A couple 100KLOC of Python, excluding 3rd party libraries? Other than Google or maybe NASA, I can't think of many places with so large python codebases.
A 100k Python project doesn't sound that bad, especially with a modular design and a great IDE (PyCharm). Good tools make reading and navigating the codebase easy. At that point, it matters a lot less how big it is, because you can trust that your tools enable you to find the answers you seek.
The project that I help maintain is a collection of Django apps totalling more than 700k lines. Thankfully I do not maintain all of it, but I am the primary maintainer for several sub-apps, totalling about 180k lines of Python. (This doesn't count the UI code, which is not in Python.)
I'm the sole author for about 12% of the part I maintain (since I wrote some of the individual sub-apps), and am familiar enough to describe probably half of the rest. There are still some parts that I really have to puzzle out ("What happens on the codepath to generate those reports?"), but that part shrinks over time.
The modular design that our original engineers used when they first built this system really helps, and the fantastic tooling in PyCharm is critical to my being able to navigate the codebase. Between grepping the python sources (thank god for shell aliases wrapping grep!) and my base familiarity with the system, I can start tracing with PyCharm's "find definition" to investigate things. Being able to run my own copy of the system, and put debugging breakpoints in (even in libraries!) is __amazing__.
