[New-bugs-announce] [issue42737] PEP 563: drop annotations for complex assign targets

[Batuhan Taskaya]

New submission from Batuhan Taskaya <isidentical at gmail.com>:

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 t.py
foo[bar]: int
$ python -m dis t.py
  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 t.py
a.b: int
$ python -m dis t.py
  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 t.py
a.b: int = 5
$ python -m dis t.py
  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.

----------
components: Interpreter Core
messages: 383729
nosy: BTaskaya, gvanrossum, lys.nikolaou, pablogsal, serhiy.storchaka
priority: normal
severity: normal
status: open
title: PEP 563: drop annotations for complex assign targets
versions: Python 3.10

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42737>
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