added error message to assert statement

youtube_transcript_api/_api.py

@@ -92,6 +92,8 @@ class YouTubeTranscriptApi(object):
         video ids, which could not be retrieved
         :rtype ({str: [{'text': str, 'start': float, 'end': float}]}, [str]}):
         """
+        assert isinstance(video_ids, list), "`video_ids` must be a list of strings"
+
         data = {}
         unretrievable_videos = []
 
@@ -126,6 +128,7 @@ class YouTubeTranscriptApi(object):
         :return: a list of dictionaries containing the 'text', 'start' and 'duration' keys
         :rtype [{'text': str, 'start': float, 'end': float}]:
         """
+        assert isinstance(video_id, str), "`video_id` must be a string"
         return cls.list_transcripts(video_id, proxies, cookies).find_transcript(languages).fetch()
     
     @classmethod

youtube_transcript_api/test/test_api.py

@@ -1,3 +1,4 @@
+from ast import Assert
 from unittest import TestCase
 from mock import patch
 
@@ -254,6 +255,24 @@ class TestYouTubeTranscriptApi(TestCase):
                 {'text': 'just something shorter, I made up for testing', 'start': 5.7, 'duration': 3.239}
             ]
         )
+    
+    def test_get_transcript__assertionerror_if_input_not_string(self):
+        """
+        Raise Assertion error if not expected type
+        """
+        with self.assertRaises(AssertionError):
+            YouTubeTranscriptApi.get_transcript(['video_id_1', 'video_id_2'])
+            # YouTubeTranscriptApi.get_transcripts(['video_id_1', 'video_id_2'])
+    
+    def test_get_transcripts__assertionerror_if_input_not_list(self):
+        """
+        Raise Assertion error if not expected type
+        """
+        with self.assertRaises(AssertionError):
+            YouTubeTranscriptApi.get_transcripts('video_id_1')
+            # YouTubeTranscriptApi.get_transcripts(['video_id_1', 'video_id_2'])
+
+
 
     @patch('youtube_transcript_api.YouTubeTranscriptApi.get_transcript')
     def test_get_transcripts(self, mock_get_transcript):