Question

I have two lists of lists – i.e.

[['1', 'expired', 'test', '0'], ['31', 'active', 'test', '1']]

as well as

[['1', 'Andrew', 'Alexander'], ['31', 'John', 'Smith']]

Lets call them list1 and list2

I want to merge list1 and list2, but ONLY when (note, this is pseudocode, trying to figure out how to program this in Python)

x[0] in list1 == x[0] in list2

I’m not sure how to write this out.

By merge I mean (pseudocode)

list[x] = list1[x] + list2[x] while x[0] in list1 == x[0] in list2

Output desired:

[['1', 'expired', 'test', '0', '1', 'Andrew', 'Alexander'], ['31', 'active', 'test', '1', '31', 'John', 'Smith']]

The only critical point is that not all of the x[0]’s are going to match up perfectly.

Answer 1

Using agf’s idea of employing a collections.defaultdict, this in O(m+n) where m and n are the lengths of the lists.

import collections
import itertools

x=[['1', 'expired', 'test', '0'], ['31', 'active', 'test', '1']]
y=[['1', 'Andrew', 'Alexander'], ['31', 'John', 'Smith']]

result=collections.defaultdict(list)
for item in itertools.chain(x,y):
    result[item[0]].append(item)
result=[list(itertools.chain.from_iterable(value)) for value in result.values()]
print(result)

yields

[['1', 'expired', 'test', '0', '1', 'Andrew', 'Alexander'], ['31', 'active', 'test', '1', '31', 'John', 'Smith']]

---

In the comments the OP says the desired output is

[['1', 'expired', 'test', '0', 'Andrew', 'Alexander'], ['31', 'active', 'test', '1', 'John', 'Smith']]

(this is different than the desired output posted in the original question.)

Then:

import collections
import itertools

x=[['1', 'expired', 'test', '0'], ['31', 'active', 'test', '1']]
y=[['1', 'Andrew', 'Alexander'], ['31', 'John', 'Smith']]

result={}
for item in itertools.chain(x,y):
    result.setdefault(item[0],item[:1]).extend(item[1:])
result=result.values()
print(result)

This is one of the few times I’ve found using setdefault more convenient than collections.defaultdict.