What happens is that in tracing mode, a copy of the original code stream is created, a new CALL_TRACE opcode is stored in it at the addresses corresponding to each source line number, then the instruction pointer is redirected to execute the modified code string. Whenever a CALL_TRACE opcode is reached, the callback is triggered. On a successful return, the original opcode at the current address is fetched from the original code string, then directly goto the dispatch code.
This code string duplication & conditional break-point setting occurs only when a trace function is set; in the "normal case", the interpreter executes a code string without SET_LINENO.
Ai! This really sounds like a hack. It may be a standard trick in the repertoire of virtual machine implementers, but it is still a hack, and makes my heart cry. I really wonder if it makes enough of a difference to warrant all that code, and the risk that that code isn't quite correct. (Is it thread-safe?) --Guido van Rossum (home page: http://www.python.org/~guido/)