M.E.Farmer wrote:
>>They're both an UNPACK_SEQUENCE byte-code.
>
> I have dissed it myself ;)
> My argument, if you can call it that, is that it is not clear that
> Python is going to do a tuple unpacking here or not !
Hmm... Well, I wouldn't say that. I think it's quite clear that
Python's doing a tuple unpacking. Just like it always does anytime
there's a comma on the left-hand side of an assignment statement.
Note that there's nothing that forces you to write the unpack sequence
like a tuple display. You can write it like a list display too, and
Python will treat it identically:
py> def unpack_tuple(t):
... [x, y] = t
...
py> dis.dis(unpack_tuple)
2 0 LOAD_FAST 0 (t)
3 UNPACK_SEQUENCE 2
6 STORE_FAST 2 (x)
9 STORE_FAST 1 (y)
12 LOAD_CONST 0 (None)
15 RETURN_VALUE
I don't know how UNPACK_SEQUENCE is implemented, but at least on the
Python level, no tuple or list is created in the unpacking process.
> Seems weird, non-intuitive that a tuple unpacking and tuple creation
> have the same bytecode.
Sorry, I don't understand. Why do you think this?
py> def create_tuple(x, y):
... (x, y)
...
py> dis.dis(create_tuple)
2 0 LOAD_FAST 0 (x)
3 LOAD_FAST 1 (y)
6 BUILD_TUPLE 2
9 POP_TOP
10 LOAD_CONST 0 (None)
13 RETURN_VALUE
py> def unpack_tuple(t):
... x, y = t
...
py> dis.dis(unpack_tuple)
2 0 LOAD_FAST 0 (t)
3 UNPACK_SEQUENCE 2
6 STORE_FAST 2 (x)
9 STORE_FAST 1 (y)
12 LOAD_CONST 0 (None)
15 RETURN_VALUE
To me, it looks like tuple creation uses the BUILD_TUPLE op-code, and
tuple unpacking uses the UNPACK_SEQUENCE op-code. Could you explain
what you meant by "a tuple unpacking and tuple creation have the same
bytecode"?
STeVe
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