diff --git a/git_time_tracker/__main__.py b/git_time_tracker/__main__.py
index e85b8ff..bb1008c 100644
--- a/git_time_tracker/__main__.py
+++ b/git_time_tracker/__main__.py
@@ -19,7 +19,8 @@ def filter_samples(
     samples: list[ActivitySample],
     sample_filter: set[str],
 ) -> list[ActivitySample]:
-    assert len(sample_filter) > 0
+    if not sample_filter:
+        raise ValueError("sample_filter must not be empty")
     return [s for s in samples if set(s.labels).intersection(sample_filter)]
 
 
diff --git a/scripts/__init__.py b/scripts/__init__.py
new file mode 100644
index 0000000..4fd3dda
--- /dev/null
+++ b/scripts/__init__.py
@@ -0,0 +1 @@
+# This file marks the scripts directory as a package.
diff --git a/scripts/download_simfiles.py b/scripts/download_simfiles.py
index 9aa5055..d1d6c88 100644
--- a/scripts/download_simfiles.py
+++ b/scripts/download_simfiles.py
@@ -32,9 +32,9 @@ def parse_results(response) -> list[Result]:
         link = cells[0].a
         if link is None:
             continue
-        id = link['href'].removeprefix('viewsimfile.php?simfileid=')
+        simfile_id = link['href'].removeprefix('viewsimfile.php?simfileid=')
         levels = cells[1].get_text().strip()
-        results.append(Result(title, int(id), levels))
+        results.append(Result(title, int(simfile_id), levels))
     return results
 
 
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..00a6a8a
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1 @@
+# This file marks the tests directory as a package.
diff --git a/tests/test_myanimelist.py b/tests/test_myanimelist.py
index 920c53a..9e7f721 100644
--- a/tests/test_myanimelist.py
+++ b/tests/test_myanimelist.py
@@ -4,7 +4,7 @@ from personal_data.fetchers.myanimelist import parse_name
 
 
 @pytest.mark.parametrize(
-    'input_str, expected_group1, expected_group2',
+    ("input_str", "expected_group1", "expected_group2"),
     [
         ('"Soundscape"', 'Soundscape', None),
         ('"Soundscape (サウンドスケープ)"', 'Soundscape', 'サウンドスケープ'),