[Edu-sig] Peeking under the hood

[Kirby Urner]

And now for something completely different...

Let's help give students a sense of what it means to say
"in Python, user-defined functions compile to bytecodes".

Here's a silly function to remove an item from a list, or
not care if it ain't there.  This one's "in place" (returns
nothing).

   >>> def rem(L,i):
	try: L.remove(i)
	except ValueError: pass

	
   >>> mylist = ['C','A','T']
   >>> rem(mylist,'C')
   >>> mylist
   ['A', 'T']
   >>> rem(mylist,'X')
   >>> mylist
   ['A', 'T']

The code object corresponding to this function may be dumped
as an attribute of rem:

   >>> rem.func_code
   <code object rem at 00B7EE80, file "<pyshell#24>", line 1>

Let's look at the bytecodes:

   >>> rem.func_code.co_code
   '\x7f\x01\x00\x7f\x02\x00y\x17\x00\x7f\x02\x00|\x00\x00i
   \x01\x00d\x01\x00d\x02\x00\x83\x00\x01\x01Wn\x19\x00\x7f
   \x03\x00\x04t\x02\x00j\n\x00o\n\x00\x01\x01\x01\x01\x7f
   \x03\x00n\x02\x00\x01Xd\x00\x00S'

The dis module lets you disassemble bytecodes into something
more readable (if not the original Python source):

   >>> import dis
   >>> import pydoc
   >>> pydoc.help(dis)
   Help on module dis:

   NAME
       dis - Disassembler of Python byte code into mnemonics.

   FILE
       d:\program files\python21\lib\dis.py

   DATA


   <<SNIP>>

   >>> dis.disassemble(rem.func_code)
           0 SET_LINENO               1

           3 SET_LINENO               2
           6 SETUP_EXCEPT            23 (to 32)

           9 SET_LINENO               2
          12 LOAD_FAST                0 (L)
          15 LOAD_ATTR                1 (remove)
          18 LOAD_CONST               1 ('i')
          21 LOAD_CONST               2 (0)
          24 CALL_FUNCTION          256
          27 POP_TOP
          28 POP_BLOCK
          29 JUMP_FORWARD            25 (to 57)

     >>   32 SET_LINENO               3
          35 DUP_TOP
          36 LOAD_GLOBAL              2 (ValueError)
          39 COMPARE_OP              10 (exception match)
          42 JUMP_IF_FALSE           10 (to 55)
          45 POP_TOP
          46 POP_TOP
          47 POP_TOP
          48 POP_TOP

          49 SET_LINENO               3
          52 JUMP_FORWARD             2 (to 57)
     >>   55 POP_TOP
          56 END_FINALLY
     >>   57 LOAD_CONST               0 (None)
          60 RETURN_VALUE

Now that's something different.

Pieces missing in this story.  Could be fleshed out.  Like Arthur,
I like to peek under the hood sometimes.

Kirby