Your first VOC contribution

So you want to contribute to an Open Source project, but you don't know how? No problem - VOC is a perfect place to start.

The idea behind VOC is to take Python bytecode, and compile it directly to Java bytecode. This means your .py file will be converted in a .class file that can be executed in a Java Virtual Machine.

This is useful in a number of possible ways:

• Writing Android applications
• Writing ElasticSearch/Lucene plugins
• Writing Minecraft plugins
• Writing web applications for situations where JavaEE is the only available deployment platform.

It’s similar to Jython in that it allows you to run Python on Java; but it’s different because there’s no runtime “executable” - it’s not a Python interpreter written in Java; it generates Java bytecode that is indistinguishable from code that was generated using javac.

My personal interest comes from wanting to write Android applications in Python; but I hope you agree that there’s lots of other potential uses.

In the following instructions, I’m going to assume you’re familiar with Github and making pull requests. I'm also going to assume some entry level Python and Java; if anything I describe here doesn’t make sense, don’t worry - I’m more than happy to fill in the gaps. At this point, I don’t know what you don’t know :-)

I’m also going to assume that you’re interested in contributing code; if your interests/skills are elsewhere (e.g., testing, documentation), let me know, and I can make some other suggestions.

Before you make your first contribution, I’d suggest taking VOC for a spin. Write a short Python program (a quick "hello world" app), use VOC to compile it to a class file, and then run it under Java. The instructions on the project page should be enough, with one exception: instead of running “pip install voc”, I’d suggest forking the project on Github, checking out that fork, and then running pip install -e . from the root directory. That will mean you’ll have the most recent version of the code, and you’ll have a Github repository ready to work in, too.

If you get stuck, let me know and I’ll help you out. And if you get stuck, that will also point to your first contribution - work out what instructions would have made you not get stuck, and contribute an update to the README.

Once you’ve got VOC working, you’ll be ready to make your first code contribution.

In order to make Java bytecode behave like Python, VOC needs to implement all the eccentricities of Python behaviour. For example, Python allows you to multiply a string by an integer, resulting in a duplicated string (e.g., “foo” * 3 => “foofoofoo”). This isn’t legal Java, however; so we need to provide a Java library that implements this beheavior.

That particular example (string times number) is already implemented - see:

https://github.com/pybee/voc/blob/master/python/common/org/python/types/Str.java#L274

This implements __mul__() (the Python multiplication method) on a “Str” object. It’s implemented in Java, but it implements Python behavior.

If you then look at:

https://github.com/pybee/voc/blob/master/tests/datatypes/test_str.py

You’ll see the tests for this behaviour. I’ve already written a test suite that tries to perform every operation on every data type. However, all of the names listed in that file are the examples that are known to fail (because the implementation doesn’t exist yet). The tests work by writing a short program that implements one mathematical operation, running the program under normal Python, and then running it again under VOC/Java. The test passes when the two executions produce identical output.

So - your first open source contribution: Pick a test that is currently listed in the test_str test file (e.g., test_multiply_float is the test of multiplying a string by a float), and modify the implementation of Str until you get the same output from CPython program as you do from VOC-compiled version of the same program. This includes error messages - if CPython raises a TypeError for a particular operation (e.g., “foo” * 3.1415), then so should VOC - and the error message should be the same too, right down to the punctuation. Run the tests, and if you get an “unexpected success”, and you don’t get any failures, you’ve done it! Delete the line from the test_str file corresponding to the test you’ve fixed (because the test is no longer expected to fail), and submit a pull request!

Then break out a celebratory beverage of your choice! You’re an open source contributor!

If you need help stepping through this, you can grab me on Twitter, or in #beeware on IRC (on Freenode). I’m freakboy3742 on both. Or, feel free to send me an email

Best of luck - and I can’t wait to see your first contribution!