Why does my script stop execution after 33178210th iteration?

I am writing a python program to find and return a list of Twin primes between two numbers.

This is my code:

#prime selector
def is_prime(num):
    if num < 2 or num % 2 == 0: 
        return False
    if num == 2: 
        return True
    upr_lmt = int((num ** 0.5) + 1) 
    for number in range(3, upr_lmt, 2): 
        if num % number == 0:
            return False
    return True


def primeLister(end_point, st_point = 2):
    primeList = []
    for i in range(st_point, end_point + 1):
        if is_prime(i):
            primeList.append(i)
    return primeList

It is saved as prime_selector.py in a folder.

This is the testcase that I have written:

# Test case
from prime_selector import primeLister

def log_newline(file_writer):
    file_writer.write("\n\n")

def log_testcase(case_num, case_desc, result, file_writer):
    file_writer.write(f"================= Test Case {case_num + 1} =================\n")
    print((f"================= Test Case {case_num + 1} ================="))
    file_writer.write(case_desc)
    print(case_desc, end = "")
    file_writer.write(result)
    print(result)

num_list = [99991,999999937]

with open("./primeLister_testcase","w") as file_writer:
    for case_num, i in enumerate(num_list):
        case_desc = f"Listing all prime numbers till {i}:\n"
        result = "".join([f" > {prime}\n" for prime in primeLister(i)])

        log_testcase(case_num, case_desc, result, file_writer)

        if case_num + 1 != len(num_list):
            log_newline(file_writer)    

The testcase is also saved in the same folder locally. The program stops abruptly during the execution of primeLister testcase after writing the primes up to 99991 in the file.

I guess that it stops during the process of listing primes between 99991 and 999999937. I've got no idea why. My computer has an Intel i5-12450H processor with 16GB RAM. If that's relevant.

I have already tried implementing an infinite loop, threading and also using exception handling. Nothing has worked so far. What am I doing wrong?

I was expecting the program to keep executing until it found all the primes within the provided range.

The problem is that your process is aiming to produce about 50 million primes for the input 999999937, accumulating them in a list, and does this in an non-optimal way, as for each integer 𝑘 between 1 and 999999937 you call is_prime, which executes √𝑘/2 iterations when 𝑘 is prime.

Then, after a long running time, you concatenate those 50 million numbers with their decimal representation into one, long string, which will be about 500 million characters long. That is half a GB if we assume a 1-byte character representation. It may be that your program runs out of memory... if it ever finishes producing the primes in the first place.

Running your code with a small adaptation, turning primeLister into a generator, and then printing the primes one by one (well, I only printed one per 100000 to save time), I could monitor the progress, I got that generating the first million of primes took about 2 minutes, and it continued like this:

I stopped there, but extrapolating, I estimate that it would take about 22 hours to finish the call to primeLister with input 999999937, and only then the string of 0.5 GB would be created from it. I suppose your system just kills the process either because of memory or time constraints. In my test it didn't get killed for the 15 minutes I waited for it. Then I killed it myself.

But it is clear that this is not practical. You should use a sieve approach.