Keep the longest chains

[Amniote]

"give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime."

So grab a paper and a pen and try to be a computer. What would you do if you need to achieve desired result manually:

- in order to keep longest lines you have to check all lines, right? So - go through file line by line
- in order to keep longest lines you have to know length of every line, right? So - determine length of every line

Now just pick the longest ones (there is ambiguity in task description and therefore I don't know what exactly and in which order should be picked).

Implement it in Python and off you go.

Thank you for your reply,

So I wrote this script for the moment:

L1=[]
L2=[]
L3=[]
L4=[]
with open ("C:/Users/lveillat/Desktop/Données stage/Fichiers tests/test_retrait_chaines_intermediaire.txt","r") as f1:

	for lignes in f1:
		lignes=lignes.rstrip('\n')
		L1.append (lignes)
		L2.append (lignes)

	for i in L1:
		if i not in L4:
		
			L4.append(i)
			L3.append(i)
		else:
			continue

		for j in L2:
			if j.startswith(i) and i != j:
				L4.append(j)
				L3.append(j)

		print(L3)
		L3.clear()

With this file:

1 2 3 4 5
1 2 3 4 5 6
4 5 2 1
4 5 2 1 8 9
4 5 2 1 8
7 8 9
7 8 9 10
7 8 9 10 11
7 8 9 45 16
7 8 9 45

And he gives me this result:

['1 2 3 4 5', '1 2 3 4 5 6']
['4 5 2 1', '4 5 2 1 8 9', '4 5 2 1 8']
['7 8 9', '7 8 9 10', '7 8 9 10 11', '7 8 9 45 16', '7 8 9 45']

It's ok for the first two lines, however for the last one, the channels 7 8 9 45 16 and 7 8 9 45 are too much (they should be in a separate list).

How could I solve this problem?