Improved dataset error introspection (#8091)

* Improved dataset error introspection

Intended to help https://github.com/ultralytics/yolov5/issues/8090

* Update general.py

utils/general.py

@@ -448,8 +448,7 @@ def check_font(font=FONT, progress=False):
 
 
 def check_dataset(data, autodownload=True):
-    # Download and/or unzip dataset if not found locally
-    # Usage: https://github.com/ultralytics/yolov5/releases/download/v1.0/coco128_with_yaml.zip
+    # Download, check and/or unzip dataset if not found locally
 
     # Download (optional)
     extract_dir = ''
@@ -463,6 +462,13 @@ def check_dataset(data, autodownload=True):
         with open(data, errors='ignore') as f:
             data = yaml.safe_load(f)  # dictionary
 
+    # Checks
+    for k in 'train', 'val', 'nc':
+        assert k in data, emojis(f"data.yaml '{k}:' field missing ❌")
+    if 'names' not in data:
+        LOGGER.warning(emojis("data.yaml 'names:' field missing ⚠, assigning default names 'class0', 'class1', etc."))
+        data['names'] = [f'class{i}' for i in range(data['nc'])]  # default names
+
     # Resolve paths
     path = Path(extract_dir or data.get('path') or '')  # optional 'path' default to '.'
     if not path.is_absolute():
@@ -472,9 +478,6 @@ def check_dataset(data, autodownload=True):
             data[k] = str(path / data[k]) if isinstance(data[k], str) else [str(path / x) for x in data[k]]
 
     # Parse yaml
-    assert 'nc' in data, "Dataset 'nc' key missing."
-    if 'names' not in data:
-        data['names'] = [f'class{i}' for i in range(data['nc'])]  # assign class names if missing
     train, val, test, s = (data.get(x) for x in ('train', 'val', 'test', 'download'))
     if val:
         val = [Path(x).resolve() for x in (val if isinstance(val, list) else [val])]  # val path