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2012-05-06T20:39:00.140
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<h2>Short Answer</h2> In Python 2.x, the <code>str</code> type holds raw bytes, so <code>dis</code> assumes that if you pass it a string it is getting compiled bytecode. It tries to disassemble the string you pass it as bytecode and -- purely due to the implementation details of Python bytecode -- succeeds for <code>i in (2,3)</code>. Obviously, though, it returns gibberish. In Python 3.x, the <code>str</code> type is for strings and the <code>bytes</code> types is for raw bytes, so <code>dis</code> can distinguish between compiled bytecode and strings -- and assumes it is getting source code if it gets a string. <hr> <h2>Long Answer</h2> Here's the thought process I followed to work this one out. <ol> <li>I tried it on my Python (3.2): <pre><code>>>> import dis >>> dis.dis("i in (2,3)") 1 0 LOAD_NAME 0 (i) 3 LOAD_CONST 2 ((2, 3)) 6 COMPARE_OP 6 (in) 9 RETURN_VALUE >>> dis.dis("i in [2,3]") 1 0 LOAD_NAME 0 (i) 3 LOAD_CONST 2 ((2, 3)) 6 COMPARE_OP 6 (in) 9 RETURN_VALUE </code></pre> Obviously, this works.</li> <li>I tried it on Python 2.7: <pre><code>>>> import dis >>> dis.dis("i in (2,3)") 0 BUILD_MAP 26912 3 JUMP_FORWARD 10272 (to 10278) 6 DELETE_SLICE+0 7 <44> 8 DELETE_SLICE+1 9 STORE_SLICE+1 >>> dis.dis("i in [2,3]") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\Python27\lib\dis.py", line 45, in dis disassemble_string(x) File "C:\Python27\lib\dis.py", line 112, in disassemble_string labels = findlabels(code) File "C:\Python27\lib\dis.py", line 166, in findlabels oparg = ord(code[i]) + ord(code[i+1])*256 IndexError: string index out of range </code></pre> Aha! Notice also that the generated bytecode in Python 3.2 is what you would expect ("load <code>i</code>, load <code>(2,3)</code>, test for membership, return the result") whereas what you have got in Python 2.7 is gibberish. Clearly, <code>dis</code> is decompiling the string as bytecode in 2.7 but compiling it as Python in 3.2.</li> <li>I had a look in the source code for <code>dis.dis</code>. Here are the key points: Python 2.7: <pre><code>elif isinstance(x, str): disassemble_string(x) </code></pre> Python 3.2: <pre><code> elif isinstance(x, (bytes, bytearray)): # Raw bytecode _disassemble_bytes(x) elif isinstance(x, str): # Source code _disassemble_str(x) </code></pre> Just for fun, let's check this by passing the same bytes to <code>dis</code> in Python 3: <pre><code>>>> dis.dis("i in (2,3)".encode()) 0 BUILD_MAP 26912 3 JUMP_FORWARD 10272 (to 10278) 6 <50> 7 <44> 8 <51> 9 <41> </code></pre> Aha! Gibberish! (Though note that it's slightly different gibberish -- the bytecode has changed with the Python version!)</li> </ol>
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1. COAnd here's the corresponding part of the doc From the Python2 documentation ( http://docs.python.org/library/dis.html ): >dis.dis([bytesource]) > Disassemble the bytesource object. > bytesource can denote either a module, a class, a method, a function, or a code object whereas in the Python 3 documentation ( http://docs.python.org/dev/library/dis.html?highlight=dis#dis) it states: >dis.dis(x=None) >Disassemble the x object. x can denote either a module, a class, a method, a function, a code object, **a string of source code** or a byte sequence of raw bytecode.
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