Question

I have a large list

[[1,.., ..],[2,...,...],[5,...,...],[1,...,...]]

I need to remove all elements that have the same first value. (keep only once)

How to do it most efficiently?

Answer 1

Keep a set of the first values seen so far, and only keep sublists if their first value isn’t in the set.

Because set.add always returns None, keys.add(sublist[0]) or sublist is the same as None or sublist which is the same as sublist, so it doesn’t affect what gets kept in the list, while allowing you to add values to the set inside a list comprehension.

keys = set()
biglist = [keys.add(sublist[0]) or sublist 
              for sublist in biglist 
                  if sublist[0] not in keys]
del keys # if you don't need it any more