question about a command like 'goto ' in Python's bytecode or it's just a compiler optimization?

higer higerinbeijing at gmail.com
Wed Jun 17 05:40:00 CEST 2009


My Python version is 2.5.2; When I reading the bytecode of some pyc
file, I always found that there are many jump command from different
position,but to the same position. You can see this situation in
following code(this bytecode is just from one .pyc file and I don't
have its source .py file):

.....
526	POP_TOP           ''
527	LOAD_FAST         'imeHandle'
530	LOAD_ATTR         'isCnInput'
533	CALL_FUNCTION_0   ''
536	JUMP_IF_FALSE     '574'
539	POP_TOP           ''
540	LOAD_FAST         'GUIDefine'
543	LOAD_ATTR         'CandidateIsOpen'
546	JUMP_IF_TRUE      '574'
549	POP_TOP           ''
550	LOAD_FAST         'GUIDefine'
553	LOAD_ATTR         'CompositionWndIsOpen'
556	JUMP_IF_TRUE      '574'
559	POP_TOP           ''
560	LOAD_FAST         'isWanNengWB'
563	JUMP_IF_FALSE     '574'
566	POP_TOP           ''
567	LOAD_FAST         'state'
570	LOAD_CONST        1
573	BINARY_AND        ''
574_0	COME_FROM         ''
574_1	COME_FROM         ''
574_2	COME_FROM         ''
574_3	COME_FROM         ''
...

>From the above bytecode,we know that line 574 is the point that many
position jumps to.So,it just looks like the 'goto' function in C, but
we know that there is none such function in Python.
One 'JUMP**' command is companied with a 'COME_FROM' command,so more
than one 'COME_FROM' OPs are listed on line 574...

But ,the question is, I have tried a lot of ways(e.g.for loop,while
loop and mixed) to re-present 'goto' style bytecodes like this, but
the result depressed me.
So,I think maybe it is just a compiler optimization in Python2.5? I'm
not sure,so I'm appreciated that if anyone can help me.



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