Tracing Your Program As It Runs

There are two ways to inject code to watch your program run: tracing and profiling. They are similar, but intended for different purposes and so have different constraints. The easiest, but least efficient, way to monitor your program is through a trace hook, which can be used for writing a debugger, code coverage monitoring, or many other purposes.

The trace hook is modified by passing a callback function to sys.settrace(). The callback is passed three arguments, frame (the stack frame from the code being run), event (a string naming the type of notification), and arg (an event-specific value). There are 7 event types for different levels of information that occur as your program is being executed.

EventWhenarg value
'call'Before a function is executed.None'line'Before a line is executed.None'return'Before a function returns.The value being returned.'exception'After an exception occurs.The (exception, value, traceback) tuple.'c_call'Before a C function is called.The C function object.'c_return'After a C function returns.None'c