Bleach v2.0 released!

What is it?

Bleach is a Python library for sanitizing and linkifying text from untrusted sources for safe usage in HTML.

Bleach v2.0 released!

Bleach 2.0 is a massive rewrite. Bleach relies on the html5lib library. html5lib 0.99999999 (8 9s) changed the APIs that Bleach was using to sanitize text. As such, in order to support html5lib >= 0.99999999 (8 9s), I needed to rewrite Bleach.

Before embarking on the rewrite, I improved the tests and added a set of tests based on XSS example strings from the OWASP site. Spending quality time with tests before a rewrite or refactor is both illuminating (you get a better understanding of what the requirements are) and also immensely helpful (you know when your rewrite/refactor differs from the original). That was time well spent.

Given that I was doing a rewrite anyways, I decided to take this opportunity to break the Bleach API to make it more flexible and easier to use:

  • added Cleaner and Linkifier classes that you can create once and reuse to reduce redundant work--suggested in #125
  • created BleachSanitizerFilter which is now an html5lib filter that can be used anywhere you can use an html5lib filter
  • created LinkifyFilter as an html5lib filter that can be used anywhere you use an html5lib filter including as part of cleaning allowing you to clean and linkify in one pass--suggested in #46
  • changed arguments for attribute callables and linkify callbacks
  • and so on

During and after the rewrite, I improved the documentation converting all the examples to doctest format so they're testable and verifiable and adding examples where there weren't any. This uncovered bugs in the documentation and pointed out some annoyances with the new API.

As I rewrote and refactored code, I focused on making the code simpler and easier to maintain going forward and also documented the intentions so I and others can know what the code should be doing.

I also adjusted the internals to make it easier for users to extend, subclass, swap out and whatever else to adjust the functionality to meet their needs without making Bleach harder to maintain for me or less safe because of additional complexity.

For API-adjustment inspiration, I went through the Bleach issue tracker and tried to address every possible issue with this update: infinite loops, unintended behavior, inflexible APIs, suggested refactorings, features, bugs, etc.

The rewrite took a while. I tried to be meticulous because this is a security library and it's a complicated problem domain and I was working on my own during slow times on work projects. When working on one's own, you don't have benefit of review. Making sure to have good test coverage and waiting a day to self-review after posting a PR caught a lot of issues. I also go through the PR and add comments explaining why I did things to give context to future me. Those habits help a lot, but probably aren't as good as a code review by someone else.

Some stats

OMG! This blog post is so boring! Walls of text everywhere so far!

There were 61 commits between v1.5 and v2.0:

  • Vadim Kotov: 1
  • Alexandr N. Zamaraev: 2
  • me: 58

I closed out 22 issues--possibly some more.

The rewrite has the following git diff --shortstat:

64 files changed, 2330 insertions(+), 1128 deletions(-)

Lines of code for Bleach 1.5:

~/mozilla/bleach> cloc bleach/ tests/
      11 text files.
      11 unique files.
       0 files ignored. v 1.60  T=0.07 s (152.4 files/s, 25287.2 lines/s)
Language                     files          blank        comment           code
Python                          11            353            200           1272
SUM:                            11            353            200           1272

Lines of code for Bleach 2.0:

~/mozilla/bleach> cloc bleach/ tests/
      49 text files.
      49 unique files.
      36 files ignored. v 1.60  T=0.13 s (101.7 files/s, 20128.5 lines/s)
Language                     files          blank        comment           code
Python                          13            545            406           1621
SUM:                            13            545            406           1621

Some off-the-cuff performance benchmarks

I ran some timings between Bleach 1.5 and various uses of Bleach 2.0 on the Standup corpus.

Here's the results:

what? time to clean and linkify
Bleach 1.5 1m33s
Bleach 2.0 (no code changes) 41s
Bleach 2.0 (using Cleaner and Linker) 10s
Bleach 2.0 (clean and linkify--one pass) 7s

How'd I compute the timings?

  1. I'm using the Standup corpus which has 42000 status messages in it. Each status message is like a tweet--it's short, has some links, possibly has HTML in it, etc.
  2. I wrote a timing harness that goes through all those status messages and times how long it takes to clean and linkify the status message content, accumulates those timings and then returns the total time spent cleaning and linking.
  3. I ran that 10 times and took the median. The timing numbers were remarkably stable and there was only a few seconds difference between the high and low for all of the sets.
  4. I wrote the median number down in that table above.
  5. Then I'd adjust the code as specified in the table and run the timings again.

I have several observations/thoughts:

First, holy moly--1m33s to 7s is a HUGE performance improvement.

Second, just switching from Bleach 1.5 to 2.0 and making no code changes (in other words, keeping your calls as bleach.clean and bleach.linkify rather than using Cleaner and Linker and LinkifyFilter), gets you a lot. Depending on whether your have attribute filter callables and linkify callbacks, you may be able to just upgrade the libs and WIN!

Third, switching to reusing Cleaner and Linker also gets you a lot.

Fourth, your mileage may vary depending on the nature of your corpus. For example, Standup status messages are short so if your text fragments are larger, you may see more savings by clean-and-linkify in one pass because HTML parsing takes more time.

How to upgrade

Upgrading should be straight-forward.

Here's the minimal upgrade path:

  1. Update Bleach to 2.0 and html5lib to >= 0.99999999 (8 9s).
  2. If you're using attribute callables, you'll need to update them.
  3. If you're using linkify callbacks, you'll need to update them.
  4. Read through version 2.0 changes for any other backwards-incompatible changes that might affect you.
  5. Run your tests and see how it goes.


If you're using html5lib 1.0b8, then you have to explicitly upgrade the version. 1.0b8 is equivalent to html5lib 0.9999999 (7 9s) and that's not supported by Bleach 2.0.

You have to explicitly upgrade because pip will think that 1.0b8 comes after 0.99999999 (8 9s) and it doesn't. So it won't upgrade html5lib for you.

If you're doing 9s, make sure to upgrade to 0.99999999 (8 9s) or higher.

If you're doing 1.0bs, make sure to upgrade to 1.0b9 or higher.

If you want better performance:

  1. Switch to reusing bleach.sanitizer.Cleaner and bleach.linkifier.Linker.

If you have large text fragments:

  1. Switch to reusing bleach.sanitizer.Cleaner and set filters to include LinkifyFilter which lets you clean and linkify in one step.

Many thanks

Many thanks to James Socol (previous maintainer) for walking me through why things were the way they were.

Many thanks to Geoffrey Sneddon (html5lib maintainer) for answering questions, helping with problems I encountered and all his efforts on html5lib which is a huge library that he works on in his spare time for which he doesn't get anywhere near enough gratitude.

Many thanks to Lonnen (my manager) who heard me talk about html5lib zero point nine nine nine nine nine nine nine nine a bunch.

Also, many thanks to Mozilla for letting me work on this during slow periods of the projects I should be working on. A bunch of Mozilla sites use Bleach, but none of mine do.

Where to go for more

For more specifics on this release, see here:

Documentation and quickstart here:

Source code and issue tracker here:

Want to comment? Send an email to willkg at bluesock dot org. Include the url for the blog entry in your comment so I have some context as to what you're talking about.