如何在不遇到MemoryError的情况下逐步编写一个巨大的bytearray文件

I am working on a tool that generates random data for testing purposes. See below the part of my code that is giving me grief. This works perfectly and faster(takes around 20 seconds) than conventional solutions when the file is around 400MB, however, once it reaches around 500MB I am getting an out of memory error. How can I pull contents from the memory and write it in a file progressively having not more than 10 MB in my memory at a single time.I am working on a tool that generates random da