But would you consider someone passionate if they said that they only care about learning a language/technology/framework that is marketable and doesn’t believe in learning for its own sake?
Unless the Python developer is actually writing C++, I don’t think he would ever need to learn the intricacies of C to be a good developer.
But take that opinion with a large grain of salt. I spent 12 years bit twiddling in C/C++, 10 in C#, and have been doing Python for a grand total of 6 months so I definitely haven’t done anything complicated with Python.
The end result is, that (1) my knowledge and skills extend way beyond any of my past or present job descriptions, and (2) (this is probably even more important for employers/productivity) that I'm capable of, and curious enough to actually want to, learning quickly/deeply and solving hard problems that might not have an obvious instant-knowledge-based solution.
I think that anyone that has ever had an interest to go "beyond" (in any way) the obvious, or immediately what's necessary in his/her day job, would built such a knowledge base - but could of course be completely unrelated to mine (e.g. networking specialist, cryptography geek, hardware/embedded engineer, ...).
Wants to stay competitive and be able to earn near the top of local market => interest => knowledge.
A more charitable interpretation of "passion" would be... I'm nominally a Python & Scala programmer (according to my CV), but I can also competently talk about OCaml, Haskell, Rust, instruction scheduling, tracing JIT compiler implementation, propagating type inference, TCP stack, C memory model, register allocation, hardware concurrency primitives and ring 0 privileges.
Compare that to another Python programmer that once tried to convince me that when a C++ program tries to access unallocated memory "the computer just throws an exception".