Question

In Python (2.7.2),why does

import dis
dis.dis("i in (2, 3)")

works as expected whereas

import dis
dis.dis("i in [2, 3]")

raises:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python2.7/dis.py", line 45, in dis
  disassemble_string(x)
File "/usr/lib/python2.7/dis.py", line 112, in disassemble_string
  labels = findlabels(code)
File "/usr/lib/python2.7/dis.py", line 166, in findlabels
 oparg = ord(code[i]) + ord(code[i+1])*256
IndexError: string index out of range

Note that this doesn’t affect Python3.

Answer 1

Short Answer

In Python 2.x, the str type holds raw bytes, so dis 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 i in (2,3). Obviously, though, it returns gibberish.

In Python 3.x, the str type is for strings and the bytes types is for raw bytes, so dis can distinguish between compiled bytecode and strings — and assumes it is getting source code if it gets a string.

---

Long Answer

Here’s the thought process I followed to work this one out.

1. I tried it on my Python (3.2):

   >>> 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
   

   Obviously, this works.

2. I tried it on Python 2.7:

   >>> 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
   

   Aha! Notice also that the generated bytecode in Python 3.2 is what you would expect (“load i, load (2,3), test for membership, return the result”) whereas what you have got in Python 2.7 is gibberish. Clearly, dis is decompiling the string as bytecode in 2.7 but compiling it as Python in 3.2.

3. I had a look in the source code for dis.dis. Here are the key points:

   Python 2.7:

   elif isinstance(x, str):
       disassemble_string(x)
   

   Python 3.2:

      elif isinstance(x, (bytes, bytearray)): # Raw bytecode
          _disassemble_bytes(x)
      elif isinstance(x, str):    # Source code
          _disassemble_str(x)
   

   Just for fun, let’s check this by passing the same bytes to dis in Python 3:

   >>> 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>
   

   Aha! Gibberish! (Though note that it’s slightly different gibberish — the bytecode has changed with the Python version!)