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Notes for Julia Contributors

Hi! If you are new to the Julia community: welcome, and thanks for trying Julia. Please be sure to respect our community standards in all interactions.

If you are already familiar with Julia itself, this blog post by Katharine Hyatt on Making your first Julia pull request is a great way to get started.

Learning Julia

The learning page has a great list of resources for new and experienced users alike.

Before filing an issue

  • Reporting a potential bug? Please read the "How to file a bug report" section to make sure that all necessary information is included.

  • Contributing code? Be sure to review the contributor checklist for helpful tips on the tools we use to build Julia.

  • Library feature requests are generally not accepted on this issue tracker. New libraries should be developed as packages. Discuss ideas for libraries at the Julia Discourse forum. Doing so will often lead to pointers to existing projects and bring together collaborators with common interests.

Contributor Checklist

  • Create a GitHub account.

  • Fork Julia.

  • Build the software and libraries (the first time takes a while, but it's fast after that). Detailed build instructions are in the README. Julia depends on several external packages; most are automatically downloaded and installed, but are less frequently updated than Julia itself.

  • Keep Julia current. Julia is a fast-moving target, and many details of the language are still settling out. Keep the repository up-to-date and rebase work-in-progress frequently to make merges simpler.

  • Learn to use git, the version control system used by GitHub and the Julia project. Try a tutorial such as the one provided by GitHub.

  • Review discussions on the Julia Discourse forum.

  • For more detailed tips, read the submission guide below.

  • Relax and have fun!

How to file a bug report

A useful bug report filed as a GitHub issue provides information about how to reproduce the error.

  1. Before opening a new GitHub issue:
  • Try searching the existing issues or the Julia Discourse forum to see if someone else has already noticed the same problem.
  • Try some simple debugging techniques to help isolate the problem.
    • Try running the code with the debug build of Julia with make debug, which produces the usr/bin/julia-debug.
    • Consider running julia-debug with a debugger such as gdb or lldb. Obtaining even a simple backtrace is very useful.
    • If Julia segfaults, try following these debugging tips to help track down the specific origin of the bug.
  1. If the problem is caused by a Julia package rather than core Julia, file a bug report with the relevant package author rather than here.

  2. When filing a bug report, provide where possible:

  • The full error message, including the backtrace.
  • A minimal working example, i.e. the smallest chunk of code that triggers the error. Ideally, this should be code that can be pasted into a REPL or run from a source file. If the code is larger than (say) 50 lines, consider putting it in a gist.
  • The version of Julia as provided by the versioninfo() command. Occasionally, the longer output produced by versioninfo(verbose = true) may be useful also, especially if the issue is related to a specific package.
  1. When pasting code blocks or output, put triple backquotes (```) around the text so GitHub will format it nicely. Code statements should be surrounded by single backquotes (`). Be aware that the @ sign tags users on GitHub, so references to macros should always be in single backquotes. See GitHub's guide on Markdown for more formatting tricks.

Submitting contributions

Writing tests

There are never enough tests. Track code coverage at Codecov, and help improve it.

  1. Go visit

  2. Browse through the source files and find some untested functionality (highlighted in red) that you think you might be able to write a test for.

  3. Write a test that exercises this functionality---you can add your test to one of the existing files, or start a new one, whichever seems most appropriate to you. If you're adding a new test file, make sure you include it in the list of tests in test/choosetests.jl. may be helpful in explaining how the testing infrastructure works.

  4. Run make test-all to rebuild Julia and run your new test(s). If you had to fix a bug or add functionality in base, this will ensure that your test passes and that you have not introduced extraneous whitespace.

  5. Submit the test as a pull request (PR).

Code coverage shows functionality that still needs "proof of concept" tests. These are important, as are tests for tricky edge cases, such as converting between integer types when the number to convert is near the maximum of the range of one of the integer types. Even if a function already has some coverage on Codecov, it may still benefit from tests for edge cases.

