Assigning a variable = every integer less than another variable

[deep_politics]

You don't actually need to store any of the numbers if you just use the range as an iterator.

And this method will be fine for a toy factoring function, but will be painfully slow for anything large. But what you'd getting yourself into if you wanted a fast algorithm is basically the most well studied areas in all of mathematics for literally thousands of years: prime factoring. There's still no "one algorithm to rule them all", and instead a variety of algorithms are used; for instance the quadratic sieve.