fix: tests

src/lerobot/datasets/aggregate.py

@@ -15,8 +15,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import contextlib
 import logging
 import shutil
+from concurrent.futures import ThreadPoolExecutor, as_completed
 from pathlib import Path
 
 import pandas as pd
@@ -107,6 +109,7 @@ def update_meta_data(
     dst_meta,
     meta_idx,
     data_idx,
+    data_file_map,
     videos_idx,
 ):
     """Updates metadata DataFrame with new chunk, file, and timestamp indices.
@@ -127,8 +130,25 @@ def update_meta_data(
 
     df["meta/episodes/chunk_index"] = df["meta/episodes/chunk_index"] + meta_idx["chunk"]
     df["meta/episodes/file_index"] = df["meta/episodes/file_index"] + meta_idx["file"]
-    df["data/chunk_index"] = df["data/chunk_index"] + data_idx["chunk"]
+    # Remap data chunk/file indices per-source-file using the actual destination
-    df["data/file_index"] = df["data/file_index"] + data_idx["file"]
+    # file chosen during data aggregation. A flat offset is incorrect when
+    # multiple source files are concatenated into a single destination file.
+    if data_file_map:
+        new_data_chunk = []
+        new_data_file = []
+        for idx in df.index:
+            src_chunk = int(df.at[idx, "data/chunk_index"])  # original source file location
+            src_file = int(df.at[idx, "data/file_index"])  # original source file location
+            dst_chunk, dst_file = data_file_map.get(
+                (src_chunk, src_file), (src_chunk + data_idx["chunk"], src_file + data_idx["file"])
+            )
+            new_data_chunk.append(dst_chunk)
+            new_data_file.append(dst_file)
+        df["data/chunk_index"] = new_data_chunk
+        df["data/file_index"] = new_data_file
+    else:
+        df["data/chunk_index"] = df["data/chunk_index"] + data_idx["chunk"]
+        df["data/file_index"] = df["data/file_index"] + data_idx["file"]
     for key, video_idx in videos_idx.items():
         # Store original video file indices before updating
         orig_chunk_col = f"videos/{key}/chunk_index"
@@ -166,7 +186,7 @@ def update_meta_data(
     return df
 
 
-def aggregate_datasets(
+def _aggregate_datasets(
     repo_ids: list[str],
     aggr_repo_id: str,
     roots: list[Path] | None = None,
@@ -175,39 +195,24 @@ def aggregate_datasets(
     video_files_size_in_mb: float | None = None,
     chunk_size: int | None = None,
 ):
-    """Aggregates multiple LeRobot datasets into a single unified dataset.
+    """Serial aggregation kernel: combines datasets into a destination dataset.
 
