grouptok trains a byte-level BPE tokenizer on sentence pairs. It then aligns the subwords of each pair with a neural word aligner and puts subwords that are often aligned into one group. Each token becomes a (group, member) pair:

 " house" -> (412, 0)      " maison" -> (412, 1)      " Haus" -> (412, 2)
 " king"  -> (977, 0)      " roi"    -> (977, 1)

A language model can then predict the group first and the member second, so translations share most of their parameters. The tokenizer itself is an ordinary BPE tokenizer (it saves a standard tokenizer.json, so it also loads in transformers), plus a groups.json that holds the grouping.

Install

pip install grouptok            # load and use a trained tokenizer (needs only `tokenizers`)
pip install "grouptok[train]"   # train one (adds torch and transformers for the aligner)
pip install "grouptok[hf]"      # GroupedTokenizer.to_hf() without the training dependencies

Quick start

from grouptok import GroupedTokenizer, TokenizerConfig

pairs = [("The king said unto the people", "Le roi dit au peuple"), ...]   # (one language, the other)
tok = GroupedTokenizer.train(pairs, TokenizerConfig(vocab_size=8192))
tok.save_pretrained("my-tokenizer")      # tokenizer.json, tokenizer_config.json, groups.json

tok = GroupedTokenizer.from_pretrained("my-tokenizer")
ids = tok.encode("The king", bos=True, eos=True)
tok.pairs(ids)                           # [(group, member), ...] for each token
tok.decode(ids)                          # 'The king'
tok.group_of(" king")                    # [' king', ' roi']
tok.readable_groups()[:5]                # groups with at least two members, as token strings
hf = tok.to_hf()                         # a transformers PreTrainedTokenizerFast

The grouping is a Grouping, a one-to-one map between token ids and (group, member) pairs, which a model needs for its embedding tables and heads:

g = tok.grouping
g.token_group, g.token_member            # per-token group and member ids (tuples of length vocab_size)
g.num_groups, g.max_group_size           # sizes of the group and member tables
g.token(group, member)                   # back to the token id

More than two languages

Pairs don't have to share a language pair: pool en–fr, en–de and fr–de pairs and train once. A token joins the group of whatever it is most often aligned with, so a group can hold " house", " maison" and " Haus".

The group size cap(max_group_size, default 8) bounds how many members a group can get.

from grouptok import GroupedTokenizer, GroupingConfig, TokenizerConfig

pairs = en_fr_pairs + en_de_pairs + fr_de_pairs
tok = GroupedTokenizer.train(pairs, TokenizerConfig(vocab_size=32000, grouping=GroupingConfig(max_group_size=8)))

Configuration

from grouptok import AlignerConfig, GroupingConfig, TokenizerConfig

TokenizerConfig(
    vocab_size=8192,
    special_tokens=("<|endoftext|>", "<|im_start|>", "<|im_end|>"),   # pad, bos, eos -> ids 0, 1, 2
    additional_tokens=("<think>", "</think>"),                        # more reserved tokens, never grouped
    chat_template=None,                                               # Jinja template saved to tokenizer_config.json
    grouping=GroupingConfig(max_group_size=8, min_link_count=5, min_dice=0.1),
    aligner=AlignerConfig(model="aneuraz/awesome-align-with-co", layer=8, threshold=1e-3,
                          batch_size=64, max_length=128, device=None, fp16=True),
)

Any Hugging Face encoder with a fast tokenizer can be the aligner (mBERT, XLM-R, awesome-align, ...). You can also pass your own object to GroupedTokenizer.train(..., aligner=...). It needs an encode(texts) method that returns the vectors [batch, length, dim] and the character spans [batch, length, 2] of its word pieces, with empty spans for special and padding positions.

Command line

grouptok train --pairs europarl.jsonl:en,fr opus.tsv --limit 200000 --out my-tokenizer --vocab-size 8192
grouptok inspect my-tokenizer --word " king" " house"
grouptok encode my-tokenizer "The king said"

--pairs takes JSONL files (the two sides are the --columns, or the first two text fields) or TSV files (one pair per line, split at the first tab). Write FILE:A,B to choose the fields of one file. grouptok train --help lists every option.

How it works

1. BPE. A byte-level BPE is trained on both sides of every pair. Reserved tokens come first.

2. Alignment. Each pair goes through the aligner. A BPE subword gets the mean vector of the aligner's word pieces that overlap it in characters. Two subwords are linked when each one's softmax over the other side's similarities exceeds threshold (awesome-align's rule).

3. Scores. The Dice score of two subwords is 2 * links / (occurrences(a) + occurrences(b)). Identical subwords on the two sides (names, numbers) are not linked.

4. Grouping. Links are visited by descending Dice, skipping pairs below min_link_count or min_dice:

   • if neither subword is grouped, they start a new group;
   • if one is, the other joins its group unless the group already has max_group_size members;
   • two existing groups are never merged.

   Every remaining token, including the reserved tokens, is a group of its own.

Files

save_pretrained writes:

• tokenizer.json: a standard tokenizers file;
• tokenizer_config.json: for transformers.AutoTokenizer;
• groups.json:

{"max_group_size": 8, "token_group": [0, 1, 2, 3, ...], "token_member": [0, 0, 0, 0, ...],
 "groups": {"412": ["Ġhouse", "Ġmaison", "ĠHaus"], ...}}

groups is only there for people reading the file. It lists the groups with several members as raw BPE tokens, where Ġ marks a leading space. Grouping.load reads token_group and token_member, and ignores any other keys.

Development

Clone the repo

git clone https://github.com/henryhale/grouptok.git
cd grouptok

Install dependencies

pip install torch --index-url https://download.pytorch.org/whl/cpu   # optional: CPU torch is enough for the tests
pip install -e ".[train,test]"
pytest

The tests run offline. They train on a 500-pair English–French sample in tests/data (kept in the repository, not in the package) with a stand-in aligner instead of a downloaded model.

License

Released under the Apache-2.0 license. See LICENSE for details.

© 2026-present Henry Hale