Reviewing Zulip code

Code review is a key part of how Zulip does development! If you’ve been contributing to Zulip’s code, we’d love for you to do reviews. This is a guide to how. (With some thoughts for writing code too.)

Protocol for authors

When you send a PR, try to think of a good person to review it – outside of the handful of people who do a ton of reviews – and @-mention them with something like “@person, would you review this?”. Good choices include

  • someone based in your timezone or a nearby timezone

  • people working on similar things, or in a loosely related area

Alternatively, posting a message in #code-review on the Zulip development community server, would help in reaching out to a wider group of reviewers. Either way, please be patient and mindful of the fact that it isn’t possible to provide a quick reply always, but that the reviewer would get to it sooner or later. Lastly, ensuring the your PR passes CI and is organized into coherent commits would help save reviewers time, which could otherwise be used to dive right into reviewing the PR’s core functionality.

Responding to a review feedback

Once you’ve received a review and resolved any feedback, it’s critical to update the GitHub thread to reflect that. Best practices are to:

  • Make sure that CI passes and the PR is rebased onto recent main.

  • Post comments on each feedback thread explaining at least how you resolved the feedback, as well as any other useful information (problems encountered, reasoning for why you picked one of several options, a test you added to make sure the bug won’t recur, etc.).

  • Mark any resolved threads as “resolved” in the GitHub UI, if appropriate.

  • Post a summary comment in the main feed for the PR, explaining that this is ready for another review, and summarizing any changes from the previous version, details on how you tested the changes, new screenshots/etc. More detail is better than less, as long as you take the time to write clearly.

If you resolve the feedback, but the PR has merge conflicts, CI failures, or the most recent comment is the reviewer asking you to fix something, it’s very likely that a potential reviewer skimming your PR will assume it isn’t ready for review and move on to other work.

If you need help or think an open discussion topic requires more feedback or a more complex discussion, move the discussion to a topic in the Zulip development community server. Be sure to provide links from the GitHub PR to the conversation (and vice versa) so that it’s convenient to read both conversations together.

Principles of code review

Anyone can review

Anyone can do a code review – you don’t have to have a ton of experience, and you don’t have to have the power to ultimately merge the PR. If you

  • read the code, see if you understand what the change is doing and why, and ask questions if you don’t; or

  • fetch the code (for Zulip server code, tools/fetch-rebase-pull-request is super handy), play around with it in your dev environment, and say what you think about how the feature works

those are really helpful contributions.

Please do reviews

Doing code reviews is an important part of making the project grow. It’s also an important skill to develop for participating in open-source projects and working in the industry in general. If you’re contributing to Zulip and have been working in our code for a little while, we would love for some of your time contributing to come in the form of doing code reviews!

For students participating in Google Summer of Code or a similar program, we expect you to spend a chunk of your time each week (after the first couple of weeks as you’re getting going) doing code reviews.

Fast replies are key

For the author of a PR, getting feedback quickly is really important for making progress quickly and staying productive. That means that if you get @-mentioned on a PR with a request for you to review it, it helps the author a lot if you reply promptly.

A reply doesn’t even have to be a full review; if a PR is big or if you’re pressed for time, then just getting some kind of reply in quickly – initial thoughts, feedback on the general direction, or just saying you’re busy and when you’ll have time to look harder – is still really valuable for the author and for anyone else who might review the PR.

People in the Zulip project live and work in many timezones, and code reviewers also need focused chunks of time to write code and do other things, so an immediate reply isn’t always possible. But a good benchmark is to try to always reply within one workday, at least with a short initial reply, if you’re working regularly on Zulip. And sooner is better.

