Contributing to OpenMMLab

Welcome to the MMCV community, we are committed to building a cutting-edge computer vision foundational library and all kinds of contributions are welcomed, including but not limited to

Fix bug

You can directly post a Pull Request to fix typo in code or documents

The steps to fix the bug of code implementation are as follows.

  1. If the modification involve significant changes, you should create an issue first and describe the error information and how to trigger the bug. Other developers will discuss with you and propose an proper solution.

  2. Posting a pull request after fixing the bug and adding corresponding unit test.

New Feature or Enhancement

  1. If the modification involve significant changes, you should create an issue to discuss with our developers to propose an proper design.

  2. Post a Pull Request after implementing the new feature or enhancement and add corresponding unit test.


You can directly post a pull request to fix documents. If you want to add a document, you should first create an issue to check if it is reasonable.

Pull Request Workflow

If you’re not familiar with Pull Request, don’t worry! The following guidance will tell you how to create a Pull Request step by step. If you want to dive into the develop mode of Pull Request, you can refer to the official documents

1. Fork and clone

If you are posting a pull request for the first time, you should fork the OpenMMLab repositories by clicking the Fork button in the top right corner of the GitHub page, and the forked repositories will appear under your GitHub profile.

Then, you can clone the repositories to local:

git clone{username}/mmcv.git

After that, you should ddd official repository as the upstream repository

git remote add upstream

Check whether remote repository has been added successfully by git remote -v

origin{username}/mmcv.git (fetch)
origin{username}/mmcv.git (push)
upstream (fetch)
upstream (push)


Here’s a brief introduction to origin and upstream. When we use “git clone”, we create an “origin” remote by default, which points to the repository cloned from. As for “upstream”, we add it ourselves to point to the target repository. Of course, if you don’t like the name “upstream”, you could name it as you wish. Usually, we’ll push the code to “origin”. If the pushed code conflicts with the latest code in official(“upstream”), we should pull the latest code from upstream to resolve the conflicts, and then push to “origin” again. The posted Pull Request will be updated automatically.

2. Configure pre-commit

You should configure pre-commit in the local development environment to make sure the code style matches that of OpenMMLab. Note: The following code should be executed under the MMCV directory.

pip install -U pre-commit
pre-commit install

Check that pre-commit is configured successfully, and install the hooks defined in .pre-commit-config.yaml.

pre-commit run --all-files


Chinese users may fail to download the pre-commit hooks due to the network issue. In this case, you could download these hooks from gitee by setting the .pre-commit-config-zh-cn.yaml

pre-commit install -c .pre-commit-config-zh-cn.yaml pre-commit run –all-files -c .pre-commit-config-zh-cn.yaml

If the installation process is interrupted, you can repeatedly run pre-commit run ... to continue the installation.

If the code does not conform to the code style specification, pre-commit will raise a warning and fixes some of the errors automatically.

If we want to commit our code bypassing the pre-commit hook, we can use the --no-verify option(only for temporarily commit.

git commit -m "xxx" --no-verify

3. Create a development branch

After configuring the pre-commit, we should create a branch based on the main branch to develop the new feature or fix the bug. The proposed branch name is username/pr_name

git checkout -b yhc/refactor_contributing_doc

In subsequent development, if the main branch of the local repository is behind the main branch of “upstream”, we need to pull the upstream for synchronization, and then execute the above command:

git pull upstream main

4. Commit the code and pass the unit test

  • MMCV introduces mypy to do static type checking to increase the robustness of the code. Therefore, we need to add Type Hints to our code and pass the mypy check. If you are not familiar with Type Hints, you can refer to this tutorial.

  • The committed code should pass through the unit test

    # Pass all unit tests
    pytest tests
    # Pass the unit test of runner
    pytest tests/test_runner/

    If the unit test fails for lack of dependencies, you can install the dependencies referring to the guidance

  • If the documents are modified/added, we should check the rendering result referring to guidance

5. Push the code to remote

We could push the local commits to remote after passing through the check of unit test and pre-commit. You can associate the local branch with remote branch by adding -u option.

git push -u origin {branch_name}

This will allow you to use the git push command to push code directly next time, without having to specify a branch or the remote repository.

