Fix alternate label handling in WDS parser to skip invalid alt labels

timm/data/parsers/parser_wds.py

@@ -106,18 +106,51 @@ def _decode(sample, image_key='jpg', image_format='RGB', target_key='cls', alt_l
     * cls byte string label to int
     * pass through JSON byte string (if it exists) without parse
     """
+    # decode class label, skip if alternate label not valid
+    if alt_label:
+        # alternative labels are encoded in json metadata
+        meta = json.loads(sample['json'])
+        class_label = int(meta[alt_label])
+        if class_label < 0:
+            # skipped labels currently encoded as -1, may change to a null/None value
+            return None
+    else:
+        class_label = int(sample[target_key])
+
+    # decode image
     with io.BytesIO(sample[image_key]) as b:
         img = Image.open(b)
         img.load()
     if image_format:
         img = img.convert(image_format)
-    if alt_label:
-        # alternative labels are encoded in json metadata
-        assert 'json' in sample
-        meta = json.loads(sample['json'])
-        return dict(jpg=img, cls=int(meta[alt_label]), json=meta)
-    else:
-        return dict(jpg=img, cls=int(sample[target_key]), json=sample.get('json', None))
+
+    # json passed through in undecoded state
+    return dict(jpg=img, cls=class_label, json=sample.get('json', None))
+
+
+def _decode_samples(
+        data,
+        image_key='jpg',
+        image_format='RGB',
+        target_key='cls',
+        alt_label='',
+        handler=wds.reraise_exception):
+    """Decode samples with skip."""
+    for sample in data:
+        try:
+            result = _decode(
+                sample, image_key=image_key, image_format=image_format, target_key=target_key, alt_label=alt_label)
+        except Exception as exn:
+            if handler(exn):
+                continue
+            else:
+                break
+
+        # null results are skipped
+        if result is not None:
+            if isinstance(sample, dict) and isinstance(result, dict):
+                result["__key__"] = sample.get("__key__")
+            yield result
 
 
 class ParserWebdataset(Parser):
@@ -217,11 +250,11 @@ class ParserWebdataset(Parser):
                 wds.tarfile_to_samples(),
             ])
         pipeline.extend([
-            wds.map(partial(
-                _decode,
+            partial(
+                _decode_samples,
                 image_key=self.image_key,
                 image_format=self.image_format,
-                alt_label=self.split_info.alt_label))
+                alt_label=self.split_info.alt_label)
         ])
         self.ds = wds.DataPipeline(*pipeline)
         self.init_count += 1