commit - 2ea06096079b0d6641dc41e5c598fb835607e54e
commit + 260a6d9c91b5d530b912e4046b84c039a685fb5b
blob - 026fdca3d5fc6d3c592623312d18bb7ee5e29c78
blob + 91159b95a15c816fe9543a63c1ddb9cd7f9098de
--- dulwich/refs.py
+++ dulwich/refs.py
@@ -34,12 +34,14 @@ from dulwich.objects import (
     valid_hexsha,
     ZERO_SHA,
     Tag,
+    ObjectID,
 )
 from dulwich.file import (
     GitFile,
     ensure_dir_exists,
 )
 
+Ref = bytes
 
 HEADREF = b"HEAD"
 SYMREF = b"ref: "
@@ -69,7 +71,7 @@ def parse_symref_value(contents):
     raise ValueError(contents)
 
 
-def check_ref_format(refname):
+def check_ref_format(refname: Ref):
     """Check if a refname is correctly formatted.
 
     Implements all the same rules as git-check-ref-format[1].
@@ -166,8 +168,8 @@ class RefsContainer(object):
 
     def import_refs(
         self,
-        base: bytes,
-        other: Dict[bytes, bytes],
+        base: Ref,
+        other: Dict[Ref, ObjectID],
         committer: Optional[bytes] = None,
         timestamp: Optional[bytes] = None,
         timezone: Optional[bytes] = None,
@@ -455,8 +457,8 @@ class DictRefsContainer(RefsContainer):
 
     def set_symbolic_ref(
         self,
-        name,
-        other,
+        name: Ref,
+        other: Ref,
         committer=None,
         timestamp=None,
         timezone=None,
@@ -507,8 +509,8 @@ class DictRefsContainer(RefsContainer):
 
     def add_if_new(
         self,
-        name: bytes,
-        ref: bytes,
+        name: Ref,
+        ref: ObjectID,
         committer=None,
         timestamp=None,
         timezone=None,
blob - 0fa4b1fd483b2f59fa583f53a414f04a50323967
blob + 89439b4b22e7fee40a30893f00779eca2eb6a6db
--- dulwich/repo.py
+++ dulwich/repo.py
@@ -741,7 +741,8 @@ class BaseRepo(object):
             return cached
         return self.object_store.peel_sha(self.refs[ref]).id
 
-    def get_walker(self, include=None, *args, **kwargs):
+    def get_walker(self, include: Optional[List[bytes]] = None,
+                   *args, **kwargs):
         """Obtain a walker for this repository.
 
         Args:
@@ -771,8 +772,6 @@ class BaseRepo(object):
 
         if include is None:
             include = [self.head()]
-        if isinstance(include, str):
-            include = [include]
 
         kwargs["get_parents"] = lambda commit: self.get_parents(commit.id, commit)