List comprehension in Python under specific circumstances

Question

lst_a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
lst_b = [[1, 4, 7], [6, 5, 4], [9, 8, 7]]

My goal is to check all nested lists in lst_a if the first entry == first entry of any element in lst_b. If it's not than copy ONLY THAT sublist. In this example he wouldn't copy lst_a[0] but 1 and 2.

I tried to achieve my goal with list comprehension but it won't work.

zero = [x[0] for x in lst_a]
if zero not in lst_b:
    # I don't know what to do here.

Creating a tuple or a dictionary isn't possible because the whole process is in a loop in which every second new data come in and I try to avoid copying duplicates to the list.

EDIT: lst_b should look like that after the whole process:

lst_b = [[1, 4, 7], [6, 5, 4], [9, 8, 7], [4, 5, 6], [7, 8, 9]]

question from:https://stackoverflow.com/questions/65830666/list-comprehension-in-python-under-specific-circumstances

Answer 1

There may be more efficient ways of doing this, but this accomplishes the goal.

>>> [a for a in lst_a if a[0] not in [b[0] for b in lst_b]]
[[4, 5, 6], [7, 8, 9]]