-    This is the main function that orchestrates the aggregation process by:
+    This function performs a single-process aggregation. It assumes it is the
-    1. Loading and validating all source dataset metadata
+    sole writer for its destination `aggr_root`.
-    2. Creating a new destination dataset with unified tasks
-    3. Aggregating videos, data, and metadata from all source datasets
-    4. Finalizing the aggregated dataset with proper statistics
-
-    Args:
-        repo_ids: List of repository IDs for the datasets to aggregate.
-        aggr_repo_id: Repository ID for the aggregated output dataset.
-        roots: Optional list of root paths for the source datasets.
-        aggr_root: Optional root path for the aggregated dataset.
-        data_files_size_in_mb: Maximum size for data files in MB (defaults to DEFAULT_DATA_FILE_SIZE_IN_MB)
-        video_files_size_in_mb: Maximum size for video files in MB (defaults to DEFAULT_VIDEO_FILE_SIZE_IN_MB)
-        chunk_size: Maximum number of files per chunk (defaults to DEFAULT_CHUNK_SIZE)
     """
-    logging.info("Start aggregate_datasets")
+    # Build metadata objects, supporting a per-dataset "root" that may be None.
-
+    # When root is provided we load from the local filesystem, otherwise from Hub cache.
-    if data_files_size_in_mb is None:
+    if roots is None:
-        data_files_size_in_mb = DEFAULT_DATA_FILE_SIZE_IN_MB
+        all_metadata = [LeRobotDatasetMetadata(repo_id) for repo_id in repo_ids]
-    if video_files_size_in_mb is None:
+    else:
-        video_files_size_in_mb = DEFAULT_VIDEO_FILE_SIZE_IN_MB
+        all_metadata = [
-    if chunk_size is None:
+            (
-        chunk_size = DEFAULT_CHUNK_SIZE
+                LeRobotDatasetMetadata(repo_id, root=root)
-
+                if root is not None
-    all_metadata = (
+                else LeRobotDatasetMetadata(repo_id)
-        [LeRobotDatasetMetadata(repo_id) for repo_id in repo_ids]
+            )
-        if roots is None
+            for repo_id, root in zip(repo_ids, roots, strict=False)
-        else [
-            LeRobotDatasetMetadata(repo_id, root=root) for repo_id, root in zip(repo_ids, roots, strict=False)
         ]
-    )
     fps, robot_type, features = validate_all_metadata(all_metadata)
     video_keys = [key for key in features if features[key]["dtype"] == "video"]
 
@@ -237,9 +242,11 @@ def aggregate_datasets(
 
     for src_meta in tqdm.tqdm(all_metadata, desc="Copy data and videos"):
         videos_idx = aggregate_videos(src_meta, dst_meta, videos_idx, video_files_size_in_mb, chunk_size)
-        data_idx = aggregate_data(src_meta, dst_meta, data_idx, data_files_size_in_mb, chunk_size)
+        data_idx, data_file_map = aggregate_data(
+            src_meta, dst_meta, data_idx, data_files_size_in_mb, chunk_size
+        )
 
-        meta_idx = aggregate_metadata(src_meta, dst_meta, meta_idx, data_idx, videos_idx)
+        meta_idx = aggregate_metadata(src_meta, dst_meta, meta_idx, data_idx, data_file_map, videos_idx)
 
         dst_meta.info["total_episodes"] += src_meta.total_episodes
         dst_meta.info["total_frames"] += src_meta.total_frames
@@ -248,6 +255,168 @@ def aggregate_datasets(
     logging.info("Aggregation complete.")
 
