How to outer join list of lists in Python?

Question

I have two lists of lists in Python that I would like joined together. The structure and desired result is as follows:

ListofLists1 = [['20200701', DF_x1], ['20200702', DF_x2], ...]
ListofLists2 = [['20200702', DF_y2], ['20200704', DF_y4], ...]
ListofLists3 = [['20200702', DF_z2], ['20200707', DF_z7], ...]
ListofLists4 = [['20200702', DF_a2], ['20200704', DF_a4], ...]

DesiredList = [['20200701', DF_x1, ''   , ''   , ''   ], 
               ['20200702', DF_x2, DF_y2, DF_z2, DF_a2], 
               ['20200704', ''   , DF_y4, DF_z4, ''   ], 
               ['20200707', ''   , ''   , ''   , DF_a7]]

As you can see, each element of the original lists contains a date and a dataframe. I would like these lists merged, such that one date corresponds with all available dataframes for that date. Each sublist contains 3 elements. This would need to be an outer join, as each list-of-lists contains elements which the other does not.

Wups · Accepted Answer · 2020-10-29 19:30:50Z

3

First create a dictionary with the values from the first list, then update it with the values from the second one:

d = {l[0]: [l[1], ''] for l in ListofLists1}

for l in ListofLists2:
    d.setdefault(l[0], ['', ''])[1] = l[1]

Converting the dictionary back to a list, if needed:

joined_list = [[k, *v] for k, v in d.items()]

answered Oct 29, 2020 at 19:30

Wups

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5 Comments

Dylan Moore Over a year ago

This worked precisely, thank you! I thought it might have something to do with dictionaries, but I don't know enough about them yet to have executed it myself. I will still need to chew on this solution a bit to understand how it all works, but I am thankful!

s3dev Over a year ago

Nice answer. Very close to the solution I had rolling around in my head; just hadn’t written the answer yet. +1 mate.

dstromberg Over a year ago

Good, but I would use a collections.defaultdict(list) instead of a plain dict. It's simpler and more concise.

Dylan Moore Over a year ago

To generalize this solution a bit, I now have 4 lists of lists, rather than 2. How would I combine all 4? See the edit above. Thanks again!

Wups Over a year ago

@DylanMoore the empty list then needs to be ['', '', '', ''] and the for loop needs to be done for list 2 to 4. Be careful updating the correct position; not always [1].

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