diff --git a/swh/loader/package/tests/test_loader.py b/swh/loader/package/tests/test_loader.py
--- a/swh/loader/package/tests/test_loader.py
+++ b/swh/loader/package/tests/test_loader.py
@@ -5,7 +5,7 @@
 
 import hashlib
 import string
-from unittest.mock import MagicMock
+from unittest.mock import Mock
 
 import attr
 import pytest
@@ -47,7 +47,7 @@
 
 def test_resolve_revision_from_artifacts():
     loader = PackageLoader(None, None)
-    loader.known_artifact_to_extid = MagicMock(
+    loader.known_artifact_to_extid = Mock(
         wraps=lambda known_artifact: known_artifact["key"].encode()
     )
 
@@ -56,7 +56,11 @@
         b"b" * 40: {"key": "extid-of-bbbb"},
     }
 
-    p_info = MagicMock()
+    class StubPackageInfo(BasePackageInfo):
+        def extid(self):
+            return return_extid
+
+    p_info = Mock(wraps=StubPackageInfo(None, None))
 
     # No known artifact -> it would be useless to compute the extid
     assert loader.resolve_revision_from_artifacts({}, p_info) is None
@@ -66,7 +70,7 @@
     p_info.extid.reset_mock()
 
     # Some artifacts, but the PackageInfo does not support extids
-    p_info.extid.return_value = None
+    return_extid = None
     assert loader.resolve_revision_from_artifacts(known_artifacts, p_info) is None
     p_info.extid.assert_called_once()
     loader.known_artifact_to_extid.assert_not_called()
@@ -74,7 +78,7 @@
     p_info.extid.reset_mock()
 
     # Some artifacts, and the PackageInfo is not one of them (ie. cache miss)
-    p_info.extid.return_value = b"extid-of-cccc"
+    return_extid = b"extid-of-cccc"
     assert loader.resolve_revision_from_artifacts(known_artifacts, p_info) is None
     p_info.extid.assert_called_once()
     loader.known_artifact_to_extid.assert_any_call({"key": "extid-of-aaaa"})
@@ -84,7 +88,7 @@
     loader.known_artifact_to_extid.reset_mock()
 
     # Some artifacts, and the PackageInfo is one of them (ie. cache hit)
-    p_info.extid.return_value = b"extid-of-aaaa"
+    return_extid = b"extid-of-aaaa"
     assert loader.resolve_revision_from_artifacts(known_artifacts, p_info) == b"a" * 40
     p_info.extid.assert_called_once()
     loader.known_artifact_to_extid.assert_called_once_with({"key": "extid-of-aaaa"})