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Contributing to Mypy

Welcome! Mypy is a community project that aims to work for a wide range of Python users and Python codebases. If you're trying Mypy on your Python code, your experience and what you can contribute are important to the project's success.

Getting started, building, and testing

If you haven't already, take a look at the project's file and the Mypy documentation, and try adding type annotations to your file and type-checking it with Mypy.


If you've run into behavior in Mypy you don't understand, or you're having trouble working out a good way to apply it to your code, or you've found a bug or would like a feature it doesn't have, we want to hear from you!

Our main forum for discussion is the project's GitHub issue tracker. This is the right place to start a discussion of any of the above or most any other topic concerning the project.

For less formal discussion we have a chat room on Some Mypy core developers are almost always present; feel free to find us there and we're happy to chat. Substantive technical discussion will be directed to the issue tracker.

(We also have an IRC channel, #python-mypy on This is lightly used, we have mostly switched to the gitter room mentioned above.)

Code of Conduct

Everyone participating in the Mypy community, and in particular in our issue tracker, pull requests, and IRC channel, is expected to treat other people with respect and more generally to follow the guidelines articulated in the Python Community Code of Conduct.

First Time Contributors

Mypy appreciates your contribution! If you are interested in helping improve mypy, there are several ways to get started:

Submitting Changes

Even more excellent than a good bug report is a fix for a bug, or the implementation of a much-needed new feature. (*) We'd love to have your contributions.

(*) If your new feature will be a lot of work, we recommend talking to us early -- see below.

We use the usual GitHub pull-request flow, which may be familiar to you if you've contributed to other projects on GitHub. For the mechanics, see our git and GitHub workflow help page, or GitHub's own documentation.

Anyone interested in Mypy may review your code. One of the Mypy core developers will merge your pull request when they think it's ready. For every pull request, we aim to promptly either merge it or say why it's not yet ready; if you go a few days without a reply, please feel free to ping the thread by adding a new comment.

For a list of mypy core developers, see the file CREDITS.

Preparing Changes

Before you begin: if your change will be a significant amount of work to write, we highly recommend starting by opening an issue laying out what you want to do. That lets a conversation happen early in case other contributors disagree with what you'd like to do or have ideas that will help you do it.

The best pull requests are focused, clearly describe what they're for and why they're correct, and contain tests for whatever changes they make to the code's behavior. As a bonus these are easiest for someone to review, which helps your pull request get merged quickly! Standard advice about good pull requests for open-source projects applies; we have our own writeup of this advice.

See also our coding conventions -- which consist mainly of a reference to PEP 8 -- for the code you put in the pull request.

Also, do not squash your commits after you have submitted a pull request, as this erases context during review. We will squash commits when the pull request is merged.

You may also find other pages in the Mypy developer guide helpful in developing your change.

Core developer guidelines

Core developers should follow these rules when processing pull requests:

  • Always wait for tests to pass before merging PRs.
  • Use "Squash and merge" to merge PRs.
  • Delete branches for merged PRs (by core devs pushing to the main repo).
  • Edit the final commit message before merging to conform to the following style (we wish to have a clean git log output):
    • When merging a multi-commit PR make sure that the commit message doesn't contain the local history from the committer and the review history from the PR. Edit the message to only describe the end state of the PR.
    • Make sure there is a single newline at the end of the commit message. This way there is a single empty line between commits in git log output.
    • Split lines as needed so that the maximum line length of the commit message is under 80 characters, including the subject line.
    • Capitalize the subject and each paragraph.
    • Make sure that the subject of the commit message has no trailing dot.
    • Use the imperative mood in the subject line (e.g. "Fix typo in README").
    • If the PR fixes an issue, make sure something like "Fixes #xxx." occurs in the body of the message (not in the subject).
    • Use Markdown for formatting.

Issue-tracker conventions

We aim to reply to all new issues promptly. We'll assign a milestone to help us track which issues we intend to get to when, and may apply labels to carry some other information. Here's what our milestones and labels mean.

Task priority and sizing

We use GitHub "labels" (see our list) to roughly order what we want to do soon and less soon. There's two dimensions taken into account: priority (does it matter to our users) and size (how long will it take to complete).

Bugs that aren't a huge deal but do matter to users and don't seem like a lot of work to fix generally will be dealt with sooner; things that will take longer may go further out.

We are trying to keep the backlog at a manageable size, an issue that is unlikely to be acted upon in foreseeable future is going to be respectfully closed. This doesn't mean the issue is not important, but rather reflects the limits of the team.

The question label is for issue threads where a user is asking a question but it isn't yet clear that it represents something to actually change. We use the issue tracker as the preferred venue for such questions, even when they aren't literally issues, to keep down the number of distinct discussion venues anyone needs to track. These might evolve into a bug or feature request.

Issues without a priority or size haven't been triaged. We aim to triage all new issues promptly, but there are some issues from previous years that we haven't yet re-reviewed since adopting these conventions.

Other labels

  • needs discussion: This issue needs agreement on some kind of design before it makes sense to implement it, and it either doesn't yet have a design or doesn't yet have agreement on one.
  • feature, bug, crash, refactoring, documentation: These classify the user-facing impact of the change. Specifically "refactoring" means there should be no user-facing effect.
  • topic- labels group issues touching a similar aspect of the project, for example PEP 484 compatibility, a specific command-line option or dependency.