at 15:40

Python: Value disappears from the list when using groupby and converting to a dictionary

1 Answers1