Improving documentation

By contributing documentation to Julia, you are agreeing to release it under the MIT License.

Julia's documentation source files are stored in the doc/ directory and all docstrings are found in base/. Like everything else these can be modified using git. Documentation is built with Documenter.jl, which uses Markdown syntax. The HTML documentation can be built locally by running

make docs

from Julia's root directory. This will rebuild the Julia system image, then install or update the package dependencies required to build the documentation, and finally build the HTML documentation and place the resulting files in doc/_build/html/.


When making changes to any of Julia's documentation it is recommended that you run make docs to check that your changes are valid and do not produce any errors before opening a pull request.

Below are outlined the three most common types of documentation changes and the steps required to perform them. Please note that the following instructions do not cover the full range of features provided by Documenter.jl. Refer to Documenter's documentation if you encounter anything that is not covered by the sections below.

Modifying files in doc/src/

Most of the source text for the Julia Manual is located in doc/src/. To update or add new text to any one of the existing files the following steps should be followed:

  1. update the text in whichever .md files are applicable;
  2. run make docs from the root directory;
  3. check the output in doc/_build/html/ to make sure the changes are correct;
  4. commit your changes and open a pull request.


The contents of doc/_build/ does not need to be committed when you make changes.

To add a new file to doc/src/ rather than updating a file replace step 1 above with

  1. add the file to the appropriate subdirectory in doc/src/ and also add the file path to the PAGES vector in doc/make.jl.

Modifying an existing docstring in base/

All docstrings are written inline above the methods or types they are associated with and can be found by clicking on the source link that appears below each docstring in the HTML file. The steps needed to make a change to an existing docstring are listed below:

  1. find the docstring in base/;
  2. update the text in the docstring;
  3. run make docs from the root directory;
  4. check the output in doc/_build/html/ to make sure the changes are correct;
  5. commit your changes and open a pull request.

Adding a new docstring to base/

The steps required to add a new docstring are listed below:

  1. find a suitable definition in base/ that the docstring will be most applicable to;

  2. add a docstring above the definition;

  3. find a suitable @docs code block in one of the doc/src/stdlib/ files where you would like the docstring to appear;

  4. add the name of the definition to the @docs code block. For example, with a docstring added to a function bar

    function bar(args...)
        # ...

    you would add the name bar to a @docs block in doc/src/stdlib/

     bar # <-- Added this one.
  5. run make docs from the root directory;

  6. check the output in doc/_build/html to make sure the changes are correct;

  7. commit your changes and open a pull request.


Examples written within docstrings can be used as testcases known as "doctests" by annotating code blocks with jldoctest.

julia> uppercase("Docstring test")

A doctest needs to match an interactive REPL including the julia> prompt. It is recommended to add the header # Examples above the doctests.

To run doctests you need to run make -C doc doctest=true from the root directory. You can use make -C doc doctest=true revise=true if you are modifying the doctests and don't want to rebuild Julia after each change (see details below about the Revise.jl workflow).

News-worthy changes

For new functionality and other substantial changes, add a brief summary to The news item should cross reference the pull request (PR) parenthetically, in the form ([#pr]). To add the PR reference number, first create the PR, then push an additional commit updating with the PR reference number. We periodically run ./julia doc/NEWS-update.jl from the julia directory to update the cross-reference links, but this should not be done in a typical PR in order to avoid conflicting commits.

Annotations for new features, deprecations and behavior changes

API additions and deprecations, and minor behavior changes are allowed in minor version releases. For documented features that are part of the public API, a compatibility note should be added into the manual or the docstring. It should state the Julia minor version that changed the behavior and have a brief message describing the change.

At the moment, this should always be done with the following compat admonition (so that it would be possible to programmatically find the annotations in the future):

!!! compat "Julia 1.X"
    This method was added in Julia 1.X.

