A linter is a program that runs on the source code of software and reports potentially problematic code to the user. Linters help enforce coding guidelines, from minor details like whitespace formatting or capitalization patterns for strings to preventing problematic coding patterns that can lead to security bugs.


Zulip does extensive linting of much of its source code, including Python/JavaScript/TypeScript files, HTML templates (Django/Handlebars), CSS files, JSON fixtures, Markdown documents, puppet manifests, and shell scripts.

For some files we simply check for small things like trailing whitespace, but for other files, we are quite thorough about checking semantic correctness.

Obviously, a large reason for linting code is to enforce the Zulip coding standards. But we also use the linters to prevent common coding errors.

We borrow some open source tools for much of our linting, and the links below will direct you to the official documentation for these projects.

Zulip also uses some home-grown code to perform tasks like validating indentation in template files, enforcing coding standards that are unique to Zulip, allowing certain errors from third party linters to pass through, and exempting legacy files from lint checks.

Running the linters

If you run ./tools/test-all, it will automatically run the linters. You can also run them individually or pass specific files:

./tools/lint web/src/compose.js
./tools/lint web/src/

./tools/lint has many useful options; you can read about them in its internal documentation using ./tools/lint --help. Of particular note are:

  • --fix: Several of our linters support automatically fixing basic issues; this option will ask tools/lint to run those.

  • --verbose: Provides detailed information on how to fix many common linter errors not covered by --fix.

  • --skip and --only: Only run certain linters.

  • -m: Only check modified files.

Finally, you can rely on our continuous integration setup to run linters for you, but it is good practice to run lint checks locally.


We provide a Git pre-commit hook that can automatically run tools/lint on just the files that changed (in a few 100ms) whenever you make a commit. This can save you a lot of time, by automatically detecting linter errors as you make them.

Note: The linters only check files that Git tracks. Remember to git add new files before running lint checks.

Our linting tools generally support the ability to lint files individually–with some caveats–and those options will be described later in this document.

General considerations

Once you have read the Zulip coding guidelines, you can be pretty confident that 99% of the code that you write will pass through the linters fine, as long as you are thorough about keeping your code clean. And, of course, for minor oversights, lint is your friend, not your foe.

Occasionally, our linters will complain about things that are more of an artifact of the linter limitations than any actual problem with your code. There is usually a mechanism where you can bypass the linter in extreme cases, but often it can be a simple matter of writing your code in a slightly different style to appease the linter. If you have problems getting something to lint, you can submit an unfinished PR and ask the reviewer to help you work through the lint problem, or you can find other people in the Zulip Community to help you.

Also, bear in mind that 100% of the lint code is open source, so if you find limitations in either the Zulip home-grown stuff or our third party tools, feedback will be highly appreciated.

Finally, one way to clean up your code is to thoroughly exercise it with tests. The Zulip test documentation describes our test system in detail.

Lint checks

Most of our lint checks get performed by ./tools/lint. These include the following checks:

  • Check Python code with ruff.

  • Check Python formatting with Black and isort.

  • Check JavaScript and TypeScript code with ESLint.

  • Check CSS, JavaScript, TypeScript, and YAML formatting with Prettier.

  • Check Python code for custom Zulip rules.

  • Check non-Python code for custom Zulip rules.

  • Check Puppet manifests with the Puppet validator.

  • Check HTML templates for matching tags and indentations.

  • Check CSS for parsability and formatting.

  • Check JavaScript code for addClass calls.

  • Running mypy to check static types in Python code. Our documentation on using mypy covers mypy in more detail.

  • Running tsc to compile TypeScript code. Our documentation on TypeScript covers TypeScript in more detail.

The rest of this document pertains to the checks that occur in ./tools/lint.


Zulip has a script called lint that lives in our “tools” directory. It is the workhorse of our linting system, although in some cases it dispatches the heavy lifting to other components such as ruff, eslint, and other home grown tools.

You can find the source code here.

In order for our entire lint suite to run in a timely fashion, the lint script performs several lint checks in parallel by forking out subprocesses.

Note that our project does custom regex-based checks on the code. The code for these types of checks mostly lives here.

Special options

You can use the -h option for lint to see its usage. One particular flag to take note of is the --modified flag, which enables you to only run lint checks against files that are modified in your Git repo. Most of the “sub-linters” respect this flag, but some will continue to process all the files. Generally, a good workflow is to run with --modified when you are iterating on the code, and then run without that option right before committing new code.

If you need to troubleshoot the linters, there is a --verbose-timing option that can give you clues about which linters may be running slow, for example.

Lint checks

The next part of this document describes the lint checks that we apply to various file types.

Generic source code checks

We check almost our entire codebase for trailing whitespace. Also, we disallow tab (\t) characters in all but two files.

We also have custom regex-based checks that apply to specific file types. For relatively minor files like Markdown files and JSON fixtures, this is the extent of our checking.

Finally, we’re checking line length in Python code (and hope to extend this to other parts of the codebase soon). You can use #ignorelinelength for special cases where a very long line makes sense (e.g. a link in a comment to an extremely long URL).

Python code

Our Python code is formatted using Black (using the options in the [] section of pyproject.toml) and isort (using the options in .isort.cfg). The lint script enforces this by running Black and isort in check mode, or in write mode with --fix.

The bulk of our Python linting gets outsourced to the “ruff” tool, which is configured in the tool.ruff section of pyproject.toml.

Zulip also has custom regex-based rules that it applies to Python code. Look for python_rules in the source code for lint. Note that we provide a mechanism to exclude certain lines of codes from these checks. Often, it is simply the case that our regex approach is too crude to correctly exonerate certain valid constructs. In other cases, the code that we exempt may be deemed not worthwhile to fix.

JavaScript code

We check our JavaScript code in a few different ways:

  • We run eslint.

  • We check code formatting with Prettier.

  • We perform custom Zulip regex checks on the code.

Puppet manifests

We use Puppet as our tool to manage configuration files, using Puppet “manifests.” To lint Puppet manifests, we use the “parser validate” option of Puppet.

HTML templates

Zulip uses two HTML templating systems:

Zulip has an internal tool that validates both types of templates for correct indentation and matching tags. You can find the code here:

We exempt some legacy files from indentation checks, but we are hoping to clean those files up eventually.


Zulip uses stylelint to lint its CSS; see our configuration for the rules we currently enforce.

Shell scripts

Zulip uses shellcheck to lint our shell scripts. We recommend the shellcheck gallery of bad code as a resource on how to not write bad shell.

Markdown, shell scripts, JSON fixtures

We mostly validate miscellaneous source files like .json, and .md files for whitespace issues.


If you want to help improve Zulip’s system for linting, here are some considerations.


We want our linters to be fast enough that most developers will feel comfortable running them in a pre-commit hook, so we run our linters in parallel and support incremental checks.


We try to catch as many common mistakes as possible, either via a linter or an automated test.


Our goal is to have most common style issues by caught by the linters, so new contributors to the codebase can efficiently fix produce code with correct style without needing to go back-and-forth with a reviewer.