How to diff the two files using Python Generator

If the values are in sequential order, you can simply note the previous value and see if the difference equals one:

prev = 0
with open('numbers.txt','r') as f:
    for line in f:
        value = int(line.strip())
        for i in range(prev, value-1):
            print('missing:', i+1)
    prev = value
# output numbers that are missing at the end of the file (see comment by @blhsing)
for i in range(prev, 1000000000000):
    print('missing:', i+1)

This should work fine in python3, as readlines is an iterator so will not load the full file at once or keep it in memory.

You can iterate over all the numbers generated by range and keep comparing the number to the next number in the file. Output the number if it's missing, or read the next number for the next match:

with open('numbers') as f:
    next_number = 0
    for n in range(1000000000001):
        if n == next_number:
            next_number = int(next(f, 0))
        else:
            print(n)

Demo (assuming target numbers from 1 to 10 instead): https://repl.it/repls/WaterloggedUntimelyCoding