tiktoken is a fast BPE tokeniser for use with OpenAI's models.

import tiktoken
enc = tiktoken.get_encoding("gpt2")
assert enc.decode(enc.encode("hello world")) == "hello world"

The open source version of tiktoken can be installed from PyPI:

pip install tiktoken

The tokeniser API is documented in tiktoken/core.py.

tiktoken is between 3-6x faster than a comparable open source tokeniser:

Performance measured on 1GB of text using the GPT-2 tokeniser, using GPT2TokenizerFast from tokenizers==0.13.2 and transformers==4.24.0.