 
+def aggregate_datasets(
+    repo_ids: list[str],
+    aggr_repo_id: str,
+    roots: list[Path] | None = None,
+    aggr_root: Path | None = None,
+    data_files_size_in_mb: float | None = None,
+    video_files_size_in_mb: float | None = None,
+    chunk_size: int | None = None,
+    num_workers: int | None = None,
+    tmp_root: Path | None = None,
+):
+    """Aggregates multiple LeRobot datasets into a single unified dataset.
+
+    This is the main function that orchestrates the aggregation process by:
+    1. Loading and validating all source dataset metadata
+    2. Creating a new destination dataset with unified tasks
+    3. Aggregating videos, data, and metadata from all source datasets
+    4. Finalizing the aggregated dataset with proper statistics
+
+    Args:
+        repo_ids: List of repository IDs for the datasets to aggregate.
+        aggr_repo_id: Repository ID for the aggregated output dataset.
+        roots: Optional list of root paths for the source datasets.
+        aggr_root: Optional root path for the aggregated dataset.
+        data_files_size_in_mb: Maximum size for data files in MB (defaults to DEFAULT_DATA_FILE_SIZE_IN_MB)
+        video_files_size_in_mb: Maximum size for video files in MB (defaults to DEFAULT_VIDEO_FILE_SIZE_IN_MB)
+        chunk_size: Maximum number of files per chunk (defaults to DEFAULT_CHUNK_SIZE)
+        num_workers: When > 1, performs a tree-based parallel reduction using a thread pool
+        tmp_root: Optional base directory to store intermediate reduction outputs
+    """
+    logging.info("Start aggregate_datasets")
+
+    if data_files_size_in_mb is None:
+        data_files_size_in_mb = DEFAULT_DATA_FILE_SIZE_IN_MB
+    if video_files_size_in_mb is None:
+        video_files_size_in_mb = DEFAULT_VIDEO_FILE_SIZE_IN_MB
+    if chunk_size is None:
+        chunk_size = DEFAULT_CHUNK_SIZE
+
+    if num_workers is None or num_workers <= 1:
+        # Run aggregation sequentially
+        _aggregate_datasets(
+            repo_ids=repo_ids,
+            aggr_repo_id=aggr_repo_id,
+            aggr_root=aggr_root,
+            roots=roots,
+            data_files_size_in_mb=data_files_size_in_mb,
+            video_files_size_in_mb=video_files_size_in_mb,
+            chunk_size=chunk_size,
+        )
+
+    # Uses a parallel fan-out/fan-in strategy when num_workers is provided
+    elif num_workers > 1:
+        # Validate across all metadata early to fail fast
+        all_metadata_for_validation = (
+            [LeRobotDatasetMetadata(repo_id) for repo_id in repo_ids]
+            if roots is None
+            else [
+                LeRobotDatasetMetadata(repo_id, root=root)
+                for repo_id, root in zip(repo_ids, roots, strict=False)
+            ]
+        )
+        validate_all_metadata(all_metadata_for_validation)
+
+        # Clamp workers to a sensible upper bound (pairs per round)
+        num_workers = min(num_workers, max(1, len(repo_ids) // 2))
+
+        # Choose a base temporary root for intermediate merge results
+        if tmp_root is not None:
+            base_tmp_root = tmp_root
+        elif aggr_root is not None:
+            base_tmp_root = aggr_root.parent / f".{aggr_repo_id}__tmp"
+        else:
+            base_tmp_root = Path.cwd() / f".{aggr_repo_id}__tmp"
+        base_tmp_root.mkdir(parents=True, exist_ok=True)
+
+        current_repo_ids: list[str] = list(repo_ids)
+        # Always maintain a roots list aligned with repo_ids. Use None for Hub-backed inputs.
+        current_roots: list[Path | None] = list(roots) if roots is not None else [None] * len(repo_ids)
+
+        try:
+            level = 0
+            while len(current_repo_ids) > 1:
+                next_repo_ids: list[str] = []
+                next_roots: list[Path | None] = []
+                futures = []
+
+                with ThreadPoolExecutor(max_workers=num_workers) as executor:
+                    group_index = 0
+                    i = 0
+                    while i < len(current_repo_ids):
+                        group_repo_ids = current_repo_ids[i : i + 2]
+                        group_roots = current_roots[i : i + 2]
+
+                        if len(group_repo_ids) == 1:
+                            # Carry over singleton to next level
+                            next_repo_ids.append(group_repo_ids[0])
+                            next_roots.append(group_roots[0])
+                            i += 1
+                            continue
+
+                        out_repo_id = f"{aggr_repo_id}__reduce_l{level}_g{group_index}"
+                        out_root = base_tmp_root / f"reduce_l{level}_g{group_index}"
+
+                        futures.append(
+                            executor.submit(
+                                _aggregate_datasets,
+                                group_repo_ids,
+                                out_repo_id,
+                                group_roots,
+                                out_root,
+                                data_files_size_in_mb,
+                                video_files_size_in_mb,
+                                chunk_size,
+                            )
+                        )
+
+                        next_repo_ids.append(out_repo_id)
+                        next_roots.append(out_root)
+
+                        i += 2
+                        group_index += 1
+
+                    for f in as_completed(futures):
+                        # Bubble up any exception raised inside tasks
+                        f.result()
+
+                # Cleanup previous level temporary outputs that won't be used again
+                base_resolved = base_tmp_root.resolve()
+                keep_set = {nr.resolve() for nr in next_roots if nr is not None}
+                for prev_root in current_roots:
+                    if prev_root is None:
+                        continue
+                    # Suppress per-iteration to keep cleaning other roots even if one fails
+                    with contextlib.suppress(Exception):
+                        pr = prev_root.resolve()
+                        if pr not in keep_set and base_resolved in pr.parents:
+                            shutil.rmtree(prev_root, ignore_errors=True)
+
+                current_repo_ids = next_repo_ids
+                current_roots = next_roots  # aligned list of Path|None after first level
+                level += 1
+
+            # Final copy/aggregation into the desired output
+            _aggregate_datasets(
+                repo_ids=current_repo_ids,
+                aggr_repo_id=aggr_repo_id,
+                roots=current_roots,
+                aggr_root=aggr_root,
+                data_files_size_in_mb=data_files_size_in_mb,
+                video_files_size_in_mb=video_files_size_in_mb,
+                chunk_size=chunk_size,
+            )
+        finally:
+            # Remove all temporary reduction artifacts
+            with contextlib.suppress(Exception):
+                shutil.rmtree(base_tmp_root, ignore_errors=True)
+
+        logging.info("Aggregation complete.")
+        return
+
+
 def aggregate_videos(src_meta, dst_meta, videos_idx, video_files_size_in_mb, chunk_size):
     """Aggregates video chunks from a source dataset into the destination dataset.
 
