Add "assert len(labels)" catch (#4257)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

ultralytics/data/dataset.py

@@ -126,6 +126,7 @@ class YOLODataset(BaseDataset):
         # Read cache
         [cache.pop(k) for k in ('hash', 'version', 'msgs')]  # remove items
         labels = cache['labels']
+        assert len(labels), f'No valid labels found, please check your dataset. {HELP_URL}'
         self.im_files = [lb['im_file'] for lb in labels]  # update im_files
 
         # Check if the dataset is all boxes or all segments

ultralytics/data/utils.py

@@ -24,7 +24,7 @@ from ultralytics.utils.checks import check_file, check_font, is_ascii
 from ultralytics.utils.downloads import download, safe_download, unzip_file
 from ultralytics.utils.ops import segments2boxes
 
-HELP_URL = 'See https://docs.ultralytics.com/yolov5/tutorials/train_custom_data'
+HELP_URL = 'See https://docs.ultralytics.com/datasets/detect for YOLO dataset format help.'
 IMG_FORMATS = 'bmp', 'dng', 'jpeg', 'jpg', 'mpo', 'png', 'tif', 'tiff', 'webp', 'pfm'  # image suffixes
 VID_FORMATS = 'asf', 'avi', 'gif', 'm4v', 'mkv', 'mov', 'mp4', 'mpeg', 'mpg', 'ts', 'wmv', 'webm'  # video suffixes
 PIN_MEMORY = str(os.getenv('PIN_MEMORY', True)).lower() == 'true'  # global pin_memory for dataloaders