splitcheck

Detect rows that leak between dataset splits, and fail CI when they do.

A row that appears in both train and test inflates every metric and is easy to introduce: a careless concat, a re-export, a duplicated record. splitcheck compares your splits and reports how much of one appears in another, both as exact matches and after normalization, so cosmetic differences do not hide a leak.

$ splitcheck check train.csv test.csv --on text --max-leakage 0.0
target  in source  exact  normalized  leakage
test    train          1           3     2.1%
splitcheck: worst leakage 2.1%

Install

$ pip install splitcheck                 # from PyPI, once released
$ pip install git+https://github.com/jmweb-org/splitcheck   # latest, available now

Reads CSV, Parquet, JSON Lines, and plain text (one row per line) through polars.

Usage

$ splitcheck check train.csv test.csv                 # compare whole rows
$ splitcheck check train.csv val.csv test.csv         # all pairs at once
$ splitcheck check train.csv test.csv --on text       # compare one column
$ splitcheck check train.csv test.csv --json          # machine-readable
$ splitcheck check train.csv test.csv --max-leakage 0.01   # allow 1%
$ splitcheck check train.csv test.csv --no-check      # report without failing

By default any leakage fails the command (--max-leakage 0.0). Raise the limit to tolerate a known, small overlap.

In CI

- run: splitcheck check data/train.parquet data/test.parquet --on text

How matching works

Each row is reduced to a string: a single column with --on, or the whole row joined tab-separated. Two matches are reported:

• exact: identical strings.
• normalized: equal after case folding, punctuation removal and whitespace collapsing, which catches the same example re-saved with cosmetic changes.

Leakage is the fraction of the target split (for example test) whose rows also appear in a source split (for example train). Pairs are checked in both directions and sorted worst-first.

Exit codes

Code	Meaning
0	Checked; leakage within the limit (or --no-check)
1	Leakage exceeded --max-leakage
2	Fewer than two files, or a file was missing or unsupported

Scope

Matching is exact or normalized-exact, not fuzzy. Near-duplicates that differ in wording (paraphrases) are not caught; see the issues for planned MinHash-based fuzzy matching.

License

MIT. See LICENSE.