[pitivi] common: Fix broken decorator

[Alexandru Băluț <alexbalut src gnome org>]

commit 0d3fe6fa72558c9348db1b4ccf1f3767392db33a
Author: Tyler Senne <tsenne2 huskers unl edu>
Date:   Sat Apr 17 16:37:17 2021 -0500

    common: Fix broken decorator

 tests/common.py                 | 7 ++++---
 tests/test_clipproperties.py    | 2 +-
 tests/test_timeline_elements.py | 2 +-
 3 files changed, 6 insertions(+), 5 deletions(-)
---
diff --git a/tests/common.py b/tests/common.py
index cc1db65ae..ee5cfb97d 100644
--- a/tests/common.py
+++ b/tests/common.py
@@ -27,6 +27,7 @@ import sys
 import tempfile
 import traceback
 import unittest
+from typing import List
 from unittest import mock
 
 from gi.repository import Gdk
@@ -188,13 +189,13 @@ class CheckedOperationDuration:
         signal.alarm(0)
 
 
-def setup_project_with_clips(assets_names=None):  # pylint: disable=unused-argument
+def setup_project_with_clips(assets_names: List[str]):
     """Sets up a Pitivi instance with the specified assets on the timeline."""
+    # Ensure this method is not being used directly as a decorator.
+    assert isinstance(assets_names, list)
 
     def decorator(func):
         nonlocal assets_names
-        if assets_names is None:
-            assets_names = ["1sec_simpsons_trailer.mp4"]
 
         def wrapper(self):
             with cloned_sample(*assets_names):
diff --git a/tests/test_clipproperties.py b/tests/test_clipproperties.py
index e16855c1e..c4e51fb3b 100644
--- a/tests/test_clipproperties.py
+++ b/tests/test_clipproperties.py
@@ -585,7 +585,7 @@ class SpeedPropertiesTest(common.TestCase):
         self.timeline_container.timeline.selection.select([clip1])
         self.assert_applied_rate(1, 2.0, clip1_duration / 2)
 
-    @common.setup_project_with_clips
+    @common.setup_project_with_clips(assets_names=["1sec_simpsons_trailer.mp4"])
     @common.setup_clipproperties
     def test_load_project_clip_speed(self):
         sources_count = 2
diff --git a/tests/test_timeline_elements.py b/tests/test_timeline_elements.py
index 39b448825..1e87d0935 100644
--- a/tests/test_timeline_elements.py
+++ b/tests/test_timeline_elements.py
@@ -412,7 +412,7 @@ class TestVideoSource(common.TestCase):
         self.assertEqual(width, 960)
         self.assertEqual(height, 400)
 
-    @common.setup_project_with_clips
+    @common.setup_project_with_clips(assets_names=["1sec_simpsons_trailer.mp4"])
     @common.setup_clipproperties
     def test_change_set_project_size(self):
         """Checks the size of the scaled clips after project settings changes."""