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On Mon, Sep 2, 2019 at 12:36 PM Alan Bawden <alan at csail.mit.edu> wrote: > > Eko palypse <ekopalypse at 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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