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Author BTaskaya
Recipients BTaskaya, gvanrossum, lys.nikolaou, pablogsal, serhiy.storchaka
Date 2020-12-25.09:32:49
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PEP 526 classifies everything but simple, unparenthesized names (a.b, (a), a[b]) as complex targets. The way the we handle annotations for them right now is, doing literally nothing but evaluating every part of it (just pushing the name to the stack, and popping, without even doing the attribute access);

$ cat
foo[bar]: int
$ python -m dis
  1           0 SETUP_ANNOTATIONS
              2 LOAD_NAME                0 (foo)
              4 POP_TOP
              6 LOAD_NAME                1 (bar)
              8 POP_TOP
             10 LOAD_NAME                2 (int)
             12 POP_TOP
             14 LOAD_CONST               0 (None)
             16 RETURN_VALUE

$ cat
a.b: int
$ python -m dis
  1           0 SETUP_ANNOTATIONS
              2 LOAD_NAME                0 (a)
              4 POP_TOP
              6 LOAD_NAME                1 (int)
              8 POP_TOP
             10 LOAD_CONST               0 (None)
             12 RETURN_VALUE

I noticed this while creating a patch for issue 42725, since I had to create an extra check for non-simple annassign targets (because compiler tries to find their scope, `int` in this case is not compiled to string). 

Since they have no real side effect but just accessing a name, I'd propose we drop this from 3.10 so that both I can simply the patch for issue 42725, and also we have consistency with what we do when the target is simple (instead of storing this time, we'll just drop the bytecode).

$ cat
a.b: int = 5
$ python -m dis
  1           0 SETUP_ANNOTATIONS
              2 LOAD_CONST               0 (5)
              4 LOAD_NAME                0 (a)
              6 STORE_ATTR               1 (b)
              8 LOAD_NAME                2 (int)
             10 POP_TOP
             12 LOAD_CONST               1 (None)
             14 RETURN_VALUE

8/10 will be gone in this case.

If agreed upon, I can propose a patch.
Date User Action Args
2020-12-25 09:32:50BTaskayasetrecipients: + BTaskaya, gvanrossum, serhiy.storchaka, lys.nikolaou, pablogsal
2020-12-25 09:32:50BTaskayasetmessageid: <>
2020-12-25 09:32:49BTaskayalinkissue42737 messages
2020-12-25 09:32:49BTaskayacreate