[3.15] gh-155426: correctly reject non-bytes keys in hmac.digest (GH-155427) (#159743)

This only affects keys of length smaller than the HMAC blocksize
since larger keys are hashed first.

(cherry picked from commit 197d332)

---------

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: miss-islington

Lib/hmac.py

@@ -267,6 +267,9 @@ def digest(key, msg, digest):
 
 
 def _compute_digest_fallback(key, msg, digest):
+    if not isinstance(key, (bytes, bytearray)):
+        raise TypeError(f"key: expected bytes or bytearray, "
+                        f"but got {type(key).__name__!r}")
     digest_cons = _get_digest_constructor(digest)
     if _is_shake_constructor(digest_cons):
         raise ValueError(f"unsupported hash algorithm {digest}")

Lib/test/test_hmac.py

@@ -1525,6 +1525,12 @@ def test_with_fallback(self):
         finally:
             cache.pop('foo')
 
+    @hashlib_helper.requires_hashdigest("md5")
+    def test_hmac_digest_reject_memoryview_for_key(self):
+        hmac = import_fresh_module("hmac", blocked=["_hashlib", "_hmac"])
+        with self.assertRaises(TypeError):
+            hmac.digest(memoryview(b"small"), b"world", "md5")
+
     @hashlib_helper.requires_openssl_hashdigest("md5")
     @bigmemtest(size=_4G + 5, memuse=2, dry_run=False)
     def test_hmac_digest_overflow_error_openssl_only(self, size):

Misc/NEWS.d/next/Library/2026-08-09-14-06-27.gh-issue-155426.zZVPMd.rst

@@ -0,0 +1,4 @@
+Make sure that the pure Python fallback of :func:`hmac.digest` rejects
+keys that are neither :class:`bytes` or :class:`bytearray`. Previously,
+an :exc:`AttributeError` was raised instead of :exc:`TypeError`.
+Patch by Bénédikt Tran.