From 44b631b80b9ed9a43c25a897b9c919c0d65b88a1 Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Date: Sat, 29 Aug 2026 10:43:26 -0700
Subject: [PATCH] Support in/not in tests on sys.platform

This is proposed for the spec in python/typing#2173. It is trivial to support in
mypy and seems useful, so let's add it.
---
 mypy/reachability.py                       | 25 +++++--
 test-data/unit/check-unreachable-code.test | 76 ++++++++++++++++++++++
 2 files changed, 97 insertions(+), 4 deletions(-)

diff --git a/mypy/reachability.py b/mypy/reachability.py
index 37c7a9715600a..1d95c49d86ecc 100644
--- a/mypy/reachability.py
+++ b/mypy/reachability.py
@@ -23,6 +23,7 @@
     MemberExpr,
     NameExpr,
     OpExpr,
+    SetExpr,
     SliceExpr,
     StrExpr,
     TupleExpr,
@@ -232,19 +233,27 @@ def consider_sys_platform(expr: Expression, platform: str) -> int:
     # - sys.platform == 'linux'
     # - sys.platform != 'win32'
     # - sys.platform.startswith('win')
+    # - sys.platform in {'linux', 'darwin'}
+    # - sys.platform not in ('win32', 'cygwin')
     if isinstance(expr, ComparisonExpr):
         # Let's not yet support chained comparisons.
         if len(expr.operators) > 1:
             return TRUTH_VALUE_UNKNOWN
         op = expr.operators[0]
-        if op not in ("==", "!="):
-            return TRUTH_VALUE_UNKNOWN
         if not is_sys_attr(expr.operands[0], "platform"):
             return TRUTH_VALUE_UNKNOWN
         right = expr.operands[1]
-        if not isinstance(right, StrExpr):
+        if op in ("==", "!=") and isinstance(right, StrExpr):
+            return fixed_comparison(platform, op, right.value)
+        if op not in ("in", "not in"):
+            return TRUTH_VALUE_UNKNOWN
+        items = contains_tuple_or_set_of_strings(right)
+        if items is None:
             return TRUTH_VALUE_UNKNOWN
-        return fixed_comparison(platform, op, right.value)
+        result = platform in items
+        if op == "not in":
+            result = not result
+        return ALWAYS_TRUE if result else ALWAYS_FALSE
     elif isinstance(expr, CallExpr):
         if not isinstance(expr.callee, MemberExpr):
             return TRUTH_VALUE_UNKNOWN
@@ -296,6 +305,14 @@ def contains_int_or_tuple_of_ints(expr: Expression) -> None | int | tuple[int, .
     return None
 
 
+def contains_tuple_or_set_of_strings(expr: Expression) -> tuple[str, ...] | None:
+    if isinstance(expr, (TupleExpr, SetExpr)) and all(
+        isinstance(item, StrExpr) for item in expr.items
+    ):
+        return tuple(item.value for item in expr.items if isinstance(item, StrExpr))
+    return None
+
+
 def contains_sys_version_info(expr: Expression) -> None | int | tuple[int | None, int | None]:
     if is_sys_attr(expr, "version_info"):
         return (None, None)  # Same as sys.version_info[:]
diff --git a/test-data/unit/check-unreachable-code.test b/test-data/unit/check-unreachable-code.test
index 42aa76b4ba951..b0fbdd1057040 100644
--- a/test-data/unit/check-unreachable-code.test
+++ b/test-data/unit/check-unreachable-code.test
@@ -474,6 +474,82 @@ reveal_type(x)  # N: Revealed type is "builtins.str"
 [builtins fixtures/ops.pyi]
 [out]
 
+[case testCustomSysPlatformMembershipTuple]
+# flags: --platform linux
+import sys
+if sys.platform in ('linux', 'darwin'):
+    x = "foo"
+else:
+    x = 3
+reveal_type(x)  # N: Revealed type is "builtins.str"
+
+if sys.platform not in ('win32', 'cygwin'):
+    y = "foo"
+else:
+    y = 3
+reveal_type(y)  # N: Revealed type is "builtins.str"
+
+if sys.platform in ('win32', 'cygwin'):
+    z = "foo"
+else:
+    z = 3
+reveal_type(z)  # N: Revealed type is "builtins.int"
+
+if sys.platform not in ('linux', 'darwin'):
+    w = "foo"
+else:
+    w = 3
+reveal_type(w)  # N: Revealed type is "builtins.int"
+[builtins fixtures/ops.pyi]
+[out]
+
+[case testCustomSysPlatformMembershipSet]
+# flags: --platform linux
+import sys
+if sys.platform in {'linux', 'darwin'}:
+    x = "foo"
+else:
+    x = 3
+reveal_type(x)  # N: Revealed type is "builtins.str"
+
+if sys.platform not in {'linux', 'darwin'}:
+    y = "foo"
+else:
+    y = 3
+reveal_type(y)  # N: Revealed type is "builtins.int"
+[builtins fixtures/set.pyi]
+[out]
+
+[case testSysPlatformMembershipUnknown]
+import sys
+
+platform = sys.platform
+if sys.platform in ('linux', platform):
+    def f() -> int: return 0
+else:
+    def f() -> str: return ''  # E: All conditional function variants must have identical signatures \
+                               # N: Original: \
+                               # N:     def f() -> int \
+                               # N: Redefinition: \
+                               # N:     def f() -> str
+[builtins fixtures/ops.pyi]
+[out]
+
+[case testSysPlatformMembershipListIsUnknown]
+# flags: --platform linux
+import sys
+
+if sys.platform in ['linux', 'darwin']:
+    def f() -> int: return 0
+else:
+    def f() -> str: return ''  # E: All conditional function variants must have identical signatures \
+                               # N: Original: \
+                               # N:     def f() -> int \
+                               # N: Redefinition: \
+                               # N:     def f() -> str
+[builtins fixtures/list.pyi]
+[out]
+
 [case testShortCircuitInExpression]
 import typing
 def make() -> bool: pass