@@ -366,6 +535,9 @@ def aggregate_data(src_meta, dst_meta, data_idx, data_files_size_in_mb, chunk_si
 
     unique_chunk_file_ids = sorted(unique_chunk_file_ids)
 
+    # Map source (chunk,file) -> destination (chunk,file) actually used during write
+    src_to_dst_file: dict[tuple[int, int], tuple[int, int]] = {}
+
     for src_chunk_idx, src_file_idx in unique_chunk_file_ids:
         src_path = src_meta.root / DEFAULT_DATA_PATH.format(
             chunk_index=src_chunk_idx, file_index=src_file_idx
@@ -373,7 +545,7 @@ def aggregate_data(src_meta, dst_meta, data_idx, data_files_size_in_mb, chunk_si
         df = pd.read_parquet(src_path)
         df = update_data_df(df, src_meta, dst_meta)
 
-        data_idx = append_or_create_parquet_file(
+        data_idx, used_chunk, used_file = append_or_create_parquet_file(
             df,
             src_path,
             data_idx,
@@ -383,11 +555,12 @@ def aggregate_data(src_meta, dst_meta, data_idx, data_files_size_in_mb, chunk_si
             contains_images=len(dst_meta.image_keys) > 0,
             aggr_root=dst_meta.root,
         )
+        src_to_dst_file[(src_chunk_idx, src_file_idx)] = (used_chunk, used_file)
 
-    return data_idx
+    return data_idx, src_to_dst_file
 
 
-def aggregate_metadata(src_meta, dst_meta, meta_idx, data_idx, videos_idx):
+def aggregate_metadata(src_meta, dst_meta, meta_idx, data_idx, data_file_map, videos_idx):
     """Aggregates metadata from a source dataset into the destination dataset.
 
     Reads source metadata files, updates all indices and timestamps,
@@ -421,10 +594,11 @@ def aggregate_metadata(src_meta, dst_meta, meta_idx, data_idx, videos_idx):
             dst_meta,
             meta_idx,
             data_idx,
+            data_file_map,
             videos_idx,
         )
 
