As you can see, when a local trace function sets f_lineno, f_lineno 
doesn't get updated on subsequent lines. Otherwise it works fine.
$ python tracer_testcase.py                                                                                                                      
12
12
12

13
14
15

The reference manual (for python 2.6) states the following: "f_lineno is 
the current line number of the frame — writing to this from within a 
trace function jumps to the given line (only for the bottom-most frame). 
A debugger can implement a Jump command (aka Set Next Statement) by 
writing to f_lineno.". This is contradictory with the shown results, 
because apparently it doesn't always represent the current line number.