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
--
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