Contributing to core functionality or base libraries

By contributing code to Julia, you are agreeing to release it under the MIT License.

The Julia community uses GitHub issues to track and discuss problems, feature requests, and pull requests (PR). You can make pull requests for incomplete features to get code review. The convention is to prefix the pull request title with "WIP:" for Work In Progress, or "RFC:" for Request for Comments when work is completed and ready for merging. This will prevent accidental merging of work that is in progress.

Note: These instructions are for adding to or improving functionality in the base library. Before getting started, it can be helpful to discuss the proposed changes or additions on the Julia Discourse forum or in a GitHub issue---it's possible your proposed change belongs in a package rather than the core language. Also, keep in mind that changing stuff in the base can potentially break a lot of things. Finally, because of the time required to build Julia, note that it's usually faster to develop your code in stand-alone files, get it working, and then migrate it into the base libraries.

Add new code to Julia's base libraries as follows (this is the "basic" approach; see a more efficient approach in the next section):

  1. Edit the appropriate file in the base/ directory, or add new files if necessary. Create tests for your functionality and add them to files in the test/ directory. If you're editing C or Scheme code, most likely it lives in src/ or one of its subdirectories, although some aspects of Julia's REPL initialization live in cli/.

  2. Add any new files to sysimg.jl in order to build them into the Julia system image.

  3. Add any necessary export symbols in exports.jl.

  4. Include your tests in test/Makefile and test/choosetests.jl.

Build as usual, and do make clean testall to test your contribution. If your contribution includes changes to Makefiles or external dependencies, make sure you can build Julia from a clean tree using git clean -fdx or equivalent (be careful – this command will delete any files lying around that aren't checked into git).

Running specific tests

There are make targets for running specific tests:

make test-bitarray

You can also use the runtests.jl script, e.g. to run test/bitarray.jl and test/math.jl:

./usr/bin/julia test/runtests.jl bitarray math

Modifying base more efficiently with Revise.jl

Revise is a package that tracks changes in source files and automatically updates function definitions in your running Julia session. Using it, you can make extensive changes to Base without needing to rebuild in order to test your changes.

Here is the standard procedure:

  1. If you are planning changes to any types or macros, make those changes and build julia using make. (This is necessary because Revise cannot handle changes to type definitions or macros.) Unless it's required to get Julia to build, you do not have to add any functionality based on the new types, just the type definitions themselves.

  2. Start a Julia REPL session. Then issue the following commands:

using Revise    # if you aren't launching it in your `.julia/config/startup.jl`
  1. Edit files in base/, save your edits, and test the functionality.

If you need to restart your Julia session, just start at step 2 above. Revise.track(Base) will note any changes from when Julia was last built and incorporate them automatically. You only need to rebuild Julia if you made code-changes that Revise cannot handle.

For convenience, there are also test-revise-* targets for every test-* target that use Revise to load any modifications to Base into the current system image before running the corresponding test. This can be useful as a shortcut on the command line (since tests aren't always designed to be run outside the runtest harness).

Contributing to the standard library

The standard library (stdlib) packages are baked into the Julia system image. When running the ordinary test workflow on the stdlib packages, the system image version overrides the version you are developing. To test stdlib packages, you can do the following steps:

  1. Edit the UUID field of the Project.toml in the stdlib package
  2. Change the current directory to the directory of the stdlib you are developing
  3. Start julia with julia --project=.
  4. You can now test the package by running pkg> test in Pkg mode.

Because you changed the UUID, the package manager treats the stdlib package as different from the one in the system image, and the system image version will not override the package.

Be sure to change the UUID value back before making the pull request.

Contributing to patch releases

The process of creating a patch release is roughly as follows:

  1. Create a new branch (e.g. backports-release-1.6) against the relevant minor release branch (e.g. release-1.6). Usually a corresponding pull request is created as well.

  2. Add commits, nominally from master (hence "backports"), to that branch. See below for more information on this process.

