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I am using python 3.7.3 on Debian 10 and I'm finding it writing .python_history in my homedir. I want to shut that off completely

According to [docs.python.org/3/library/site.html] it's possible:

That much, I already found. I spent some time playing with putting various things in sys.__interactivehook__ and have so far failed to find the magic incantation necessary to stop it from writing .python_history - I'm perfectly willing to believe one exists, but I have not managed to find it, either in documentation or on my own. Everything I've tried either errored out during startup or still wrote .python_history. Mouse