Walrus operators unrecognised in dictionary initialisation of a class attribute

[masasakano]

It seems Walrus operators (in Python 3.8+) are not recognised by mypy when used in initialising a dictionary of a class attribute. Here is a test code.

class TryWalrus:
    CKEY = { 
        'KY': (p:='a_'),
         'ANOTHER': p+'content',
        }

print(TryWalrus.CKEY['ANOTHER']) # => a_content

This outputs the following:

$ mypy try_walrus.py
try_walrus.py:4: error: Name 'p' is not defined
Found 1 error in 1 file (checked 1 source file)

If the variable CKEY is out of the class definition, i.e. if directly in Main, it does not raise an error.

I tested it with Python 3.8.1 and mypy version 0.770+dev.8888b1aedf6d4ba543ae12f538f190ce5e65c20d (yesterday's head).

[dhood]

This will also happen with a list e.g. l = [x2 := 1, x2 + 2, x2 + 3] (for this case also it will only happen if it is in the class scope).

I have noticed, however, that it only happens on RE-USE of the variable from the assignment expression within the initialisation. Otherwise the walrus operator is ok.

class Example:
    # l = [x2 := 1, x2 + 2, x2 + 3] # not OK
    l = [x2 := 1, 2, 3] # OK

[bunny-therapist]

I just (2023) ran into this exact problem with

mypy 1.4.1 (compiled: yes)
Python 3.11.0

[it176131]

Any proposed fix—besides # type: ignore[name-defined]—for this? I ran into it with the following versions:

Python 3.12.2
mypy   1.10.0

[JelleZijlstra]

Anyone impacted should feel free to work on a PR to fix this. I haven't looked at the code but I imagine it's a relatively easy fix.