This TCO is in how the CPython interpreter works, not in making Python itself tail recursive. Some of the C code in the interpreter has been reorganized to put

This TCO is in how the CPython interpreter works, not in making Python itself tail recursive. Some of the C code in the interpreter has been reorganized to put some calls into tail position where the C compiler turns them into jumps. That avoids some call/return overhead and makes the interpreter run a little faster. It's still interpreting the same language with the same semantics.