tktkt 2025.12.1

Tokeniser toolkit: a collection of Pythonic subword tokenisers and text preprocessing tools.

TkTkT: the ToKeniser ToolKiT

A collection of Pythonic subword tokenisers and text preprocessing tools, with full backwards- and forwards-compatibility with HuggingFace tokenizers. One package to rule them all.

Quick navigation:

• Installation
• Features
• Examples
• Architecture

Features

Supported tokenisers

All subword tokenisers are defined under tktkt.models. Many of these can be instantiated without much background knowledge using the factory classes in tktkt.factories. Also, any HuggingFace tokeniser can be wrapped into a TkTkT tokeniser, and any TkTkT tokeniser can be wrapped into a HuggingFace tokeniser.

Currently, the package implements:

• Byte-pair encoding (BPE) tokenisers:
  • Classical BPE (Sennrich et al., 2016), with added support for any word boundary marker (Ġ, _, </w>, ...) and n-ary merges (byte-tuple encoding, BTE).
  • BPE-dropout (Provilkov et al., 2020)
  • BPE-knockout (Bauwens & Delobelle, 2024)
  • PickyBPE (Chizhov et al., 2024)
  • ScaffoldBPE (Lian et al., 2025)
  • TrimmedBPE (Cognetta et al., 2024)
  • Other experimental variants I implemented just for fun:
    • BPE-breakdown: BPE which starts randomly undoing merges after it finishes deterministically, similar to StochasTok.
    • Non-geometric BPE-dropout: BPE-dropout, but rather than picking merges geometrically, picks them uniformly.
    • EnsuredBPE: BPE where the last merges have been replaced by the merges necessary to ensure that a given list of strings is in the vocabulary.
    • ShuffledBPE: BPE but with merge priorities shuffled, although types are never shuffled to a priority before the ancestors in their merge tree.
• Unigram language model (ULM), dubbed KudoPiece in TkTkT (Kudo, 2018):
  • Wrapper around the SentencePiece package, or
  • Native implementation in TkTkT
• Greedy tokenisers:
  • MaxMatch (Hiraoka, 2022), a.k.a. left-to-right greedy tokenisation, and also right-to-left (Bauwens, 2023 and later Uzan et al., 2024)
  • FLOTA (Hofmann et al., 2022), i.e. random-access longest-first tokenisation.
  • Other experimental variants:
    • Last-BPE-first: random-access youngest-first tokenisation (specifically for BPE vocabularies).
    • Left-to-right-to-left greedy: L2R2L_Greedy
• GRaMPa (Bauwens et al., 2025): randomised segmentation constrained by a vocabulary.
• SaGe (Yehezkel & Pinter, 2023) vocabularisation.
• Derivative leverager (DeL) (Hofmann et al., 2021), both training and segmentation.
• Other, less interesting tokenisers:
  • Character/byte N-grams.
  • Lempel-Ziv-Welch (LZW) as a tokeniser (Zouhar et al., 2023).

Currently work in progress:

• Morfessor family
• VOLT

Multiplexing

TkTkT is the only package that supports multiplexing multiple tokenisers into one big tokeniser that alternates between each of them. There are multiplexers that do this deterministically (e.g. choosing the tokeniser that compresses the input the most) or stochastically (e.g. choosing among a set of tokenisers uniformly).

Evaluation metrics

TkTkT currently supports the following intrinsic tokeniser evaluation metrics:

• Fertility statistics: how many tokens the tokeniser produces per word, and how many segmentations its vocabulary could produce in theory.
• Morphological boundary recognition: using the tokeniser as a binary classifier for whether two morphemes meet at each position in a word.
• Information-theoretic measures, including Rényi entropy and Rényi efficiency.
• Window-based metrics like MATTR.
• Bigram metrics to quantify the richness of token contexts, like accessor variety.
• Comparisons between two tokenisers: how much they tokenise words exactly the same, and how much their split points overlap.

