Py4J - A Bridge between Python and Java

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# 7. Contributing to Py4J

Summary

- General Conventions
- Python Coding Conventions
- Java Coding Conventions
- Testing Python Code
- Testing Java Code
- Code Coverage
- Branch name and commit message format
  - Squashing commits and git
- License and Copyrights

Thanks a lot for using Py4J and wanting to contribute! Your help is greatly appreciated!

The following is a set of guidelines for contributing to Py4J. These are just guidelines, not rules, use your best judgment and feel free to propose changes to this document in a pull request.

These guidelines have evolved since Py4J started and past commits may not always follow them.

In general, if your contribution does not follow these guidelines, we will review your contribution, but expect that we will ask you for some changes and refer you to specific parts of this guide!

## 7.1. General Conventions

Contributions to Py4J are always welcome. To accelerate the review process, make sure that your contribution includes:

1. At least one unit test in Python and/or Java.
2. A docstring or javadoc comment if you create new public classes, methods or functions.
3. Some code examples in the py4j-web documentation subproject if you are introducing new features.

## 7.2. Python Coding Conventions

We follow pep8 rather stricly:

1. We use spaces instead of tab.
2. We use four-space indents.
3. Line length is 80
4. Code must pass the default flake8 tests (pep8 + pyflakes)

Code must be compatible with from Python 2.6 to the newest released version of Python.

If external libraries must be used, they should be wrapped in a mechanism that by default does not require them (e.g., conditional imports, graceful degradation, etc.).

Libraries used for testing and contributing (flake8, nose, tox, Sphinx) can be installed with pip:

    pip install -r requirements.txt

    # Run flake8
    flake8

## 7.3. Java Coding Conventions

We use Eclipse code formatting conventions (see py4j.formatter.xml and py4j.importorder) that can be used in Eclipse and IntelliJ IDEs.

In summary:

1. We use tabs rather than spaces to indent the Java code.
2. Most expressions (conditionals, loops, try/catch) are always wrapped with curly brackets.

You can format your code using gradle and Java 8:

    # To check Java coding conventions:
    ./gradlew spotlessCheck

    # To format your code:
    ./gradlew spotlessJavaApply

We use FindBugs on the main source code (not the test source code) and any warnings must be corrected before a pull request will be accepted.

    # To run findbugs
    ./gradlew findbugsMain

## 7.4. Testing Python Code

On the Python side, we use nose to run the test suite and tox to run the test suite across the supported python versions.

    # make sure that the jar file is created
    cd py4j-java
    ./gradlew clean
    ./gradlew assemble

    # install test requirements
    cd py4j-python
    pip install -r requirements.txt

    # Run the full test suite
    nosetests

    # Run only one particular test
    nosetests py4j.tests.java_gateway_test:GatewayLauncherTest.testRedirectToDeque

    # Run all tests on all supported pythons.
    # Typically only do this if the automated build failed
    # on one version of python.
    tox

    # Run flake8 checks
    flake8

New code or bug fix should ideally be accompanied by a test case.

Because we start a JVM for most test cases, it may happen that some test fails because the process was not ready to receive a request. This is a problem we have been working on for some time now and it has been a few months since the test suite failed because of synchronization issues. If your tests constantly fail, then something is wrong with your test!

## 7.5. Testing Java Code

We use JUnit to write test cases.

    cd py4j-java
    ./gradlew clean
    ./gradlew test

## 7.6. Code Coverage

We have started to keep track of code coverage on both the Python and Java libraries. Unfortunately, most of the Java code is being tested by the Python library so the reported code coverage is smaller than the actual coverage.

When reviewing a contribution, we will always require that both the Java code and the Python code are tested, but it does not matter if the Java code is tested by the Python code.

## 7.7. Branch name and commit message format

In general, it is best to first open an issue and then, refer to the issue in your commit:

1. Your branch should have the format `issue-XYZ-branch-name` where XYZ is the issue number and branch-name is a short description.

2. Commits that refer to an issue will have the format `refs #XYZ -- message` where XYZ is the issue number.

3. Once a pull request is approved, we ask you to rebase your changes against the master branch and squash your commits into one meaningful commit (see below for tips on how to do this). The format of the commit would be:

       fixes #XYZ -- short description below 72 characters

       Longer description that lists all the changes that occured
       on multiple lines of 79 characters.

### 7.7.1. Squashing commits and git

To squash your commits, you can use the git rebase command:

    # Squash the last three commits into 1
    git rebase -i HEAD~3
    # An editor will open. Change the word "pick" to "squash"
    # except for the very first commit at the top of the list.
    # After you save and exit, you will be prompted again to
    # change the commit message of the squashed commit.

The Django contributing guide has a good tutorial on using git to contribute and in particular, squashing commits.

## 7.8. License and Copyrights

Py4J does not have an official Contributor License Agreement (CLA), but it is assumed that as soon as you make a contribution (patch, code suggestion through any medium, pull requests) to Py4J, you accept that your code will be redistributed under the current license used by Py4J, i.e., the new BSD license. This is an irrevocable right to ensure that developers can use Py4J without the fear of seeing parts removed in the future.

You maintain the full copyrights for your contributions: you are only providing a license to distribute your code without further restrictions.

The copyright statement in the License has been standardized to:

`Copyright (c) 2009-2016, Barthelemy Dagenais and individual contributors. All rights reserved.``

Individual contributors are identified in the AUTHORS file. If you have contributed to Py4J and your name is not in AUTHORS, please open a pull request!

If you are working for a company while contributing to Py4J, make sure that the code is yours or that your company agrees with this implied CLA.

This approach is heavily inspired from the Django Contributor License Agreement..

If you have any question, do not hesitate to contact the founder of the project, Barthelemy.

### Questions/Feedback?

- Try the FAQ -- It answers many questions.
- Search for information in the archives of the py4j-users mailing list, or post a question.
- If you notice errors with this documentation, open a ticket and let us know. Please, only open a ticket for feedback or improvements on the docs. To get help, use the links above!
- Request professional services to get a custom feature implemented quickly or to receive commercial support.

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