Is bytecode machine (in)dependent?

[Robert McLay]

I'm trying to understand bytecodes generated on different machines.
I understand that the bytecodes can change between version.  But since
I'm told that .pyc files are version dependent but not machine
dependent, I'm wondering why the bytecodes are machine dependent.

my friend and I created this simple example to explore the problem.
The example code is:

   #!/usr/bin/env python
   #
   import sys, os

   def main():
     x = 1.234
     print x

   if ( __name__ == '__main__'): 
     main()


Using sib.py from Vendorid 1.0 generates different bytecodes under linux
and sgi.  At its core sib.py is using:

        # Compile the code using prefix and marshall it.
        compiled_code = compile(source_code, prefix, 'exec')
        marshalled_code = marshal.dumps(compiled_code)

to get the bytecodes.

So why are the byte codes different?  Is it that the intel-linux is
little endian and the SGI is big endian and the numerical constant
(1.234) is stored different depending on the endian-ness?


This was generated under intel-linux using python 2.4.2




     /*==========================================================================*/
     /* Frozen main script for test                                              */
     /* Generated from test.py                                                   */
     /* This is generated code; Do not modify it!                                */
     /*--------------------------------------------------------------------------*/
     unsigned char M___main__[] =
     {
         99,0,0,0,0,0,0,0,0,2,0,0,0,64,0,0,
         0,115,55,0,0,0,100,0,0,107,0,0,90,0,0,100,
         0,0,107,1,0,90,1,0,100,1,0,132,0,0,90,2,
         0,101,3,0,100,2,0,106,2,0,111,11,0,1,101,2,
         0,131,0,0,1,110,1,0,1,100,0,0,83,40,3,0,
         0,0,78,99,0,0,0,0,1,0,0,0,1,0,0,0,
         67,0,0,0,115,15,0,0,0,100,1,0,125,0,0,124,
         0,0,71,72,100,0,0,83,40,2,0,0,0,78,102,5,
         49,46,50,51,52,40,1,0,0,0,116,1,0,0,0,120,
         40,1,0,0,0,82,0,0,0,0,40,0,0,0,0,40,
         0,0,0,0,116,23,0,0,0,47,104,111,109,101,47,118,             /* This line */
         112,97,114,114,47,115,105,98,47,116,101,115,116,46,112,121,
         116,4,0,0,0,109,97,105,110,5,0,0,0,115,4,0,
         0,0,0,1,6,1,116,8,0,0,0,95,95,109,97,105,
         110,95,95,40,4,0,0,0,116,3,0,0,0,115,121,115,
         116,2,0,0,0,111,115,82,2,0,0,0,116,8,0,0,
         0,95,95,110,97,109,101,95,95,40,3,0,0,0,82,4,
         0,0,0,82,2,0,0,0,82,5,0,0,0,40,0,0,
         0,0,40,0,0,0,0,82,1,0,0,0,116,1,0,0,
         0,63,3,0,0,0,115,6,0,0,0,18,2,9,4,13,
         1,
     };  x

 And under SGI (python 2.4.2) it created :

    /*==========================================================================*/
    /* Frozen main script for test                                              */
    /* Generated from test.py                                                   */
    /* This is generated code; Do not modify it!                                */
    /*--------------------------------------------------------------------------*/
    unsigned char M___main__[] =
    {
        99,0,0,0,0,0,0,0,0,2,0,0,0,64,0,0,
        0,115,55,0,0,0,100,0,0,107,0,0,90,0,0,100,
        0,0,107,1,0,90,1,0,100,1,0,132,0,0,90,2,
        0,101,3,0,100,2,0,106,2,0,111,11,0,1,101,2,
        0,131,0,0,1,110,1,0,1,100,0,0,83,40,3,0,
        0,0,78,99,0,0,0,0,1,0,0,0,1,0,0,0,
        67,0,0,0,115,15,0,0,0,100,1,0,125,0,0,124,
        0,0,71,72,100,0,0,83,40,2,0,0,0,78,102,5,
        49,46,50,51,52,40,1,0,0,0,116,1,0,0,0,120,
        40,1,0,0,0,82,0,0,0,0,40,0,0,0,0,40,
        0,0,0,0,116,23,0,0,0,47,87,111,114,107,47,118,              /* This line */
        112,97,114,114,47,115,105,98,47,116,101,115,116,46,112,121,
        116,4,0,0,0,109,97,105,110,5,0,0,0,115,4,0,
        0,0,0,1,6,1,116,8,0,0,0,95,95,109,97,105,
        110,95,95,40,4,0,0,0,116,3,0,0,0,115,121,115,
        116,2,0,0,0,111,115,82,2,0,0,0,116,8,0,0,
        0,95,95,110,97,109,101,95,95,40,3,0,0,0,82,4,
        0,0,0,82,2,0,0,0,82,5,0,0,0,40,0,0,
        0,0,40,0,0,0,0,82,1,0,0,0,116,1,0,0,
        0,63,3,0,0,0,115,6,0,0,0,18,2,9,4,13,
        1,
    };


The difference between the two is very slight:

18c18
<     0,0,0,0,116,23,0,0,0,47,104,111,109,101,47,118,
---
>     0,0,0,0,116,23,0,0,0,47,87,111,114,107,47,118,