used-before-assignment false positive with walrus

[nickdrozd]

Bug description

from collections import defaultdict

KEYS: dict[int, dict[int, int]] = defaultdict(dict)

def f(x: int, y: int, z: int) -> int:
    try:
        return (keys := KEYS[x])[y]
    except KeyError:
        keys[y] = z  # [used-before-assignment] FALSE POSITIVE
        return z

print(f(1, 2, 3))
print(f(4, 5, 6))
print(f(4, 5, 6))

Configuration

No response

Command used

pylint asdf.py

Pylint output

pylint asdf.py 
************* Module asdf
asdf.py:9:8: E0601: Using variable 'keys' before assignment (used-before-assignment)

------------------------------------------------------------------
Your code has been rated at 5.45/10 (previous run: 5.45/10, +0.00)

Expected behavior

KEYS[x] succeeds, and so does keys := KEYS[x]. keys[y] may fail with KeyError, but by that time keys is defined, so its use in the except block works.

It is important that KEYS is defaultdict. If it were a regular dictionary, KEYS[x] might fail, in which case keys would indeed be undefined in the except block. Is that something inference can determine? Possibly this should be possibly-used-before-assignment instead.

Pylint version

pylint 4.0.0-dev0
astroid 3.4.0-dev0
Python 3.12.3 (tags/v3.12.3:f6650f9ad73, May 28 2024, 17:31:51) [GCC 9.4.0]

OS / Environment

No response

Additional dependencies

No response

[jacobtylerwalls]

keys[y] may fail with KeyError, but by that time keys is defined, so its use in the except block works.

Right, but what about this:

    try:
        return (keys := KEYS[x][0])[y]

Now it will fail with KeyError but before keys is defined:

  File "/Users/jwalls/Desktop/a.py", line 9, in f
    keys[y] = z  # [used-before-assignment] FALSE POSITIVE
    ^^^^
UnboundLocalError: cannot access local variable 'keys' where it is not associated with a value

---

I think the idea here is to keep the principle as simple/stupid as possible: except blocks shouldn't depend on try statements succeeding.