While I was modifying test_trace, it threw an exception when I accidentally

made it try to set the line number from the trace callback for a 'call' event.
This patch makes the error message a little more helpful in that case, and
makes it a little less likely that a future editor will make the same mistake
in test_trace.

Lib/test/test_trace.py

@@ -471,7 +471,7 @@ class JumpTracer:
     def trace(self, frame, event, arg):
         if not self.done and frame.f_code == self.function.func_code:
             firstLine = frame.f_code.co_firstlineno
-            if frame.f_lineno == firstLine + self.jumpFrom:
+            if event == 'line' and frame.f_lineno == firstLine + self.jumpFrom:
                 # Cope with non-integer self.jumpTo (because of
                 # no_jump_to_non_integers below).
                 try:

Objects/frameobject.c

@@ -127,7 +127,8 @@ frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno)
 	if (!f->f_trace)
 	{
 		PyErr_Format(PyExc_ValueError,
-			     "f_lineno can only be set by a trace function");
+			     "f_lineno can only be set by a"
+			     " line trace function");
 		return -1;
 	}