python goto with system trace function

goto.py

import sys

def j(lineno):
    frame = sys._getframe().f_back
    called_from = frame

    def hook(frame, event, arg):
        if event == 'line' and frame == called_from:
            try:
                frame.f_lineno = lineno
            except ValueError as e:
                print "jump failed:", e
            while frame:
                frame.f_trace = None
                frame = frame.f_back
            return None
        return hook

    while frame:
        frame.f_trace = hook
        frame = frame.f_back
    sys.settrace(hook)


def foo():
    a = 1
    j(30)
    a = 2
    print 1
    print 2
    if a == 1:
        j(28)
    print 4


foo()

output

2
1
2
4