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.
diff --git a/Lib/test/test_trace.py b/Lib/test/test_trace.py
index b5db9a7..a8f5163 100644
--- a/Lib/test/test_trace.py
+++ b/Lib/test/test_trace.py
@@ -471,7 +471,7 @@
     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: