Yeah this is covered in pretty much every style guide, tutorial, and reference. And is an easy screener in an interview. I'm also surprised anyone using Python as more than a toy experiment had this issue, especially in such a critical service.
I've worked on a lot of Python as a hobbyist and student, much of it in real world use - probably at least 100kloc - and I've never encountered this. Where is this information? I'm worried that I'm missing some other "obvious" things. I've of course run into python's shallow vs deep copies, but I don't remember default values being shared between invocations.