How to calculate the difference between two lists of lists in Python

Question

I have two lists like the following:

A = [[1, 2, 3], [1, 2, 4], [4, 5, 6]]

and

B = [[1, 2, 3], [1, 2, 6], [4, 5, 6], [4, 3, 6]]

And I wish to calculate the difference, which is equal to the following:

A - B =[[1, 2, 4]]

In other words, I want to treat A and B as a set of lists (all of the sample size, in this example it is 3) and find the difference (ie, remove all lists in B, which are also in A and return the rest.).

Is there a faster way than using multiple for loops for this?

Answer 1

Simple list comprehension will do the trick:

[a for a in A if a not in B]

output:

[[1, 2, 4]]

Answer 2

If you convert the second list to a set first, then membership tests are asymptotically faster; the downside is you have to convert the rows to tuples so that they can be in a set. (Consider having the rows as tuples instead of lists in the first place.)

def list_of_lists_subtract(a, b):
    b_set = {tuple(row) for row in b}
    return [row for row in a if tuple(row) not in b_set]

Note that "asymptotically faster" only means this should be faster for large inputs; the simpler version will likely be faster for small inputs. If performance is critical then it's up to you to benchmark the alternatives on realistic data.

Answer 3

You can try this.

• Convert the first list of lists to a set of tuples S1
• Convert the second list of lists to a set of tuples S2
• Use the difference method or simply S1 - S2 to get the set of tuples that are present in S1 but not in S2
• Convert the result obtained to the desired format (in this case, a list of lists).

# (Untested)

A = [[1, 2, 3], [1, 2, 4], [4, 5, 6]]
B = [[1, 2, 3], [1, 2, 6], [4, 5, 6], [4, 3, 6]]

set_A = set([tuple(item) for item in A])
set_B = set([tuple(item) for item in B])

difference_set = set_A - set_B

difference_list = [list(item) for item in sorted(difference_set)]

print(difference_list)