False-positive `name-error` in match statement

[sizmailov]

Source foo.py:

import bar

match 0:
    case 0:
        x: int = bar.f(0)

Command:

pytype --disable import-error foo.py 

Error:

File "foo.py", line 5, in <module>: Name '__annotations__' is not defined [name-error]

System info:

$ python --version
Python 3.10.6
$ pytype --version
2023.05.24

[martindemello]

the problem is the match statement compiles to this:

  1           0 LOAD_CONST               0 (0)

  2           2 LOAD_CONST               0 (0)
              4 COMPARE_OP               2 (==)
              6 POP_JUMP_IF_FALSE       12 (to 24)

  3           8 LOAD_CONST               1 (1)
             10 STORE_NAME               0 (x)
             12 LOAD_NAME                1 (int)
             14 LOAD_NAME                2 (__annotations__)
             16 LOAD_CONST               2 ('x')
             18 STORE_SUBSCR
             20 LOAD_CONST               3 (None)
             22 RETURN_VALUE

  2     >>   24 LOAD_CONST               3 (None)
             26 RETURN_VALUE

where it calls __annotations__[x] = int without having generated a SETUP_ANNOTATIONS first. for some reason this does not raise an error at runtime, but pytype is technically correct that the bytecode contains a name error. i suspect the interpreter special-cases __annotations__ in LOAD_NAME and we will have to do the same.

[martindemello]

since this is being fixed in cpython, and since there will genuinely be a runtime error when a module with this code is imported, i don't think we should try to work around this from pytype's side.

[JelleZijlstra]

Just a note that we'll only fix this in 3.11+ in CPython, as 3.10 is in security fix-only mode.

[martindemello]

ah, thanks for the note. for anyone running into the issue, the workaround is simply to add a top-level annotated assignment, to force the compiler to emit a SETUP_ANNOTATIONS. e.g. this works:

import bar

match 0:
    case 0:
        x: int = bar.f(0)

fix_annotations: bool = True

[martindemello]

this has been fixed in cpython at head and there's nothing more we can do for 3.10- so closing