[Mobile-sig] Progress update on Mobile Python work

[Russell Keith-Magee]

Hi all,

This SIG has been a bit quiet of late, but over the last few months (and
especially over the Christmas/New Year break) I’ve made some significant
progress on support for Python on mobile platforms (especially Android), so
I thought I’d give an update of where things (at least, from where I stand).

iOS Support
===========

iOS support is getting quite strong. There is a patch against CPython 3.4.2
sources:

http://bugs.python.org/issue23670

This patch provides an XCode project that runs the Python test suite. There
are still a handful of bugs; these are mostly related to edge cases in
ctypes.

The issues that previously existed with libffi have now been resolved. The
biggest question with libffi support now is whether there is an opportunity
to merge libffi support on OS X with the new libffi support on iOS. The
generated code for libffi_osx is several years old at this point, and it
would be easy to replace the current libffi_osx generated code with an
updated version sharing sources with iOS. The only catch is that libffi no
longer supports PowerPC architectures in their OS X builds (because Apple
no longer supports those architectures).

There are two impediments currently preventing this patch from being merged
into trunk.

The first is a bug, reported during the 3.5 alpha, that is still lingering:

http://bugs.python.org/issue22625

This bug was introduced in the process of fixing support for parallel
builds in the root Makefile. This problem prevents *any* cross platform
build from working. I don’t have enough Makefile mojo to sort this one out.

The second is the absence of a feedback from someone on the core team. The
patch really needs a review from someone to indicate which of the design
decisions I’ve made are acceptable to the CPython core team.

In the meantime, I’ve been maintaining an external project to make it easy
to build a Python framework for use in an iPhone (or iPad, or AppleTV, or
Apple Watch) app:

https://github.com/pybee/Python-iOS-support

You can download a pre-compiled version of the libraries from the project
page. Add an Objective C bridging library (like Rubicon), and you can write
your iOS (or tvOS) app in Python.

Android Support
===============

Cyd Haselton has been working on an analogous patch for CPython to add
support for Android. From what I can see, he’s having some difficulties;
the code is segfaulting, and debugging the source of the segfault is
proving difficult.

I experimented with CPython on Android briefly at the end of 2014; I came
to the conclusion that it wasn’t really viable, because once you have
CPython, you need to access Java libraries to actually write an
application. That means you have to use JNI, and while JNI *exists* on
Android, it is constrained to the point that it makes writing apps nigh on
impractical.

Instead, I’ve been working on an alternate approach: transpiling CPython
3.4 bytecode directly to Java class files. The result is a project called
VOC:

https://github.com/pybee/voc

This is a different approach to that used by Jython. There’s no Python
interpreter at runtime - what
you push to the phone is a pure Java class file implementing Python
language semantics. The price you pay is that there’s no exec(), eval() or
REPL - but if you’re writing an app for distribution on a phone, that
probably doesn’t matter.

Performance is passable, but not great - PyStone indicates that it’s
roughly an order of magnitude slower than CPython on the same machine.
However I haven’t been focussing on performance to date, and there’s a lot
of low hanging opportunities for optimisation. It’s also worth noting that
mobile apps (other than games) generally won’t be CPU bound - they’re
network bound - so this performance issue isn’t as critical as it may seem.

Here’s a sample project:

https://gist.github.com/freakboy3742/3c6b74e8506d47d9bd97

When you compile that code with VOC, you get 3 class files (one for each
class, and one for the module itself). If you add those class files to an
Android project and upload to a phone, you get this:

https://twitter.com/PyBeeWare/status/683258762816192513/video/1

There’s still a lot of work to do on VOC; there are some language
constructs that it doesn’t handle yet (most notably generators and context
managers), and I need to port the parts of the standard library that are
written in C. However, as the video shows, even without those parts, you
can write simple Android applications in Python.

Packaging
=========

One last thing - I’ve also created project called Briefcase to tie all
these pieces together.

https://github.com/pybee/briefcase

Briefcase is an extension to distutils that adds setup.py targets to
package a project for distribution as an app. iOS and OS/X support is quite
stable. A stub command exists for Android, but it isn’t fully operational
yet. Apple TV support would be trivial to add - most of the pieces are
already there, they just need to be tied together.

Summary
=======

So - that’s where, IMHO, Python on Mobile is at right now.

If you’ve got any questions, comments, or feedback - let me know.

Yours,
Russ Magee %-)
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