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Summary

With batched=True, a Datasets mapping function receives batches rather than individual examples. A batch may produce more or fewer rows than it receives. [S1, S2]

Details

Within one returned batch, every output column must have the same length. Keeping an old column at its original length while adding a longer column makes an invalid table. Removing that old column or replacing it avoids the mismatch. The batch_size argument controls how many input examples reach each call. [S1, S2]

Claims

• id: C1 claim: "A mapping output batch may differ in length from its input." source: S1 evidence: "return a batch of size M"
• id: C2 claim: "All columns returned within one batch must have equal lengths." source: S1 evidence: "all values of the output dictionary should have the same length"
• id: C3 claim: "batched=True switches map to batch processing." source: S2 evidence: "Operate on batches by setting batched=True"

Sources

• [S1] Batch mapping. Hugging Face contributors. https://raw.githubusercontent.com/huggingface/datasets/main/docs/source/about_map_batch.mdx. Apache-2.0; license: https://raw.githubusercontent.com/huggingface/datasets/main/LICENSE. Accessed 2026-11-05.
• [S2] Process. Hugging Face contributors. https://raw.githubusercontent.com/huggingface/datasets/main/docs/source/process.mdx. Apache-2.0; license: https://raw.githubusercontent.com/huggingface/datasets/main/LICENSE. Accessed 2026-11-05.

Attribution and notices

This note is CC BY 4.0. Source material has been condensed and restated; short evidence excerpts retain their source license.

Hugging Face Datasets documentation is Apache-2.0. This note restates the documentation and includes only short evidence excerpts. See the accompanying source license.

@misc{iil_note_zebgf7a4,
  author = {{Librarian}},
  title = {{Batch mapping can change a dataset row count}},
  howpublished = {iinventedlight Library},
  url = {https://iinventedlight.com/library/notes/hf-datasets-batch-map/},
  note = {Permanent ID iil:note:zebgf7a4. License: CC-BY-4.0.},
}

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