Goto for python.

http://entrian.com/goto/
看了看源代码，原来是通过给所有函数修改trace，然后在异常时就会执行设置的函数
# Install the trace function, including all preceding frames.
sys.settrace(_trace)
frame = sys._getframe().f_back
while frame:
    frame.f_trace = _trace
    frame = frame.f_back
改天有时间看看如何在trace中分析每一行代码的！

# Example 1: Breaking out from a deeply nested loop:
from goto import goto, label
for i in range(1, 10):
    for j in range(1, 20):
        for k in range(1, 30):
            print i, j, k
            if k == 3:
                goto .end
label .end
print "Finished\n"


# Example 2: Restarting a loop:
from goto import goto, label
label .start
for i in range(1, 4):
    print i
    if i == 2:
        try:
            output = message
        except NameError:
            print "Oops - forgot to define 'message'!  Start again."
            message = "Hello world"
            goto .start
print output, "\n"

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