  3. Run the BaseBenchmarks.jl benchmark suite and PkgEval.jl package ecosystem exerciser against that branch. Nominally BaseBenchmarks.jl and PkgEval.jl are invoked via Nanosoldier.jl from the pull request associated with the backports branch. Fix any issues.

  4. Once all test and benchmark reports look good, merge the backports branch into the corresponding release branch (e.g. merge backports-release-1.6 into release-1.6).

  5. Open a pull request that bumps the version of the relevant minor release to the next patch version, e.g. as in this pull request.

  6. Ping @JuliaLang/releases to tag the patch release and update the website.

  7. Open a pull request that bumps the version of the relevant minor release to the next prerelease patch version, e.g. as in this pull request.

Step 2 above, i.e. backporting commits to the backports-release-X.Y branch, has largely been automated via Backporter: Backporter searches for merged pull requests with the relevant backport-X.Y tag, and attempts to cherry-pick the commits from those pull requests onto the backports-release-X.Y branch. Some commits apply successfully without intervention, others not so much. The latter commits require "manual" backporting, with which help is generally much appreciated. Backporter generates a report identifying those commits it managed to backport automatically and those that require manual backporting; this report is usually copied into the first post of the pull request associated with backports-release-X.Y and maintained as additional commits are automatically and/or manually backported.

When contributing a manual backport, if you have the necessary permissions, please push the backport directly to the backports-release-X.Y branch. If you lack the relevant permissions, please open a pull request against the backports-release-X.Y branch with the manual backport. Once the manual backport is live on the backports-release-X.Y branch, please remove the backport-X.Y tag from the originating pull request for the commits.

Code Formatting Guidelines

General Formatting Guidelines for Julia code contributions

  • 4 spaces per indentation level, no tabs
  • use whitespace to make the code more readable
  • no whitespace at the end of a line (trailing whitespace)
  • comments are good, especially when they explain the algorithm
  • try to adhere to a 92 character line length limit
  • use upper camel case convention for modules, type names
  • use lower case with underscores for method names
  • it is generally preferred to use ASCII operators and identifiers over Unicode equivalents whenever possible
  • in docstring refer to the language as "Julia" and the executable as "julia"

General Formatting Guidelines For C code contributions

  • 4 spaces per indentation level, no tabs
  • space between if and ( (if (x) ...)
  • newline before opening { in function definitions
  • f(void) for 0-argument function declarations
  • newline between } and else instead of } else {
  • if one part of an if..else chain uses { } then all should
  • no whitespace at the end of a line

Git Recommendations For Pull Requests

  • Avoid working from the master branch of your fork, creating a new branch will make it easier if Julia's master changes and you need to update your pull request.
  • Try to squash together small commits that make repeated changes to the same section of code so your pull request is easier to review. A reasonable number of separate well-factored commits is fine, especially for larger changes.
  • If any conflicts arise due to changes in Julia's master, prefer updating your pull request branch with git rebase versus git merge or git pull, since the latter will introduce merge commits that clutter the git history with noise that makes your changes more difficult to review.
  • Descriptive commit messages are good.
  • Using git add -p or git add -i can be useful to avoid accidentally committing unrelated changes.
  • When linking to specific lines of code in discussion of an issue or pull request, hit the y key while viewing code on GitHub to reload the page with a URL that includes the specific version that you're viewing. That way any lines of code that you refer to will still make sense in the future, even if the content of the file changes.
  • Whitespace can be automatically removed from existing commits with git rebase.
    • To remove whitespace for the previous commit, run git rebase --whitespace=fix HEAD~1.
    • To remove whitespace relative to the master branch, run git rebase --whitespace=fix master.

Git Recommendations For Pull Request Reviewers

  • When merging, we generally like squash+merge. Unless it is the rare case of a PR with carefully staged individual commits that you want in the history separately, in which case merge is acceptable, but usually prefer squash+merge.