Idle bytecode query on apparently unreachable returns

Tom Anderson twic at urchin.earth.li
Mon Oct 10 01:20:13 CEST 2005


Evening all,

Here's a brief chat with the interpretator:

Python 2.4.1 (#2, Mar 31 2005, 00:05:10)
[GCC 3.3 20030304 (Apple Computer, Inc. build 1666)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def fib(x):
...     if (x == 1):
...             return 1
...     else:
...             return x * fib((x - 1))
...
>>> import dis
>>> dis.dis(fib)
   2           0 LOAD_FAST                0 (x)
               3 LOAD_CONST               1 (1)
               6 COMPARE_OP               2 (==)
               9 JUMP_IF_FALSE            8 (to 20)
              12 POP_TOP

   3          13 LOAD_CONST               1 (1)
              16 RETURN_VALUE
              17 JUMP_FORWARD            19 (to 39)
         >>   20 POP_TOP

   5          21 LOAD_FAST                0 (x)
              24 LOAD_GLOBAL              1 (fib)
              27 LOAD_FAST                0 (x)
              30 LOAD_CONST               1 (1)
              33 BINARY_SUBTRACT
              34 CALL_FUNCTION            1
              37 BINARY_MULTIPLY
              38 RETURN_VALUE
         >>   39 LOAD_CONST               0 (None)
              42 RETURN_VALUE

I'm no bytecode connoisseur, but having read 
<http://docs.python.org/lib/bytecodes.html>, i more or less get this.

What puzzles me, though, are bytecodes 17, 39 and 42 - surely these aren't 
reachable? Does the compiler just throw in a default 'return None' 
epilogue, with routes there from every code path, even when it's not 
needed? If so, why?

tom

-- 
News flash: there's no deep meaning or hidden message BECAUSE DAVID LYNCH IS INSANE



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