6. Create a Pull Request

(1) Create a pull request in GitHub’s Pull request interface

(2) Modify the PR description according to the guidelines so that other developers can better understand your changes

Find more details about Pull Request description in pull request guidelines.


(a) The Pull Request description should contain the reason for the change, the content of the change, and the impact of the change, and be associated with the relevant Issue (see documentation

(b) If it is your first contribution, please sign the CLA

(c) Check whether the Pull Request pass through the CI

MMCV will run unit test for the posted Pull Request on different platforms (Linux, Window, Mac), based on different versions of Python, PyTorch, CUDA to make sure the code is correct. We can see the specific test information by clicking Details in the above image so that we can modify the code.

(3) If the Pull Request passes the CI, then you can wait for the review from other developers. You’ll modify the code based on the reviewer’s comments, and repeat the steps 4-5 until all reviewers approve it. Then, we will merge it ASAP.

7. Resolve conflicts

If your local branch conflicts with the latest main branch of “upstream”, you’ll need to resolove them. There are two ways to do this:

git fetch --all --prune
git rebase upstream/main


git fetch --all --prune
git merge upstream/main

If you are very good at handling conflicts, then you can use rebase to resolve conflicts, as this will keep your commit logs tidy. If you are not familiar with rebase, then you can use merge to resolve conflicts.


Unit test

If you cannot run the unit test of some modules for lacking of some dependencies, such as video module, you can try to install the following dependencies:

# Linux
sudo apt-get update -y
sudo apt-get install -y libturbojpeg
sudo apt-get install -y ffmpeg

# Windows
conda install ffmpeg

We should also make sure the committed code will not decrease the coverage of unit test, we could run the following command to check the coverage of unit test:

python -m coverage run -m pytest /path/to/test_file
python -m coverage html
# check file in htmlcov/index.html

Document rendering

If the documents are modified/added, we should check the rendering result. We could install the dependencies and run the following command to render the documents and check the results:

pip install -r requirements/docs.txt
cd docs/zh_cn/
# or docs/en
make html
# check file in ./docs/zh_cn/_build/html/index.html

Code style


We adopt PEP8 as the preferred code style.

We use the following tools for linting and formatting:

  • flake8: A wrapper around some linter tools.

  • isort: A Python utility to sort imports.

  • yapf: A formatter for Python files.

  • codespell: A Python utility to fix common misspellings in text files.

  • mdformat: Mdformat is an opinionated Markdown formatter that can be used to enforce a consistent style in Markdown files.

  • docformatter: A formatter to format docstring.

Style configurations of yapf and isort can be found in setup.cfg.

We use pre-commit hook that checks and formats for flake8, yapf, isort, trailing whitespaces, markdown files, fixes end-of-files, double-quoted-strings, python-encoding-pragma, mixed-line-ending, sorts requirments.txt automatically on every commit. The config for a pre-commit hook is stored in .pre-commit-config.

C++ and CUDA

We follow the Google C++ Style Guide.

PR Specs

  1. Use pre-commit hook to avoid issues of code style

  2. One short-time branch should be matched with only one PR

  3. Accomplish a detailed change in one PR. Avoid large PR

    • Bad: Support Faster R-CNN

    • Acceptable: Add a box head to Faster R-CNN

    • Good: Add a parameter to box head to support custom conv-layer number

  4. Provide clear and significant commit message

  5. Provide clear and meaningful PR description

    • Task name should be clarified in title. The general format is: [Prefix] Short description of the PR (Suffix)

    • Prefix: add new feature [Feature], fix bug [Fix], related to documents [Docs], in developing [WIP] (which will not be reviewed temporarily)

    • Introduce main changes, results and influences on other modules in short description

    • Associate related issues and pull requests with a milestone

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