On Mon, Sep 2, 2019 at 12:36 PM Alan Bawden <a...@csail.mit.edu> wrote:
>
> Eko palypse <ekopaly...@gmail.com> writes:
>
> > Am Montag, 2. September 2019 00:49:05 UTC+2 schrieb Hongyi Zhao:
> > > Hi,
> > >
> > > What's differences:
> > >
> > > a,b = 2,3 and [a,b] = [2,3]
> > >
> > > Regards
> >
> > In this example the result is the same but the second one
> > builds, internally, an additional list, therefore isn't as sufficient
> > as the first one.
>
> It looks to me like they generate identical code. The first one calls the
> construction of a tuple, where the second one calls for the construction of
> a list. It would be surprising if the compiler optimized the tuple
> away, but failed to optimize the list away!
>
Well, you can find out with the 'dis' module.
>>> def f():
... a, b = 2, 3
... a, b = [2, 3]
...
>>> dis.dis(f)
2 0 LOAD_CONST 1 ((2, 3))
2 UNPACK_SEQUENCE 2
4 STORE_FAST 0 (a)
6 STORE_FAST 1 (b)
3 8 LOAD_CONST 2 (2)
10 LOAD_CONST 3 (3)
12 BUILD_LIST 2
14 UNPACK_SEQUENCE 2
16 STORE_FAST 0 (a)
18 STORE_FAST 1 (b)
20 LOAD_CONST 0 (None)
22 RETURN_VALUE
This is with CPython 3.9. It's entirely possible that other Pythons
and/or other versions of CPython may give different results, but with
this particular interpreter, the list is not optimized away.
Tuples get optimized away in quite a number of situations. Exchanging
is done on the stack:
>>> def f():
... x, y = y, x
...
>>> dis.dis(f)
2 0 LOAD_FAST 0 (y)
2 LOAD_FAST 1 (x)
4 ROT_TWO
6 STORE_FAST 1 (x)
8 STORE_FAST 0 (y)
10 LOAD_CONST 0 (None)
12 RETURN_VALUE
There are a lot of assumptions the interpreter can make about them,
including that they cannot nest recursively [1], and that, when
constructed from constants, they are themselves constant. Any time you
think it'll make no difference, go with a tuple.
ChrisA
[1] Technically they can, but you have to use the CPython API or
equivalent - vanilla Python code can't do it.
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