Interpreter instructions in CPython are atomic (due to the GIL, actually) and calling into extension functions is a single instruction (incidentally). This make

Interpreter instructions in CPython are atomic (due to the GIL, actually) and calling into extension functions is a single instruction (incidentally). This makes it impossible to corrupt built-in data structures, even if no locks are used. In some simple cases this is sufficient, esp. if no one's going to run it with !CPython... This is also neat for other reasons (think sandboxing).