Fast tokenizers optimized for research and production. Rust-based implementation tokenizes 1GB in <20 seconds. Supports BPE, WordPiece, and Unigram algorithms. Train custom vocabularies, track alignments, handle padding/truncation. Integrates seamlessly with transformers. Use when you need high-performance tokenization or custom tokenizer training.
Named entity recognition (map predictions back to text)
Question answering (extract answer spans)
Token classification (align labels to original positions)
Integration with transformers
Load with AutoTokenizer
from transformers import AutoTokenizer
# AutoTokenizer automatically uses fast tokenizers
tokenizer = AutoTokenizer.from_pretrained("bert-base-uncased")
# Check if using fast tokenizer
print(tokenizer.is_fast) # True
# Access underlying tokenizers.Tokenizer
fast_tokenizer = tokenizer.backend_tokenizer
print(type(fast_tokenizer)) # <class 'tokenizers.Tokenizer'>
Convert custom tokenizer to transformers
from tokenizers import Tokenizer
from transformers import PreTrainedTokenizerFast
# Train custom tokenizer
tokenizer = Tokenizer(BPE())
# ... train tokenizer ...
tokenizer.save("my-tokenizer.json")
# Wrap for transformers
transformers_tokenizer = PreTrainedTokenizerFast(
tokenizer_file="my-tokenizer.json",
unk_token="[UNK]",
pad_token="[PAD]",
cls_token="[CLS]",
sep_token="[SEP]",
mask_token="[MASK]"
)
# Use like any transformers tokenizer
outputs = transformers_tokenizer(
"Hello world",
padding=True,
truncation=True,
max_length=512,
return_tensors="pt"
)
Common patterns
Train from iterator (large datasets)
from datasets import load_dataset
# Load dataset
dataset = load_dataset("wikitext", "wikitext-103-raw-v1", split="train")
# Create batch iterator
def batch_iterator(batch_size=1000):
for i in range(0, len(dataset), batch_size):
yield dataset[i:i + batch_size]["text"]
# Train tokenizer
tokenizer.train_from_iterator(
batch_iterator(),
trainer=trainer,
length=len(dataset) # For progress bar
)
Performance: Processes 1GB in ~10-20 minutes
Enable truncation and padding
# Enable truncation
tokenizer.enable_truncation(max_length=512)
# Enable padding
tokenizer.enable_padding(
pad_id=tokenizer.token_to_id("[PAD]"),
pad_token="[PAD]",
length=512 # Fixed length, or None for batch max
)
# Encode with both
output = tokenizer.encode("This is a long sentence that will be truncated...")
print(len(output.ids)) # 512
Multi-processing
from tokenizers import Tokenizer
from multiprocessing import Pool
# Load tokenizer
tokenizer = Tokenizer.from_file("tokenizer.json")
def encode_batch(texts):
return tokenizer.encode_batch(texts)
# Process large corpus in parallel
with Pool(8) as pool:
# Split corpus into chunks
chunk_size = 1000
chunks = [corpus[i:i+chunk_size] for i in range(0, len(corpus), chunk_size)]
# Encode in parallel
results = pool.map(encode_batch, chunks)
Speedup: 5-8× with 8 cores
Performance benchmarks
Training speed
Corpus Size
BPE (30k vocab)
WordPiece (30k)
Unigram (8k)
10 MB
15 sec
18 sec
25 sec
100 MB
1.5 min
2 min
4 min
1 GB
15 min
20 min
40 min
Hardware: 16-core CPU, tested on English Wikipedia
Tokenization speed
Implementation
1 GB corpus
Throughput
Pure Python
~20 minutes
~50 MB/min
HF Tokenizers
~15 seconds
~4 GB/min
Speedup
80×
80×
Test: English text, average sentence length 20 words
Memory usage
Task
Memory
Load tokenizer
~10 MB
Train BPE (30k vocab)
~200 MB
Encode 1M sentences
~500 MB
Supported models
Pre-trained tokenizers available via from_pretrained():