Preprocessing

TkTkT has a rich set of text mappings and pretokenisers that preprocess text before it is tokenised, including support for stochastic perturbation. Unlike other libraries, preprocessors are objects, not regular expressions. This allows much more powerful processing than regex, whilst being more easy to read. See if you can understand this arguably complicated transformation:

from tktkt.preparation.splitters import *
from tktkt.preparation.mappers import PseudoByteMapping
from tktkt.factories.preprocessing import RobertaSpaceMarker


class ExamplePretokeniser(PretokeniserSequence):
    def __init__(self):
        super().__init__([
            IsolatePunctuation(HyphenMode.EXCLUDED, protect_apostrophes_without_spaces=True),
            OnWhitespace(destructive=True),
            IsolateEnglishContractions(do_nt=True),

            MapperAsPretokeniser(PseudoByteMapping()),
            AddWordBoundary(RobertaSpaceMarker),

            IsolateDigits(),
            IsolatePunctuation(HyphenMode.ONLY)
        ])

TkTkT also comes with language-specific pretokenisation like Japanese word segmentation and Thai word segmentation.

Visualisers

The following tokenisation procedures can be visualised:

• BPE/BTE: the final merge tree (in regular LaTeX), as well as an animated progression of the merges (in LaTeX Beamer).

Architecture

Main interfaces

The goal of TkTkT is to provide a straightforward Pythonic interface for everything-tokenisation, and to be as object-oriented as possible. The main interfaces are found under tktkt.interfaces.

Inference

Fundamentally, all tokenisers are a Tokeniser that have a Preprocessor.

• The Tokeniser class has two important methods:

  • .tokenise(pretoken: str) -> List[str]: segments a string as-is into parts.
  • .prepareAndTokenise(text: str) -> List[str]: applies the tokeniser's preprocessor and then tokenises each pre-token separately.

• The Preprocessor class is a pipeline of three components:

  1. a non-invertible text mapping
  2. an invertible text mapping
  3. a pretokeniser that splits strings into smaller strings.

To map tokens (string segments) to identifiers (integers) for indexing into an embedding matrix, this interface is extended to the TokeniserWithVocabulary. This class makes use of a Vocab object which keeps types used by the tokeniser separated from the specials used by a downstream language model. This prevents token injection attacks, which HuggingFace transformers/tokenizers is vulnerable to (see the bottom of this README).

Training

To learn the parameters of a Tokeniser (e.g. BPE merges), there is the Vocabulariser class. It can learn from word-count files or from corpora of sentences. It takes a Preprocessor exactly like Tokeniser, except Vocabulariser is for training the tokeniser (vocabularisation) and Tokeniser is for inference (segmentation).

Loading

To make it easier to load the results of a vocabularisation run from storage back into Python, there are Deserialisers to do this for you.

For ease-of-use, many Tokeniser classes have a TokeniserFactory defined for them that simplify the instantiation process. Often, a TokeniserFactory will take a Deserialiser to provide it any files.

Submodules

The packages is divided into the following submodules:

• tktkt.interfaces: contains the main parent classes from which all other classes derive.
  • The most important classes are TextMapper, Pretokeniser, Preprocessor, Vocabulariser, Tokeniser, Deserialiser, and TokeniserFactory.
• tktkt.preparation: contains all the text preprocessing tools.
• tktkt.models: contains all the tokenisation (i.e. vocabularisation and/or segmentation) algorithms.
• tktkt.evaluation: contains procedures with which to quantify a Tokeniser through inference.
• tktkt.factories: contains a bunch of pre-defined constructor calls, for both vocabularies and tokenisers:
  • tktkt.factories.deserialisation: contains classes that load the files for specific tokenisers.
  • tktkt.factories.tokenisers: contains tokeniser factories.
  • tktkt.factories.preprocessing: contains a bunch of pre-defined preprocessors so you don't have to. Check out the ModernEnglishPreprocessor, for example.
  • tktkt.factories.evaluation: contains pre-built tokeniser evaluation pipelines.
• tktkt.wrappers: contains classes that wrap around existing tokenisers to equip them with more features.
  • tktkt.wrappers.multiplexing: alternate between multiple tokenisers within the same sentence.
  • tktkt.wrappers.hashingvocab: add a string-to-integer mapping to a Tokeniser that can produce any substring, turning it into a TokeniserWithFiniteIdRange.
• tktkt.visualisation: contains procedures to generate explanatory LaTeX code about some models.
• tktkt.util: contains tools peripheral to tokenisation, like string formatting, combinatoric calculations, iterable functions, timing, etc...

