Lets generate fibonacci sequence until our harddrive fills up.

fibo.py

#### The Fibo generator
import math
import os

def fibo(a=0, b=1):
    if a == 0 and b == 1:
        skip = False
    else:
        skip = True
    if not skip:
        yield a
    while True:
        if not skip:
            yield b
        else:
            skip = False
        a, b = b, a + b


def write_fibo_and_index(db, index, fib):
    size = math.ceil(fib.bit_length()/8)
    db.write(fib.to_bytes(size, "little"))
    index.write(db.tell().to_bytes(5, "little"))
    return size


def read_index(index):
    return int.from_bytes(index.read(5), "little")



with open("fibos", "a+b") as db:
    with open("index", "a+b") as index:
        # figure out if we are resuming or not
        last_index = os.stat(index.name).st_size - 5
        if last_index < 0:
            # We are starting from scatch
            a,b = 0, 1
            start = 0
            # Write the first index
            index.write((0).to_bytes(5, "little"))
        else:
            # Lets get a and b from the db
            index.seek(last_index-10)  # go back two indexes
            a_start = read_index(index)
            b_start = read_index(index)
            next_start = read_index(index)
            a_size = b_start - a_start
            b_size = next_start - b_start
            # Lets read a and b
            db.seek(a_start)
            a = int.from_bytes(db.read(a_size), "little")
            b = int.from_bytes(db.read(b_size), "little")
            start = index.tell()//5-1

        for c, f in enumerate(fibo(a, b),start = start):
            size = write_fibo_and_index(db, index, f)
            print(f"Fibo {c} is {size} bytes!")

            
            
##### Reading Fibos
import sys


def read_index(index):
    return int.from_bytes(index.read(5), "little")


seq = int(sys.argv[1])
sys.set_int_max_str_digits(0)

with open("index", "rb") as index:
    index.seek(seq*5)
    loc = read_index(index)
    size = read_index(index) - loc
    with open("fibos", "rb") as db:
        db.seek(loc)
        f = int.from_bytes(db.read(size), "little")
        print(f)

I used 5 bytes for the index because using more than 1TB for fibo numbers is extreme :P