diff --git a/swh/loader/git/loader.py b/swh/loader/git/loader.py
--- a/swh/loader/git/loader.py
+++ b/swh/loader/git/loader.py
@@ -11,7 +11,7 @@
 import os
 import pickle
 import sys
-from typing import Any, Dict, Iterable, List, Optional, Set
+from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Tuple, Type
 
 import dulwich.client
 from dulwich.object_store import ObjectStoreGraphWalker
@@ -127,11 +127,11 @@
 
     def __init__(
         self,
-        url,
-        base_url=None,
-        ignore_history=False,
-        repo_representation=RepoRepresentation,
-        config=None,
+        url: str,
+        base_url: Optional[str] = None,
+        ignore_history: bool = False,
+        repo_representation: Type[RepoRepresentation] = RepoRepresentation,
+        config: Optional[Dict[str, Any]] = None,
     ):
         """Initialize the bulk updater.
 
@@ -141,6 +141,9 @@
             data.
 
         """
+        if config is None:
+            config = {}
+
         super().__init__(logging_class="swh.loader.git.BulkLoader", config=config)
         self.origin_url = url
         self.base_url = base_url
@@ -148,10 +151,15 @@
         self.repo_representation = repo_representation
 
         # state initialized in fetch_data
-        self.remote_refs = []
-        self.symbolic_refs = {}
+        self.remote_refs: Dict[bytes, bytes] = {}
+        self.symbolic_refs: Dict[bytes, bytes] = {}
 
-    def fetch_pack_from_origin(self, origin_url, base_snapshot, do_activity):
+    def fetch_pack_from_origin(
+        self,
+        origin_url: str,
+        base_snapshot: Optional[Snapshot],
+        do_activity: Callable[[bytes], None],
+    ) -> FetchPackReturn:
         """Fetch a pack from the origin"""
         pack_buffer = BytesIO()
 
@@ -167,7 +175,7 @@
 
         size_limit = self.config["pack_size_bytes"]
 
-        def do_pack(data):
+        def do_pack(data: bytes) -> None:
             cur_size = pack_buffer.tell()
             would_write = len(data)
             if cur_size + would_write > size_limit:
@@ -201,7 +209,9 @@
             pack_size=pack_size,
         )
 
-    def list_pack(self, pack_data, pack_size):
+    def list_pack(
+        self, pack_data, pack_size
+    ) -> Tuple[Dict[bytes, bytes], Dict[bytes, Set[bytes]]]:
         id_to_type = {}
         type_to_ids = defaultdict(set)
 
@@ -214,7 +224,7 @@
 
         return id_to_type, type_to_ids
 
-    def prepare_origin_visit(self, *args, **kwargs):
+    def prepare_origin_visit(self, *args, **kwargs) -> None:
         self.visit_date = datetime.datetime.now(tz=datetime.timezone.utc)
         self.origin = Origin(url=self.origin_url)
 
@@ -228,10 +238,12 @@
             return None
         return Snapshot.from_dict(snapshot)
 
-    def prepare(self, *args, **kwargs):
+    def prepare(self, *args, **kwargs) -> None:
+        assert self.origin is not None
+
         base_origin_url = origin_url = self.origin.url
 
-        prev_snapshot = None
+        prev_snapshot: Optional[Snapshot] = None
 
         if not self.ignore_history:
             prev_snapshot = self.get_full_snapshot(origin_url)
@@ -248,8 +260,10 @@
         else:
             self.base_snapshot = Snapshot(branches={})
 
-    def fetch_data(self):
-        def do_progress(msg):
+    def fetch_data(self) -> bool:
+        assert self.origin is not None
+
+        def do_progress(msg: bytes) -> None:
             sys.stderr.buffer.write(msg)
             sys.stderr.flush()
 
@@ -263,13 +277,11 @@
         self.remote_refs = fetch_info.remote_refs
         self.symbolic_refs = fetch_info.symbolic_refs
 
-        origin_url = self.origin.url
-
         self.log.info(
-            "Listed %d refs for repo %s" % (len(self.remote_refs), origin_url),
+            "Listed %d refs for repo %s" % (len(self.remote_refs), self.origin.url),
             extra={
                 "swh_type": "git_repo_list_refs",
-                "swh_repo": origin_url,
+                "swh_repo": self.origin.url,
                 "swh_num_refs": len(self.remote_refs),
             },
         )
@@ -283,8 +295,9 @@
         # No more data to fetch
         return False
 
-    def save_data(self):
+    def save_data(self) -> None:
         """Store a pack for archival"""
+        assert isinstance(self.visit_date, datetime.datetime)
 
         write_size = 8192
         pack_dir = self.get_save_data_path()
@@ -305,14 +318,14 @@
         with open(os.path.join(pack_dir, refs_name), "xb") as f:
             pickle.dump(self.remote_refs, f)
 
-    def get_inflater(self):
+    def get_inflater(self) -> PackInflater:
         """Reset the pack buffer and get an object inflater from it"""
         self.pack_buffer.seek(0)
         return PackInflater.for_pack_data(
             PackData.from_file(self.pack_buffer, self.pack_size)
         )
 
-    def has_contents(self):
+    def has_contents(self) -> bool:
         return bool(self.type_to_ids[b"blob"])
 
     def get_content_ids(self) -> Iterable[Dict[str, Any]]:
@@ -470,9 +483,9 @@
            the one we retrieved at the beginning of the run"""
         eventful = False
 
-        if self.base_snapshot:
+        if self.base_snapshot and self.snapshot:
             eventful = self.snapshot.id != self.base_snapshot.id
-        else:
+        elif self.snapshot:
             eventful = bool(self.snapshot.branches)
 
         return {"status": ("eventful" if eventful else "uneventful")}
@@ -493,7 +506,7 @@
         help="Ignore the repository history",
         default=False,
     )
-    def main(origin_url, base_url, ignore_history):
+    def main(origin_url: str, base_url: str, ignore_history: bool) -> Dict[str, Any]:
         loader = GitLoader(
             origin_url, base_url=base_url, ignore_history=ignore_history,
         )