As someone who does a decent part of development in python, I'd say you are using the wrong language, if you can't test your edge cases and have huge liabilities.
Python code is inherently almost-untestable and fragile. These days, when coding something critical and non-trivial, I choose a memory-safe language with static typing and type inference, ADTs, pattern matching and try to write simple yet pure functional code with well defined semantics, that works almost by definition.
> If you have a liability situation, maybe you could work to rectify it
Well yes, sure, of course.
And like you said, maybe Python isn't the right language in the first place for mission-critical life-is-on-the-line software.
But if you have already gotten yourself into a position where some piece of your business infrastructure is dependent on an obscure bit of hard-to-port-to-Python-3-and-maintain-exact-behaviour Python code, then it is exactly the "2to3 transition that's shaking up your house of cards", no?
And, furthermore, like you said, if you find yourself in this position, you should be looking at some other language entirely rather than porting to Py3, eh?
Note that I am not against using python in mission-critical code.
I was referring to untestable code with a myriad of edge-cases, in which case you have a problem that will surface sooner or later, be it 2to3 transition or something else.
If the code is truly static, you can ignore the transition and deprecation. Otherwise you should probably work on documentation/testing/refactoring and/or porting to another language.
2to3 transition was handled badly, up to about 2.7 and 3.4 or so, but the pains described here seem mostly self-inflicted, and I don't see it as an argument against the needed changes.
Python code is inherently almost-untestable and fragile. These days, when coding something critical and non-trivial, I choose a memory-safe language with static typing and type inference, ADTs, pattern matching and try to write simple yet pure functional code with well defined semantics, that works almost by definition.