Print out all pairs of numbers whose sum is equal to X

numberpairs.py

"""
You are given a sorted array of positive integers and a number 'X'.
Print out all pairs of numbers whose sum is equal to X.
Print out only unique pairs and the pairs should be in ascending order

Input:

Your program should accept as its first argument a filename.
This file will contain a comma separated list of sorted numbers
and then the sum 'X', separated by semicolon. Ignore all empty lines.
If no pair exists, print the string NULL e.g.

echo -e "1,2,3,4,6;5\n2,4,5,6,9,11,15;20\n1,2,3,4;50" | python3 numberpairs.py
"""

from sys import stdin

for line in stdin.readlines():
    l, n = line.strip().split(';')
    n = int(n)
    # only grab the numbers that are less than or equal to n
    l = [int(i) for i in l.strip().split(',') if int(i) <= n]
    while l:
        last = l[-1]
        x = n - last
        i = 0 
        found = False
        while not found and i < len(l) - 1:
            if l[i] == x:
                print('{},{};'.format(l[i], last), end='')
                found = True
            i += 1
        if not found:
            print('NULL', end='')
        # to save time for next iteration, the list can be
        # cut down to exclude the numbers used already.
        l = l[i:len(l)-1]
    print()