Installation

Simply run

pip install "tktkt[github] @ git+https://github.com/bauwenst/TkTkT"

where you should leave out the [github] suffix only if you have editable installations of any of my other packages, like bpe_knockout (but you probably don't).

Examples

HuggingFace compatibility

In the example below, a BPE tokeniser is loaded from the HuggingFace hub as a PreTrainedTokenizerFast and converted into a TkTkT Tokeniser object. Then, this object is itself converted into a HuggingFace PreTrainedTokenizer again.

# Backwards-compatibility:
from transformers import AutoTokenizer
from tktkt.models.huggingface.wrapper import HuggingFaceTokeniser

hf_roberta = AutoTokenizer.from_pretrained("roberta-base")
tktkt_roberta = HuggingFaceTokeniser(hf_roberta)

###
sentence = " That's so supercalifragilisticexpialidocious, Günther!"
print("Full tokenisation pipeline:")
print("\tHF Tk:", hf_roberta.tokenize(sentence))  # Note the lack of autocompletion on this.
print("\tTkTkT:", tktkt_roberta.prepareAndTokenise(sentence))
print("Only the preprocessing:")
print("\tTkTkT:", tktkt_roberta.preprocessor.do(sentence))
###

# Forwards-compatibility:
from tktkt.interfaces.huggingface import TktktToHuggingFace

hf_tktkt_roberta = TktktToHuggingFace(tktkt_roberta, specials_from=hf_roberta)
print(hf_tktkt_roberta.tokenize(sentence))

Training and instantiating BPE

Here's a minimal working example to train a BPE tokeniser on the first 100 000 examples of an English Wikipedia dataset:

from datasets import load_dataset
from tktkt.factories.preprocessing import ModernEnglishPreprocessor, KudoSpaceMarker
from tktkt.models.bpe.vocabularisation import BPEVocabulariser

corpus        = load_dataset("Salesforce/wikitext", "wikitext-103-raw-v1", split="train", streaming=True).take(100_000)
preprocessor  = ModernEnglishPreprocessor(marker=KudoSpaceMarker)
vocabulariser = BPEVocabulariser(preprocessor=preprocessor, vocab_size=32_768)
bpe_folder = vocabulariser.vocabulariseFromHf(corpus, text_field="text")

That's just 7 lines of code to get a tokeniser from a corpus! To load the result into a HuggingFace-accelerated tokeniser, we can call

from tktkt.models.huggingface.bpe import HuggingFaceBPETokeniser

tokeniser = HuggingFaceBPETokeniser(
    preprocessor=preprocessor,
    vocab=vocabulariser.load(bpe_folder), 
    merges=vocabulariser.loadMerges(bpe_folder)
)

or, if we have made the results available in a Deserialiser, we can use a TokeniserFactory to do this for us in a one-liner. I once trained BPE across the first 3 million examples in SlimPajama, and thus we can run:

from tktkt.factories.deserialisation import BPE32ki_SlimPajama3M
from tktkt.factories.tokenisers import Factory_BPE

tokeniser = Factory_BPE(files=BPE32ki_SlimPajama3M()).buildTokeniser()

Note that the preprocessor comes with the deserialiser, so the factory doesn't require that you specify it.

Training and instantiating ULM (a.k.a. KudoPiece)

Let's now say you want to train and load an English ULM tokeniser. You are, of course, scared of the sentencepiece library because its Python interface is a thin wrapper around a command-line call, not allowing autocompletion in your IDE. In TkTkT, you would proceed as follows (note that ULM is called "KudoPiece" in TkTkT because many tokenisers are based on a language model of unigrams).

