From ab99c46baea5546cb666c140b3e99fafa9e159d4 Mon Sep 17 00:00:00 2001
From: Bhuvansh855 <bhuvanshkataria@gmail.com>
Date: Thu, 18 Jun 2026 13:33:17 +0530
Subject: [PATCH 1/2] Fix type narrowing for variable swaps after isinstance
 checks

---
 mypy/checker.py                      | 14 ++++++++++++++
 test-data/unit/check-isinstance.test | 18 ++++++++++++++++++
 test-data/unit/check-narrowing.test  | 18 ++++++++++++++++++
 3 files changed, 50 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index 33705c98e10c3..6ba65436bfd7b 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -4175,7 +4175,21 @@ def check_assignment_to_multiple_lvalues(
                     lr_pairs.append((star_lv.expr, rv_list))
                 lr_pairs.extend(zip(right_lvs, right_rvs))
 
+                captured_pairs: list[tuple[Lvalue, Expression]] = []
+
                 for lv, rv in lr_pairs:
+                    if isinstance(rv, NameExpr):
+                        narrowed = self.binder.get(rv)
+
+                        if narrowed is not None:
+                            captured_pairs.append(
+                                (lv, self.temp_node(narrowed, context=rv))
+                            )
+                            continue
+
+                    captured_pairs.append((lv, rv))
+
+                for lv, rv in captured_pairs:
                     self.check_assignment(lv, rv, infer_lvalue_type)
         else:
             self.check_multi_assignment(lvalues, rvalue, context, infer_lvalue_type)
diff --git a/test-data/unit/check-isinstance.test b/test-data/unit/check-isinstance.test
index acd81839fcdc7..ccd1d624f3711 100644
--- a/test-data/unit/check-isinstance.test
+++ b/test-data/unit/check-isinstance.test
@@ -3377,3 +3377,21 @@ def f2(x: A | None, t: type[A]):
     else:
         reveal_type(x)  # N: Revealed type is "None"
 [builtins fixtures/isinstancelist.pyi]
+
+[case testTupleSwapAfterIsinstance]
+class Base: pass
+class Sub1(Base): pass
+class Sub2(Base): pass
+
+def f(a: Base, b: Base) -> None:
+    if isinstance(b, Sub1) and isinstance(a, Sub2):
+        t = (b, a)
+
+        reveal_type(t)  # N: Revealed type is "tuple[__main__.Sub1, __main__.Sub2]"
+
+        a, b = t
+
+        reveal_type(a)  # N: Revealed type is "__main__.Sub1"
+        reveal_type(b)  # N: Revealed type is "__main__.Sub2"
+
+[builtins fixtures/isinstancelist.pyi]
diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 7bab7baa6cebd..39c161222397f 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -4281,3 +4281,21 @@ def func(y: H) -> H:
     else:
         return y
 [builtins fixtures/primitives.pyi]
+
+[case testIsInstanceVariableSwapNarrowing]
+class Base:
+    pass
+
+class Sub1(Base):
+    pass
+
+class Sub2(Base):
+    pass
+
+def f(a: Base, b: Base) -> None:
+    if isinstance(b, Sub1) and isinstance(a, Sub2):
+        a, b = b, a
+        reveal_type(a)  # N: Revealed type is "__main__.Sub1"
+        reveal_type(b)  # N: Revealed type is "__main__.Sub2"
+
+[builtins fixtures/isinstancelist.pyi]

From fd822a084cbf01e4c102cdb9620c6d56608688d2 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Thu, 18 Jun 2026 08:07:26 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/checker.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index 6ba65436bfd7b..32d9dbc8f1b32 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -4182,9 +4182,7 @@ def check_assignment_to_multiple_lvalues(
                         narrowed = self.binder.get(rv)
 
                         if narrowed is not None:
-                            captured_pairs.append(
-                                (lv, self.temp_node(narrowed, context=rv))
-                            )
+                            captured_pairs.append((lv, self.temp_node(narrowed, context=rv)))
                             continue
 
                     captured_pairs.append((lv, rv))