Things to look for

  • The CI build. The tests need to pass. One can investigate any failures and figure out what to fix by clicking on a red X next to the commit hash or the Detail links on a pull request. (Example: in #17584, click the red X before 49b10a3 to see the build jobs for that commit. You can see that there are 7 build jobs in total. All the 7 jobs run in GitHub Actions. You can see what caused the job to fail by clicking on the failed job. This will open up a page in the CI that has more details on why the job failed. For example this is the page of the Ubuntu 18.04 Bionic (Python 3.6, backend + frontend) job. See our docs on continuous integration to learn more.

  • Technical design. There are a lot of considerations here: security, migration paths/backwards compatibility, cost of new dependencies, interactions with features, speed of performance, API changes. Security is especially important and worth thinking about carefully with any changes to security-sensitive code like views.

  • User interface and visual design. If frontend changes are involved, the reviewer will check out the code, play with the new UI, and verify it for both quality and consistency with the rest of the Zulip UI. We highly encourage posting screenshots to save reviewers time in getting a feel for what the feature looks like – you’ll get a quicker response that way.

  • Error handling. The code should always check for invalid user input. User-facing error messages should be clear and when possible be actionable (it should be obvious to the user what they need to do in order to correct the problem).

  • Testing. The tests should validate that the feature works correctly, and specifically test for common error conditions, bad user input, and potential bugs that are likely for the type of change being made. Tests that exclude whole classes of potential bugs are preferred when possible (e.g., the common test suite between the Zulip server’s frontend and backend Markdown processors, or the GetEventsTest test for buggy race condition handling).

  • Translation. Make sure that the strings are marked for translation.

  • Clear function, argument, variable, and test names. Every new piece of Zulip code will be read many times by other developers, and future developers will grep for relevant terms when researching a problem, so it’s important that variable names communicate clearly the purpose of each piece of the codebase.

  • Duplicated code. Code duplication is a huge source of bugs in large projects and makes the codebase difficult to understand, so we avoid significant code duplication wherever possible. Sometimes avoiding code duplication involves some refactoring of existing code; if so, that should usually be done as its own series of commits (not squashed into other changes or left as a thing to do later). That series of commits can be in the same pull request as the feature that they support, and we recommend ordering the history of commits so that the refactoring comes before the feature. That way, it’s easy to merge the refactoring (and minimize risk of merge conflicts) if there are still user experience issues under discussion for the feature itself.

  • Completeness. For refactorings, verify that the changes are complete. Usually one can check that efficiently using git grep, and it’s worth it, as we very frequently find issues by doing so.

  • Documentation updates. If this changes how something works, does it update the documentation in a corresponding way? If it’s a new feature, is it documented, and documented in the right place?

  • Good comments. It’s often worth thinking about whether explanation in a commit message or pull request discussion should be included in a comment, /docs, or other documentation. But it’s better yet if verbose explanation isn’t needed. We prefer writing code that is readable without explanation over a heavily commented codebase using lots of clever tricks.

  • Coding style. See the Zulip code-style documentation for details. Our goal is to have as much of this as possible verified via the linters and tests, but there’s always going to be unusual forms of Python/JavaScript style that our tools don’t check for.

  • Clear commit messages. See the Zulip version control documentation for details on what we look for.

Zulip server

Some points specific to the Zulip server codebase:

  • Testing – Backend. We are trying to maintain ~100% test coverage on the backend, so backend changes should have negative tests for the various error conditions.

  • Testing – Frontend. If the feature involves frontend changes, there should be frontend tests. See the test writing documentation for more details.

  • mypy annotations. New functions should be annotated using mypy and existing annotations should be updated. Use of Any, ignore, and unparameterized containers should be limited to cases where a more precise type cannot be specified.


To make it easier to review pull requests, if you’re working in the Zulip server codebase, use our git tool tools/fetch-rebase-pull-request to check out a pull request locally and rebase it onto main.

If a pull request just needs a little fixing to make it mergeable, feel free to do that in a new commit, then push your branch to GitHub and mention the branch in a comment on the pull request. That’ll save the maintainer time and get the PR merged quicker.

Additional resources

We also strongly recommend reviewers to go through the following resources.