First we instantiate a preprocessor, and call the trainer with relevant training arguments:

from tktkt.factories.preprocessing import ModernEnglishPreprocessor_SentencePieceCompatible, BoundaryMarkerLocation
from tktkt.models.kudopiece.vocabularisation import *

### Your data iterator goes here.
sentence_corpus: Iterable[str] = ...
###

preprocessor = ModernEnglishPreprocessor_SentencePieceCompatible(
    marker_location=BoundaryMarkerLocation.START
)

trainer = KudoPieceVocabulariser(
    preprocessor=preprocessor,
    final_vocab_size=40_000,
    arguments=KudoPieceArguments(character_coverage=0.9995),
    file_stem="tutorial"
)
model_path = trainer.vocabulariseFromStringIterable(sentence_corpus)

Once the final model is stored to disk, we can load it as an object (and give it a basic preprocessor). Note that all models are stored under tktkt.paths.TkTkTPaths.pathToModels().

from tktkt.models.kudopiece.segmentation import KudoPieceTokeniser

# # If you need to recover the path:
# from tktkt.paths import TkTkTPaths
# model_path = TkTkTPaths.pathToModels() / "kudopiece" / "tutorial_xxxx-yy-zz_aa-bb-cc.model"

tokeniser = KudoPieceTokeniser(preprocessor=preprocessor, model_file=model_path)

print(tokeniser.prepareAndTokenise("Hello there, my good friend!"))

Custom preprocessing

TkTkT preprocesses text into pretokens not with a regular expression, but with a sequence of Python objects that can perform any operation they want on the current pretokens. It is hence strictly more expressive than regex-based pretokenisation. For example:

from tktkt.factories.preprocessing import *

toy_preprocessor = Preprocessor(
    Lowercaser(),
    Replace("!", "."),
    PretokeniserSequence([
        OnWhitespace(),
        IsolatePunctuation(),
        AddWordBoundary(KudoSpaceMarker)
    ])
)

print(toy_preprocessor.do("This example will be preprocessed (even without a tokeniser)!"))

This can then be used to instantiate any TkTkT tokeniser, whose functionality is decoupled from the preprocessor. For example:

from tktkt.models.greedy.directional import L2R_Greedy, Vocab
from tktkt.factories.specials import NoSpecials

tokeniser = L2R_Greedy(
    preprocessor=toy_preprocessor,
    vocab=Vocab(
        ["a", "b", "c", "d", "ab", "ba", ".", ",", "▁"],
        specials=NoSpecials(),
        unk_id=0
    )
)

print(tokeniser.prepareAndTokenise("A bad cab, ABBA!"))
print(tokeniser.tokenise("abc."))

There are many more preprocessing classes available, some pre-made. Check out the ModernEnglishPreprocessor for typical modern use-cases.

Why does this package exist if we have HuggingFace tokenizers?

First of all, note again that TkTkT has backwards compatibility with HuggingFace tokenizers. There are wrapper classes for tokenisers under tktkt.models.huggingface and for normalisers/pretokenisers under tktkt.preparation.huggingface.

Here's a non-exhaustive list of reasons:

