diff --git a/babelizer/errors.py b/babelizer/errors.py
index 8a06edb..edf1abd 100644
--- a/babelizer/errors.py
+++ b/babelizer/errors.py
@@ -1,13 +1,15 @@
 """Exceptions raised by the *babelizer*."""
 
+from __future__ import annotations
+
 
 class BabelizeError(Exception):
     """An exception that the babelizer can handle and show to the user."""
 
-    def __init__(self, message):
+    def __init__(self, message: str):
         self._message = message
 
-    def __str__(self):
+    def __str__(self) -> str:
         """Render a user-readable error message."""
         return self._message
 
diff --git a/babelizer/utils.py b/babelizer/utils.py
index c32f51d..eaa6dc4 100644
--- a/babelizer/utils.py
+++ b/babelizer/utils.py
@@ -1,8 +1,11 @@
 """Utility functions used by the babelizer."""
 
+from __future__ import annotations
+
 import pathlib
 import subprocess
 import sys
+from collections.abc import Iterator
 from contextlib import contextmanager
 from contextlib import suppress
 
@@ -36,7 +39,7 @@ def setup_py(*args):
     return [sys.executable, "setup.py"] + list(args)
 
 
-def get_setup_py_version():
+def get_setup_py_version() -> str | None:
     """Get babelized package version.
 
     Returns
@@ -64,7 +67,7 @@ def get_setup_py_version():
 
 
 @contextmanager
-def save_files(files):
+def save_files(files: Iterator[str]):
     """Generate repository files through a context.
 
     Parameters