A slight change to SET_LINENO-less tracing.

This makes things a touch more like 2.2.  Read the comments in
Python/ceval.c for more details.

Include/frameobject.h

@@ -29,6 +29,8 @@ typedef struct _frame {
     PyObject *f_exc_type, *f_exc_value, *f_exc_traceback;
     PyThreadState *f_tstate;
     int f_lasti;		/* Last instruction if called */
+    /* As of 2.3 f_lineno is only valid when tracing is active (i.e. when
+       f_trace is set) -- at other times use PyCode_Addr2Line instead. */
     int f_lineno;		/* Current line number */
     int f_restricted;		/* Flag set if restricted operations
 				   in this scope */

Lib/test/test_trace.py

@@ -57,7 +57,7 @@ no_pop_tops.events = [(0, 'call'),
                       (3, 'line'),
                       (4, 'line'),
                       (2, 'line'),
-                      (6, 'return')]
+                      (2, 'return')]
 
 def no_pop_blocks():
     while 0:

Modules/_hotshot.c

@@ -888,12 +888,10 @@ tracer_callback(ProfilerObject *self, PyFrameObject *frame, int what,
 
     case PyTrace_LINE:
         if (self->linetimings)
-            return pack_lineno_tdelta(self, PyCode_Addr2Line(frame->f_code, 
-							     frame->f_lasti),
+            return pack_lineno_tdelta(self, frame->f_lineno),
 				      get_tdelta(self));
         else
-            return pack_lineno(self, PyCode_Addr2Line(frame->f_code,
-						      frame->f_lasti));
+            return pack_lineno(self, frame->f_lineno);
 
     default:
         /* ignore PyTrace_EXCEPTION */

Objects/frameobject.c

@@ -17,7 +17,6 @@ static PyMemberDef frame_memberlist[] = {
 	{"f_globals",	T_OBJECT,	OFF(f_globals),	RO},
 	{"f_lasti",	T_INT,		OFF(f_lasti),	RO},
 	{"f_restricted",T_INT,		OFF(f_restricted),RO},
-	{"f_trace",	T_OBJECT,	OFF(f_trace)},
 	{"f_exc_type",	T_OBJECT,	OFF(f_exc_type)},
 	{"f_exc_value",	T_OBJECT,	OFF(f_exc_value)},
 	{"f_exc_traceback", T_OBJECT,	OFF(f_exc_traceback)},
@@ -37,14 +36,49 @@ frame_getlineno(PyFrameObject *f, void *closure)
 {
 	int lineno;
 
-	lineno = PyCode_Addr2Line(f->f_code, f->f_lasti);
+	if (f->f_trace)
+		lineno = f->f_lineno;
+	else
+		lineno = PyCode_Addr2Line(f->f_code, f->f_lasti);
 
 	return PyInt_FromLong(lineno);
 }
 
+static PyObject *
+frame_gettrace(PyFrameObject *f, void *closure)
+{
+	PyObject* trace = f->f_trace;
+
+	if (trace == NULL)
+		trace = Py_None;
+
+	Py_INCREF(trace);
+
+	return trace;
+}
+
+static int
+frame_settrace(PyFrameObject *f, PyObject* v, void *closure)
+{
+	/* We rely on f_lineno being accurate when f_trace is set. */
+
+	PyObject* old_value = f->f_trace;
+
+	Py_XINCREF(v);
+	f->f_trace = v;
+	
+	if (v != NULL)
+		f->f_lineno = PyCode_Addr2Line(f->f_code, f->f_lasti);
+
+	Py_XDECREF(old_value);
+
+	return 0;
+}
+
 static PyGetSetDef frame_getsetlist[] = {
 	{"f_locals",	(getter)frame_getlocals, NULL, NULL},
 	{"f_lineno",	(getter)frame_getlineno, NULL, NULL},
+	{"f_trace",	(getter)frame_gettrace, (setter)frame_settrace, NULL},
 	{0}
 };
 

Python/ceval.c

@@ -2909,7 +2909,7 @@ maybe_call_line_trace(int opcode, Py_tracefunc func, PyObject *obj,
 		 >>   21 LOAD_CONST               0 (None)
 		      24 RETURN_VALUE
 
-	   If a is false, execution will jump to instruction at offset
+	   If 'a' is false, execution will jump to instruction at offset
 	   15 and the co_lnotab will claim that execution has moved to
 	   line 3.  This is at best misleading.  In this case we could
 	   associate the POP_TOP with line 4, but that doesn't make
@@ -2920,21 +2920,32 @@ maybe_call_line_trace(int opcode, Py_tracefunc func, PyObject *obj,
 	   current instruction offset matches the offset given for the
 	   start of a line by the co_lnotab.
 
-	   This also takes care of the situation where a is true.
+	   This also takes care of the situation where 'a' is true.
 	   Execution will jump from instruction offset 12 to offset 21.
 	   Then the co_lnotab would imply that execution has moved to line
 	   5, which is again misleading.
+
+	   Why do we set f_lineno when tracing?  Well, consider the code
+	   above when 'a' is true.  If stepping through this with 'n' in
+	   pdb, you would stop at line 1 with a "call" type event, then
+	   line events on lines 2 and 3, then a "return" type event -- but
+	   you would be shown line 5 during this event.  This is a change
+	   from the behaviour in 2.2 and before, and I've found it
+	   confusing in practice.  By setting and using f_lineno when
+	   tracing, one can report a line number different from that
+	   suggested by f_lasti on this one occasion where it's desirable.
 	*/
 
 	if ((frame->f_lasti < *instr_lb || frame->f_lasti >= *instr_ub)) {
 		PyCodeObject* co = frame->f_code;
-		int size, addr;
+		int size, addr, line;
 		unsigned char* p;
 
 		size = PyString_GET_SIZE(co->co_lnotab) / 2;
 		p = (unsigned char*)PyString_AS_STRING(co->co_lnotab);
 
 		addr = 0;
+		line = co->co_firstlineno;
 
 		/* possible optimization: if f->f_lasti == instr_ub
 		   (likely to be a common case) then we already know
@@ -2951,12 +2962,14 @@ maybe_call_line_trace(int opcode, Py_tracefunc func, PyObject *obj,
 			if (addr + *p > frame->f_lasti)
 				break;
 			addr += *p++;
-			p++;
+			line += *p++;
 			--size;
 		}
-		if (addr == frame->f_lasti)
+		if (addr == frame->f_lasti) {
+			frame->f_lineno = line;
 			call_trace(func, obj, frame, 
 				   PyTrace_LINE, Py_None);
+		}
 		*instr_lb = addr;
 		if (size > 0) {
 			while (--size >= 0) {