-        meta_idx = append_or_create_parquet_file(
+        meta_idx, _m_used_chunk, _m_used_file = append_or_create_parquet_file(
             df,
             src_path,
             meta_idx,
@@ -478,7 +652,7 @@ def append_or_create_parquet_file(
             to_parquet_with_hf_images(df, dst_path)
         else:
             df.to_parquet(dst_path)
-        return idx
+        return idx, idx["chunk"], idx["file"]
 
     src_size = get_parquet_file_size_in_mb(src_path)
     dst_size = get_parquet_file_size_in_mb(dst_path)
@@ -489,17 +663,19 @@ def append_or_create_parquet_file(
         new_path.parent.mkdir(parents=True, exist_ok=True)
         final_df = df
         target_path = new_path
+        used_chunk, used_file = idx["chunk"], idx["file"]
     else:
         existing_df = pd.read_parquet(dst_path)
         final_df = pd.concat([existing_df, df], ignore_index=True)
         target_path = dst_path
+        used_chunk, used_file = idx["chunk"], idx["file"]
 
     if contains_images:
         to_parquet_with_hf_images(final_df, target_path)
     else:
         final_df.to_parquet(target_path)
 
-    return idx
+    return idx, used_chunk, used_file
 
 
 def finalize_aggregation(aggr_meta, all_metadata):

src/lerobot/datasets/dataset_tools.py

@@ -234,6 +234,7 @@ def merge_datasets(
     datasets: list[LeRobotDataset],
     output_repo_id: str,
     output_dir: str | Path | None = None,
+    num_workers: int | None = None,
 ) -> LeRobotDataset:
     """Merge multiple LeRobotDatasets into a single dataset.
 
@@ -257,6 +258,7 @@ def merge_datasets(
         aggr_repo_id=output_repo_id,
         roots=roots,
         aggr_root=output_dir,
+        num_workers=num_workers,
     )
 
     merged_dataset = LeRobotDataset(
@@ -329,7 +331,7 @@ def modify_features(
 
     if repo_id is None:
         repo_id = f"{dataset.repo_id}_modified"
-    output_dir = Path(output_dir) if output_dir is not None else HF_LEROBOT_HOME / repo_id
+    output_dir = Path(output_dir, exists_ok=True) if output_dir is not None else HF_LEROBOT_HOME / repo_id
 
     new_features = dataset.meta.features.copy()
 

src/lerobot/datasets/lerobot_dataset.py

@@ -940,11 +940,26 @@ class LeRobotDataset(torch.utils.data.Dataset):
         return query_timestamps
 
     def _query_hf_dataset(self, query_indices: dict[str, list[int]]) -> dict:
-        return {
+        """
-            key: torch.stack(self.hf_dataset[q_idx][key])
+        Query dataset for indices across keys, skipping video keys.
-            for key, q_idx in query_indices.items()
+
-            if key not in self.meta.video_keys
+        Tries column-first [key][indices] for speed, falls back to row-first.
-        }
+
+        Args:
+            query_indices: Dict mapping keys to index lists to retrieve
+
+        Returns:
+            Dict with stacked tensors of queried data (video keys excluded)
+        """
+        result: dict = {}
+        for key, q_idx in query_indices.items():
+            if key in self.meta.video_keys:
+                continue
+            try:
+                result[key] = torch.stack(self.hf_dataset[key][q_idx])
+            except (KeyError, TypeError, IndexError):
+                result[key] = torch.stack(self.hf_dataset[q_idx][key])
+        return result
 
     def _query_videos(self, query_timestamps: dict[str, list[float]], ep_idx: int) -> dict[str, torch.Tensor]:
         """Note: When using data workers (e.g. DataLoader with num_workers>0), do not call this function

src/lerobot/scripts/lerobot_edit_dataset.py

@@ -103,6 +103,7 @@ class SplitConfig:
 class MergeConfig:
     type: str = "merge"
     repo_ids: list[str] | None = None
+    num_workers: int | None = None
 
 
 @dataclass
@@ -215,6 +216,7 @@ def handle_merge(cfg: EditDatasetConfig) -> None:
         datasets,
         output_repo_id=cfg.repo_id,
         output_dir=output_dir,
+        num_workers=cfg.operation.num_workers,
     )
 
     logging.info(f"Merged dataset saved to {output_dir}")