I need to be able to find the first common list (which is a list of coordinates in this case) between a variable amount of lists.
i.e. this list
>>> [[[1,2],[3,4],[6,7]],[[3,4],[5,9],[8,3],[4,2]],[[3,4],[9,9]]]
should return
>>> [3,4]
If easier, I can work with a list of all common lists(coordinates) between the lists that contain the coordinates.
I can't use sets or dictionaries because lists are not hashable(i think?).
Correct, list objects are not hashable because they are mutable. tuple objects are hashable (provided that all their elements are hashable). Since your innermost lists are all just integers, that provides a wonderful opportunity to work around the non-hashableness of lists:
>>> lists = [[[1,2],[3,4],[6,7]],[[3,4],[5,9],[8,3],[4,2]],[[3,4],[9,9]]]
>>> sets = [set(tuple(x) for x in y) for y in lists]
>>> set.intersection(*sets)
set([(3, 4)])
Here I give you a set which contains tuples of the coordinates which are present in all the sublists. To get a list of list like you started with:
[list(x) for x in set.intersection(*sets)]
does the trick.
To address the concern by @wim, if you really want a reference to the first element in the intersection (where first is defined by being first in lists[0]), the easiest way is probably like this:
#... Stuff as before
intersection = set.intersection(*sets)
reference_to_first = next( (x for x in lists[0] if tuple(x) in intersection), None )
This will return None if the intersection is empty.