At a previous employer, I left behind maybe not millions of lines of Python 2 code, but certainly somewhere in the hundreds of thousands range. There was also a fair amount of reliance on 3rd party libs for databases, IBM MQ and tibrv that had native components. We also had our own Python libs that had native components written using Boost Python.

Large Python projects are possible, but if you don't have good discipline and organization, it can quickly become a mess (probably true regardless of the language).

I'm not sure how much effort it'd be to migrate all of that to Python 3, but I'm sure it's not trivial. But, I also don't think it'd be a multi year effort. It also wasn't a big monolithic project, so a migration could be done piecemeal.

You made me curious so I looked around. I couldn't easily find an open Python project with 1M+ LOC.

Python 3.7 has 542,292 lines of Python, according to David A. Wheeler's 'SLOCCount' . And another 418,581 lines of C.

NumPy has 159,208 lines of C and 113,842 of Python.

SciPy has 174,580 lines of Python, 83,155 of C, and 79,514 of Fortran.

Pandas has 225,417 lines of Python.

Put them all together and that's 1M lines of Python to run the standard Python data stack.

I wasn't talking hypothetical, I was talking out of my own experience. Yes, these codebases exist and, yes, maintenance isn't easy, especially because of dynamic typing.

OpenStack has millions of lines of Python, I believe.