From 382829dffeea82157ec00d3086868d778e1a4e92 Mon Sep 17 00:00:00 2001
From: TheMuffinMan1320 <76186189+TheMuffinMan1320@users.noreply.github.com>
Date: Thu, 13 Aug 2026 14:47:43 -0500
Subject: [PATCH] Fix crash on walrus assignment shadowing partial type in
 comprehension (#20792)

Evaluating an rvalue can itself resolve a variable's partial type as a
side effect, e.g. when a walrus assignment inside a comprehension
reassigns the same partial-typed name. In that case,
try_infer_partial_generic_type_from_assignment would try to delete the
already-removed entry from partial_types, raising a KeyError. Guard
against this by checking that the var is still tracked as partial
after evaluating the rvalue.

Fixes #20792
---
 mypy/checker.py                    | 4 ++++
 test-data/unit/check-python38.test | 9 +++++++++
 2 files changed, 13 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index b1c15d20c329b..a92b43ebebf9d 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -3710,6 +3710,10 @@ def try_infer_partial_generic_type_from_assignment(
                 return
             rvalue_type = self.expr_checker.accept(rvalue)
             rvalue_type = get_proper_type(rvalue_type)
+            if var not in partial_types:
+                # Evaluating rvalue may have already resolved this partial type
+                # (e.g. a walrus assignment to the same variable inside rvalue).
+                return
             if isinstance(rvalue_type, Instance):
                 if rvalue_type.type == typ.type and is_valid_inferred_type(
                     rvalue_type, self.options
diff --git a/test-data/unit/check-python38.test b/test-data/unit/check-python38.test
index d2ede55840cfe..a83b1a7d312fb 100644
--- a/test-data/unit/check-python38.test
+++ b/test-data/unit/check-python38.test
@@ -455,6 +455,15 @@ def check_partial_list() -> None:
     reveal_type(z)  # N: Revealed type is "builtins.list[builtins.int]"
 [builtins fixtures/list.pyi]
 
+[case testWalrusPartialTypeShadowedInComprehension]
+# https://github.com/python/mypy/issues/20792
+errors = []
+errors = [
+    i for i in [1, 2, 3] if (errors := [1])
+]
+reveal_type(errors)  # N: Revealed type is "builtins.list[builtins.int]"
+[builtins fixtures/list.pyi]
+
 [case testWalrusAssignmentAndConditionScopeForLiteral]
 # flags: --warn-unreachable