You pay enough attention to "is it fast" and you'll find yourself writing everything in assembler
Not everything. Not even then. Just the good bits maybe. Sometimes none of it. It might make me prefer C# over Java for example, because C# can lay down structs linearly in memory so I can throw them to the graphics card directly. Or maybe I'd do all the hard work in C and SWIG it so I could use Ruby.
But what I certainly wouldn't do is attempt to write a physics simulation in python, "idiomatic" or not.