Message1596

The tracing facility in python is used for profiling and debugging programs.
	
Profilers will usually trace all lines, so, setting sys.settrace and continually returning the trace_dispatch function is ok, but debuggers need to use frame.f_trace, to reenable tracing on parent frames after entering a new context that will have the debugger enabled, to handle step_return or normal steps (the debugger may actually work around that, but without that feature, there is no way of making any optimization to the debugger, as all function calls will have to be traced, which in the debugger case can make it almost useless).

Below is a sample program and the outputs in jython 2.1/2.2 and python2.2/2.4:

 1class spaceship:
 2            
 3    def m1(self):
 4        if 1:
 5            pass
 6        self.m2()
 7        
 8    def m2(self):
 9        if 1:
10            pass
11        self.m3()
12        
13    def m3(self):
14        if 1:
15            pass
16    
17if __name__ == '__main__':
18    
19    def trace_dispatch(frame, event, arg):
20        print frame.f_code.co_name, frame.f_lineno, event, arg
21        frame.f_trace = trace_dispatch
22        return None
23    
24    import sys
25    sys.settrace(trace_dispatch) 
26
27    tc = spaceship()
28    tc.m1()
29    tc.m2()
30    tc.m3()



jython output (2.1 and 2.2b):
m1 0 call None
m2 0 call None
m3 0 call None
m2 0 call None
m3 0 call None
m3 0 call None

python output (2.4):
m1 3 call None
m1 5 line None
m1 6 line None
m2 8 call None
m2 10 line None
m2 11 line None
m3 13 call None
m3 15 line None
m3 15 return None
m2 11 return None
m1 6 return None
m2 8 call None
m2 10 line None
m2 11 line None
m3 13 call None
m3 15 line None
m3 15 return None
m2 11 return None
m3 13 call None
m3 15 line None
m3 15 return None


python output (2.2):
m1 3 call None
m1 3 line None
m1 4 line None
m1 5 line None
m1 6 line None
m2 8 call None
m2 8 line None
m2 9 line None
m2 10 line None
m2 11 line None
m3 13 call None
m3 13 line None
m3 14 line None
m3 15 line None
m3 15 return None
m2 11 return None
m1 6 return None
m2 8 call None
m2 8 line None
m2 9 line None
m2 10 line None
m2 11 line None
m3 13 call None
m3 13 line None
m3 14 line None
m3 15 line None
m3 15 return None
m2 11 return None
m3 13 call None
m3 13 line None
m3 14 line None
m3 15 line None
m3 15 return None