1. The HuggingFace tokenizers package has horrifically un(der)documented Python interfaces. Some classes even accept arguments that aren't in their signature.
2. The tokenizers package is implemented in Rust and hence there is no possibility of inspecting implementations in any Python IDE. And given that the implementations are buggy, this is a massive problem.
3. The transformers package forces special tokens (<|endoftext|>, [CLS], [SEP], ...) to be treated as if they are user input. That's a security vulnerability.
   • Special tokens should never be treated like text. They should be seen as IDs without a name. They are purely for adding extra embedding vectors to the input of a downstream language model.
   • Yet, in the PreTrainedTokenizerBase class, specials must be declared using only a string with no identifier, and the point at which these strings receive their identifier is when they are run through the exact same method that converts the tokens from user input to identifiers.
4. The tokenizers interface does not allow separating preprocessing from the actual tokenisation algorithm.
   • The PreTrainedTokenizerBase class, from which the "slow" (Pythonic) PreTrainedTokenizer and "fast" (Rustic) PreTrainedTokenizerFast classes both inherit, only declares an end-to-end .tokenize() method (equivalent to TkTkT's .prepareAndTokenise()). The interface for these subclasses is different enough that both lack features of the other:
     • Whereas PreTrainedTokenizer does declare a ._tokenize() (equivalent to TkTkT's .tokenise()), I challenge you to find the equivalent for PreTrainedTokenizerFast. Best you'll find is .backend_tokenizer.model.tokenize(), which outputs unusable objects of class tokenizers.Token.
     • Whereas PreTrainedTokenizerFast has fields .backend_tokenizer.pre_tokenizer and .backend_tokenizer.normalizer (untyped of course, so you can't get autocompletion on their methods unless you manually assign them to a variable and annotate it yourself), PreTrainedTokenizer has no access to a pretokeniser. Preprocessing has to be defined inside ._tokenize(), which means you're doing two steps of preprocessing (one inside .tokenize() and one inside ._tokenize()) making this ._tokenize() no longer equivalent to TkTkT's .tokenise().
     • For PreTrainedTokenizerFast, the .backend_tokenizer.pre_tokenizer and .backend_tokenizer.normalizer fields can both be None, rather than an identity transform like in TkTkT, meaning you always have to check if they exist. Also funny: even when they are not None, you can't check if they exist with a simple if t.backend_tokenizer.normalizer: ... because somehow that's always False.
   • Also, the PreTrainedTokenizerBase interface is not defined with @abstractmethod but with an ever-increasing amount of raise NotImplementedError methods. In other words: it's hard to know which methods need to be implemented and there's no enforcement mechanism to ensure everything has been implemented.
5. The tokenizers.pre_tokenizers submodule has technical debt that can't be patched. Some examples:
   • The mapping from Unicode codepoints to UTF-8 bytes, as first used in GPT-2, is only implemented in the ByteLevel pretokeniser. Yet, it is concerned with more than this, since it splits on spaces and punctuation (optionally prefixed by a space) before applying the mapping. This is wrong for at least three reasons:
     • Users of the byte mapping don't necessary want the string to be split;
     • It synonymises prefixed spaces (converted to Ġ) with start-of-word boundaries whilst actually all words (even those directly preceded by punctuation) should be marked with such a boundary;
     • It assumes that such boundaries should always be at the start of a word.
   • The GPT-2 convention of having a word boundary at the start of (almost) all words is hardcoded throughout transformers and tokenizers (with options that commonly look like add_prefix_space) even though the original BPE paper used word boundaries at the end of words (</w>). Only supporting the start-of-word convention is bad because this deteriorates downstream performance for e.g. Germanic languages, where a compound has its head at the end and hence it should be allowed to tokenise the head with the exact same tokens as it would be if it was isolated.
   • There is literally a normaliser class called Precompiled which is just one big object stored in base64 in the tokeniser config JSON. No access to it in Python, no interface, no description of what it does. A black box. Probably a holdover from adapting the sentencepiece package to HuggingFace, yet TkTkT doesn't do it that way.
6. Did you know that their RoBERTa BPE implementation removes the highest-priority merge from the tokeniser unless the merge file is preceded by a #version tag? This doesn't conform to the BPE standard, and almost cost me a paper.
7. In the little documentation that does exist (e.g. for WordPiece and KudoPiece), there are so many theoretical inaccuracies that we shouldn't even have confidence in anything that isn't a BPE tokeniser implemented by them. Their explanation for KudoPiece, an algorithm which itself was already poorly explained originally, is mathematically absurd.
8. They offer very few core models (basically only BPE and KudoPiece, which sentencepiece already offers and keeps much more updated) whilst there exist many more in the literature, and the likelihood that someone who knows the literature comes along to implement all of them in C++ is rather low.

There is also the pyonmttok package which has better design than tokenizers, but also sticks to BPE and KudoPiece.

Pronunciation

The acronym stands for ToKeniser ToolKiT and is supposed to be pronounced fast, like a beatboxer mimicking hi-hats (kind of like "tuh-kuh-tuh-kuh-ts" but as fast as you can). It is mandatory that you do this.

If you are Brazilian, you may pronounce it "tuca tuca" while playing the official TkTkT theme song (yes, the demented state of modern-day tokeniser implementations will leave you with an equally demented taste in music).