How to merge two lists into a list of multiple lists?

Question

Given two lists lst1 and lst2:

lst1 = ['a']
lst2 = [['b'],
        ['b', 'c'],
        ['b', 'c', 'd']]

I'd like to merge them into a list of multiple lists with a desired output like this:

desiredList = [['a', ['b']],
              ['a', ['b', 'c']],
              ['a', ['b', 'c', 'd']]]

Here is one of my attempts that comes close using lst1 + lst2 and list.append():

lst3 = []
for elem in lst2:
    new1 = lst1
    new2 = elem
    theNew = new1 + new2
    lst3.append(theNew)

print(lst3)

#Output:
#[['a', 'b'],
#['a', 'b', 'c'],
#['a', 'b', 'c', 'd']]

Expanding on this, I thought another variation with theNew = new1.append(new2)would do the trick. But no:

lst3 = []
for elem in lst2:
    new1 = lst1
    new2 = elem
    #print(new1 + new2)
    #theNew = new1 + new2
    theNew = new1.append(new2)

    lst3.append(theNew)
print(lst3)

# Output:
[None, None, None]

And you'll get the same result with extend.

I guess this should be really easy, but I'm at a loss.

Thank you for any suggestions!

`list.append()` is in-place and `return`s `None` so never assign to it with `=` — Chris_Rands, Feb 06 '18 at 11:05

score 1 · Accepted Answer · answered Feb 06 '18 at 11:01

1

You could achieve your desired output with itertools.zip_longest with a fillvalue:

>>> from itertools import zip_longest
>>> list(zip_longest(lst1, lst2, fillvalue=lst1[0]))
[('a', ['b']), ('a', ['b', 'c']), ('a', ['b', 'c', 'd'])]

Or if you need a list of lists:

>>> [list(item) for item in zip_longest(lst1, lst2, fillvalue=lst1[0])]
[['a', ['b']], ['a', ['b', 'c']], ['a', ['b', 'c', 'd']]]

Note this assumes that lst1 always contains a single element as in your example.

answered Feb 06 '18 at 11:01

Chris_Rands

38,994
14
83
119

1

Or just: `[[*lst1, other] for other in lst2]` – Jon Clements Feb 06 '18 at 11:13
@Jon Clements. As far as I can tell, this has to be the most elegant solution. Would you like to write it up as an answer? – vestland Feb 08 '18 at 07:53

score 1 · Answer 2 · answered Feb 06 '18 at 11:08

1

Or you can use use append, but you need to create new copy of the lst1:

lst3 = []
for elem in lst2:
    theNew = lst1[:]
    theNew.append(new2)
    lst3.append(theNew)
print(lst3)

answered Feb 06 '18 at 11:08

Nikolay Tymtsiv

21
1

score 1 · Answer 3 · answered Feb 06 '18 at 11:10

1

from itertools import product

list(product(lst1,lst2))
>>>[('a', ['b']), ('a', ['b', 'c']), ('a', ['b', 'c', 'd'])]

[lst1 + [new] for new in lst2]
>>>[['a', ['b']], ['a', ['b', 'c']], ['a', ['b', 'c', 'd']]]

answered Feb 06 '18 at 11:10

Veera Balla Deva

790
6
19

score 0 · Answer 4 · answered Feb 06 '18 at 11:21

0

This might help

desiredlist = list(map(lambda y:[lst1,y],lst2))

answered Feb 06 '18 at 11:21

Pawanvir singh

373
6
17

How to merge two lists into a list of multiple lists?

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