fix: Improve type annotations in SignatureVerifier

Receive headers as abstract/immutable `collections.abc.Mapping` instead of `Dict` to more accurately declare compatibility with dict-like objects (common for holding headers in HTTP frameworks), and mark timestamp/signature args as `Optional` in `is_valid` to reflect reality and fix the typechecker errors in `is_valid_request`.

Author: BHSPitMonkey

slack_sdk/signature/__init__.py

@@ -2,8 +2,9 @@
 
 import hashlib
 import hmac
+from collections.abc import Mapping
 from time import time
-from typing import Dict, Optional, Union
+from typing import Optional, Union
 
 
 class Clock:
@@ -26,23 +27,23 @@ def __init__(self, signing_secret: str, clock: Clock = Clock()):
     def is_valid_request(
         self,
         body: Union[str, bytes],
-        headers: Dict[str, str],
+        headers: Mapping[str, str],
     ) -> bool:
         """Verifies if the given signature is valid"""
         if headers is None:
             return False
         normalized_headers = {k.lower(): v for k, v in headers.items()}
         return self.is_valid(
             body=body,
-            timestamp=normalized_headers.get("x-slack-request-timestamp", None),  # type: ignore[arg-type]
-            signature=normalized_headers.get("x-slack-signature", None),  # type: ignore[arg-type]
+            timestamp=normalized_headers.get("x-slack-request-timestamp", None),
+            signature=normalized_headers.get("x-slack-signature", None),
         )
 
     def is_valid(
         self,
         body: Union[str, bytes],
-        timestamp: str,
-        signature: str,
+        timestamp: Optional[str],
+        signature: Optional[str],
     ) -> bool:
         """Verifies if the given signature is valid"""
         if timestamp is None or signature is None: