Contribution Guide

Chainer is an open source software hosted on GitHub and welcomes contributors to take part in the development of the framework. This is a document aimed towards such contributors. Anyone who for instance would like to file an issue or send a pull request (PR) is encouraged to go through it.


As announced, Chainer is under the maintenance phase and further development will be limited to bug-fixes and maintenance only. Pull-requests for new features, enhancements, or backward-compatibility breaking changes will not be accepted.

Issues and Pull Requests

First steps in contributing to Chainer often involve filing an issue or creating a PR. This section describes how to do so.

How to File an Issue

To file an issue on GitHub, you often only need to follow instructions given by the template. Write precise explanations on how you want Chainer to behave or include necessary and sufficient conditions to reproduce the bugs. Feature requests should include what you want to do and preferably why. You may additionally suggest how.


If you have a question regarding the usage of Chainer, it is recommended that you send a post to StackOverflow or the Chainer User Group instead of the issue tracker. The issue tracker is not a place to share knowledge on practices.

How to Send a Pull Request

If you can write code to fix an issue, it is encouraged to send a PR.

In that case, confirm the following points before starting to write any code.

  • Read Coding Guidelines and Unit Testing.

  • Check the appropriate branch to which you should send a PR, following Git Branches. If you are unsure about which branch to target, choose the master branch. The current source tree of the chosen branch is the starting point of your change.

After writing your code (including unit tests and hopefully documentations!), send a PR on GitHub. You have to write a precise explanation of what and how in the description; this is the first documentation of your code and an important part of your PR.

However, even if your code is not complete, you can send a PR as a work-in-progress (WIP) PR by prefixing the PR title with [WIP]. If you just describe the PR, the core team and other contributors can join the discussion about how to proceed with it. WIP PRs may occasionally be useful for discussing based on concrete code.

When a PR is created (or updated), it is automatically tested in one of our CI environments, namely Travis CI. There are other CI environments as well often manually triggered by the reviewer. The various CIs are required to test for instance different platforms or CUDA environments. Once the tests in all CI environments pass and/or the PR is approved by the reviewer, the PR will be merged.


If you are planning to add a new feature or modify existing APIs, it is recommended that you open an issue and discuss the design first. Following the consequences of the discussions, you can send a PR that is smoothly reviewed in a shorter time.

Issue/Pull Request Labels

Issues and PRs are labeled on GitHub so that they can be grouped, filtered and better maintained. For instance, a label can indicate that a ticket needs response from the PR author, or that an issue needs immediate action in case of a critical bug. Please refer to the list of lables on GitHub.

Coding Guidelines

We follow PEP 8 and partially OpenStack Style Guidelines as basic style guidelines. Any contributions in terms of code are expected to follow these guidelines.

You can use the autopep8 and the flake8 commands to check whether or not your code follows the guidelines. In order to avoid confusion from using different tool versions, we pin the versions of those tools. Install them with the following command (from within the top directory of the Chainer repository):

$ pip install -e '.[stylecheck]'

And check your code with:

$ autopep8 path/to/your/
$ flake8 path/to/your/

autopep8 can automatically correct Python code to conform to the PEP 8 style guide:

$ autopep8 --in-place path/to/your/

The flake8 command lets you know parts of your code that are not following the style guidelines.

Note that flake8 command is not perfect. It does not check some of the style guidelines. Here is a (not-exhaustive) list of the rules that flake8 cannot check.

  • Relative imports are prohibited. [H304]

  • Importing non-module symbols is prohibited.

  • Import statements must be organized into three parts: standard libraries, third-party libraries, and internal imports. [H306]

In addition, we restrict the usage of shortcut aliases in any global-scope code. In particular, you cannot use shortcut aliases to designate a parent class in global-scope class definitions. When you want to make a class inheriting another class defined in another module, you have to spell out the full module name instead of importing a module that provides an alias.

For example, the following code is not allowed.

import chainer

class MyLink(chainer.Link): ...

Instead, import and use that.


class MyLink( ...

If you feel the code too verbose, you can also use from import or import as.

from chainer import link

class MyLink(link.Link): ...


From v3.0, we allow shortcut aliases used inside of functions and methods that are not called from any global scope code. For example, you can write chainer.Variable instead of chainer.variable.Variable inside of functions and methods. Use of such aliases was prohibited in the past for avoiding confusing errors related to cyclic dependencies; we relaxed the rule so that the library code looks similar to user code.

When you use such shortcut aliases, please be careful of cyclic imports. One of the typical pitfalls is a way to import chainer.functions. An import like import chainer.functions as F within modules under chainer.functions does not work. An import like from chainer import functions works well with Python 3, but does not with Python 2. We recommend that you use import chainer.functions and spell out like in your methods.

Unit Testing

Testing is one of the most important aspects of your PR. You should write test cases and verify your implementation by following the testing guide above. If you modify code related to existing unit tests, you must run appropriate commands and confirm that the tests still pass.

