---
title: Batch mapping can change a dataset row count
slug: hf-datasets-batch-map
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source: https://iinventedlight.com/library/notes/hf-datasets-batch-map.md
source_license: Apache-2.0
attribution: Original synthesis by iinventedlight, drafted by Librarian with AI assistance; based on the attributed sources
  below.
author: Librarian
client: Librarian
date: '2026-11-05'
checked_on: '2026-11-05'
as_of: '2026-11-05'
status: published
confidence: medium
category: coding-references
tags:
- hf
- datasets
- batch
untested: Reference note. No code or commands were run to validate behavior.
sources:
- id: S1
  title: Batch mapping
  publisher: Hugging Face contributors
  url: https://raw.githubusercontent.com/huggingface/datasets/main/docs/source/about_map_batch.mdx
  accessed: '2026-11-05'
  license: Apache-2.0
  license_url: https://raw.githubusercontent.com/huggingface/datasets/main/LICENSE
  license_verified: '2026-11-05'
  green: true
  attribution: Hugging Face contributors; Batch mapping; Apache-2.0
  snapshot: _fetched/hf-batch.txt
- id: S2
  title: Process
  publisher: Hugging Face contributors
  url: https://raw.githubusercontent.com/huggingface/datasets/main/docs/source/process.mdx
  accessed: '2026-11-05'
  license: Apache-2.0
  license_url: https://raw.githubusercontent.com/huggingface/datasets/main/LICENSE
  license_verified: '2026-11-05'
  green: true
  attribution: Hugging Face contributors; Process; Apache-2.0
  snapshot: _fetched/hf-process.txt
---
# Batch mapping can change a dataset row count

## 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.