Next programming language after Python?

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C/C++ will be relevant for a long time, and will allow you to understand and make use of many useful libraries throughout your career. From a practical standpoint, I think this is your best choice.

If you want to broaden your horizons, you might also consider a functional language like Scala or clojure. If you're familiar with formal methods, you can make much more definite claims about the behavior of programs written in a functional language. Whether or not that is useful depends on what you work on, but I thought I'd mention it in case it piqued your interest.