Note that we are using pytest and the mock package for testing. They are not included in Chainer and need to be installed as follows:

$ pip install pytest mock

How to Run Tests

You can run all unit tests with the following command from the root directory of the Chainer:

$ python -m pytest

Or specify a test script that you want to run:

$ python -m pytest path/to/your/

You can also run all unit tests under a specific directory:

$ python -m pytest tests/chainer_tests/<directory name>

Some tests require CUDA and cuDNN by default. In order to run unit tests that do not require CUDA and cuDNN, set an environment variable and filter using test marks as follows:

$ python -m pytest path/to/your/ -m='not cudnn'

Some GPU tests involve multiple GPUs. If you want to run GPU tests with insufficient number of GPUs, specify the number of available GPUs to CHAINER_TEST_GPU_LIMIT. For example, if you only have a single GPU, launch pytest with the following command to skip multi-GPU tests:

$ python -m pytest path/to/gpu/

Some tests spend too much time. If you want to skip such tests, pass -m='not slow' option to the command:

$ python -m pytest path/to/your/ -m='not slow'

Test File and Directory Naming Conventions

Tests are found in the tests/chainer_tests directory. In order to enable the test runner to find test scripts correctly, we are using a special naming convention for the test subdirectories and the test scripts.

  • The name of each subdirectory of tests must end with the _tests suffix.

  • The name of each test script must start with the test_ prefix.

When we write a test for a module, we use the appropriate path and file name for the test script whose correspondence to the tested module is clear. For example, if you want to write a test for a module chainer.x.y.z, the test script must be located at tests/chainer_tests/x_tests/y_tests/

How to Write Tests

There are many examples of unit tests under the tests directory, so reading some of them is a good and recommended way to learn how to write tests for Chainer. They use the unittest package of the standard library, while some tests are additionally using utilities from chainer.testing.

In addition to the Coding Guidelines mentioned above, the following rules apply to the test code:

  • All test classes must inherit from unittest.TestCase.

  • Use unittest features to write tests, except for the following cases:

    • Use assert statement instead of self.assert* methods (e.g., write assert x == 1 instead of self.assertEqual(x, 1)).

    • Use with pytest.raises(...): instead of with self.assertRaises(...):.


We are incrementally applying the above style. Some existing tests may be using the old style (self.assertRaises, etc.), but all newly written tests should follow the above style.

Even if your patch includes GPU-related code, your tests should not fail without GPU capability. Test functions that require CUDA must be tagged with the chainer.testing.attr.gpu decorator:

import unittest
from chainer.testing import attr

class TestMyFunc(unittest.TestCase):

    def test_my_gpu_func(self):

The functions tagged with the gpu decorator are skipped if CHAINER_TEST_GPU_LIMIT=0 environment variable is set. We also have the chainer.testing.attr.cudnn decorator to let pytest know that the test depends on cuDNN. The test functions decorated with cudnn are skipped if -m='not cudnn' is given.

The test functions decorated with gpu must not depend on multiple GPUs. In order to write tests for multiple GPUs, use the chainer.testing.attr.multi_gpu() decorator instead:

import unittest
from chainer.testing import attr

class TestMyFunc(unittest.TestCase):

    @attr.multi_gpu(2)  # specify the number of required GPUs here
    def test_my_two_gpu_func(self):

If your test requires too much time, add the chainer.testing.attr.slow decorator. The test functions decorated with slow are skipped if -m='not slow' is given:

import unittest
from chainer.testing import attr

class TestMyFunc(unittest.TestCase):

    def test_my_slow_func(self):


If you want to specify more than two attributes, use and operator like -m='not cudnn and not slow'. See detail in the documentation of pytest.


When adding a new feature to the framework, you should also document it in the reference so that other users can find it in the official documentation. For example, if you are adding a new function under chainer.functions, Functions should be updated.

The documentation source is stored under docs directory and written in reStructuredText format.

To build the documentation, you need to install Sphinx:

$ pip install sphinx sphinx_rtd_theme


Docstrings (documentation comments in the source code) are collected from the installed Chainer module. If you have edited docstrings in checked-out source files and want to see those changes reflected in the generated html, Chainer must be installed in develop mode to see those changes reflected in the generated documentation. To do this use pip install -e . from the the top of the Chainer directory.

Then you can build the documentation in HTML format locally:

$ cd docs
$ make html

HTML files are generated under build/html directory. Open index.html with the browser and see if it is rendered as expected.


If you are unsure about how to write the documentation or failed to build it locally, you can submit a PR without documentation. Reviewers will help you with it.

Other Forms of Contribution

There are several other ways in which you can contribute to Chainer without directly working with the code base. Following are such contributions.

Development Cycle

This section explains the development process of Chainer.


The versioning of Chainer follows PEP 440 and a part of Semantic versioning. The version number consists of three or four parts: X.Y.Zw where X denotes the major version, Y denotes the minor version, Z denotes the revision number, and the optional w denotes the pre-release suffix. While the major, minor, and revision numbers follow the rule of semantic versioning, the pre-release suffix follows PEP 440, the Python community standards.

Note that a major update basically does not contain compatibility-breaking changes from the last release candidate (RC). This is not a strict rule, though; if there is a critical bug in the API that need to be fixed for the major version, breaking changes may be introduced.

For more on backward compatibility, please refer to the API Compatibility Policy.

Release Cycle

A milestone for each upcoming release is published on GitHub. The GitHub milestones are used to group issues and PRs belonging to a release.

Git Branches

master branch is used for Chainer v7.x development.