In the blink of an eye, Google Summer of Code (GSoC) 2018 has come to an end. During the three months long coding period, I have contributed several patches in VOC repository of BeeWare, all working towards the ultimate end goal of running asyncio module in VOC. In this blog post (which is my first actual blog post by the way 😄), I will document what I have done so far, why I couldn't make it to the end goal (yea, unfortunately I couldn't get asyncio to work at the end of GSoC 2018), and what's left that needs to be done in order to achieve the end goal (or at least make part of asyncio work).

Building Foundation

The first error that the transpiler throws when attempting to compile asyncio module was "No handler for YieldFrom", so it makes sense to start from this issue first.

Another feature related to generator was Yield expression. Before GSoC 2018, Yield statement in VOC was just a statement, meaning yield could not be used as expression. Generator methods such as generator.send, generator.throw and generator.close were not supported as well. Those features are what make asynchronous programming with generator possible, so I spent a few weeks to extend generator functionality in VOC, laying down the path to asyncio module.

PRs related to generator are listed below:

  • PR #821 : Added support for Yield from statement (merged)
  • PR #823 : Added generator send method (merged)
  • PR #831 : Support exceptions handling in generator (merged)

Nonlocal Statement

Nonlocal statement was another syntax not supported by VOC. After completion of generator's features, implementing this is the next step towards compiling asyncio module.

Implementing this feature took about 3 ~ 4 weeks as this is not as trivial as it seems. I took several approaches on this, while some of them do work, the code is not pretty and hacky, which could come back to bite me/other contributors in the long run. After many discussions with Russell, I refactored the closure mechanism in VOC and took a much cleaner approach in nonlocal implementations. I must admit that I took some short-cuts for the sake of "making nonlocal works" in the process of implementing nonlocal statement, resulting in poor design and messy codes. Many thanks to Russell, who helped me to improve my coding style and told me not to be discouraged when I'm stuck. 😄

Related PRs:

  • PR #854 : Nonlocal statement support (merged)
  • PR #873 : Added closure related test cases (merged)

The Collections Module

Next item on my hit list was pure Java implementations of the collections module. asyncio module depends on 3 data structures from collections, namely defauldict, Deque and OrderedDict. Two of them (defaultdict and Deque) are implemented in C in CPython, plus they have good analog in Java, so it makes senses to implement the module in Java. Porting defauldict, Deque and OrderedDict to Java in VOC is relatively straight-forward, taking about 1.5 weeks to complete.

Related PRs:

  • PR #874 : Implement collections.defauldict (merged)
  • PR #896 : Implements collections.Deque (merged)
  • PR #897 : Implements collections.OrderedDict` (merged)

Other PRs submitted during GSoC 2018

  • PR #817 : Added coroutine related exception class [WIP] (closed due to not needed)
  • PR #836 : Changed Bool construction to use getBool instead (merged)
  • PR #847 : Add custom exceptions test cases (closed due to more comprehensive handling in PR #844)
  • PR #849 : Fixed Unknown constant type <class 'frozenset'> in function definition (merged)
  • PR #858 : Added test case for Issue #857 (merged)
  • PR #860 : Added test case for Issue #859 (merged)
  • PR #862 : Added test case for Issue #861 (merged)
  • PR #867 : Fixed Issue #866 RunTimeError when generator is nested in more than 1 level of function definition (merged)
  • PR #868 : Fixed Issue #861 Redefining nested function from other function overrides original nested function (merged)
  • PR #879 : Fixed Incompatible Stack Height caused by expression statement (merged)
  • PR #901 : Added test case for Issue #900 (merged)
  • PR #788 : Implements asyncio.coroutines [WIP] (open, the dream 😎)

Issues submitted during GSoC 2018

  • Issue #861 : Redefining nested function from other function overrides original nested function (fixed in PR #868)
  • Issue #866 : RunTimeError when generator is nested in more than 1 level of function definition (fixed in PR #867)
  • Issue #828 : Finally block of generator is not executed during garbage collection (open)
  • Issue #857 : Complex datatype in set cause java.lang.StackOverflowError (open)
  • Issue #859 : Duplicated values of equivalent but different data types in set (open)
  • Issue #900 : Exception in nested try-catch suite is 'leaked' to another enclosing try-catch suite (open)
  • Issue #827 : Maps reserved Java keywords to Python built-in function/method call (closed)

Towards The Ultimate End Goal

Unfortunately, three months of GSoC coding period was not enough for me to bring asyncio module to VOC. The nonlocal statement implementation was the biggest blocker for me mainly because I didn't think thoroughly before writing code. If I were to plan carefully and lay out a general coding direction, I would've completed it in much shorter time and have time for other implementations. An advice for the aspiring and upcoming GSoC-er, don't rush your code, make sure you know 100% about what you're doing before diving into the codes.

With that said, following are the list of modules to be implemented/ported to Java before asyncio will work in VOC:

  • socket module (a bit tricky since Java doesn't support Unix domain socket natively)
  • selectors module (high level I/O operations)
  • threading module (might be easier to implement this first since threading in Python is an emulation of Java's Thread)
  • time module (partially implemented in VOC)

Final Thoughts

Huge thanks to my mentor, Russell Keith-Magee for accepting my proposal, providing guidance and encouraging me when things didn't go as intended. It is truly an honor to be a part of the BeeWare community. I had a blast contributing to BeeWare project, and I'm sure I will stick around as a regular contributor. Also shout out to the BeeWare community for answering my queries and reviewing my pull requests. 😄