From 8abc3ff07179e1281fb8bce68d0506f7aac76230 Mon Sep 17 00:00:00 2001
From: Eric Traut <eric@traut.com>
Date: Tue, 11 Feb 2025 09:49:37 -0800
Subject: [PATCH] Exempted `__slots__` symbol when determining whether a class
 is a callback protocol. This addresses #9878. (#9879)

---
 .../src/analyzer/typeEvaluator.ts             | 30 +++++++++++++------
 .../src/tests/samples/callbackProtocol5.py    |  1 +
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/packages/pyright-internal/src/analyzer/typeEvaluator.ts b/packages/pyright-internal/src/analyzer/typeEvaluator.ts
index f4427db0d6aa..8a6ea47508b7 100644
--- a/packages/pyright-internal/src/analyzer/typeEvaluator.ts
+++ b/packages/pyright-internal/src/analyzer/typeEvaluator.ts
@@ -25928,19 +25928,31 @@ export function createTypeEvaluator(
         for (const mroClass of objType.shared.mro) {
             if (isClass(mroClass) && ClassType.isProtocolClass(mroClass)) {
                 for (const field of ClassType.getSymbolTable(mroClass)) {
-                    if (field[0] !== '__call__' && !field[1].isIgnoredForProtocolMatch()) {
-                        let fieldIsPartOfFunction = false;
+                    const fieldName = field[0];
+                    const fieldSymbol = field[1];
 
-                        if (prefetched?.functionClass && isClass(prefetched.functionClass)) {
-                            if (ClassType.getSymbolTable(prefetched.functionClass).has(field[0])) {
-                                fieldIsPartOfFunction = true;
-                            }
-                        }
+                    // We're expecting a __call__ method. We will also ignore a
+                    // __slots__ definition, which is (by convention) ignored for
+                    // protocol matching.
+                    if (fieldName === '__call__' || fieldName === '__slots__') {
+                        continue;
+                    }
 
-                        if (!fieldIsPartOfFunction) {
-                            return undefined;
+                    if (fieldSymbol.isIgnoredForProtocolMatch()) {
+                        continue;
+                    }
+
+                    let fieldIsPartOfFunction = false;
+
+                    if (prefetched?.functionClass && isClass(prefetched.functionClass)) {
+                        if (ClassType.getSymbolTable(prefetched.functionClass).has(field[0])) {
+                            fieldIsPartOfFunction = true;
                         }
                     }
+
+                    if (!fieldIsPartOfFunction) {
+                        return undefined;
+                    }
                 }
             }
         }
diff --git a/packages/pyright-internal/src/tests/samples/callbackProtocol5.py b/packages/pyright-internal/src/tests/samples/callbackProtocol5.py
index 6f3c498dd102..249f7d799425 100644
--- a/packages/pyright-internal/src/tests/samples/callbackProtocol5.py
+++ b/packages/pyright-internal/src/tests/samples/callbackProtocol5.py
@@ -54,6 +54,7 @@ class CallbackProto2(Protocol):
     __module__: str
     __qualname__: str
     __annotations__: dict[str, Any]
+    __slots__ = ()
 
     def __call__(self) -> None:
         ...