[Mobile-sig] python-for-android 0.4 released, and general update

[Alexander Taylor]

Hi all,

Per the title, we (the Kivy devs) have just released python-for-android
(p4a) 0.4 [1]. This release in particular is a major one because it's
the culmination of a lot of recent work rewriting the python-for-android
packager to be more general, not just for Kivy's use but to support
other kinds of Python apps. I've meant to say something about it here
for a while, and this seems like a good milestone to bring it up.

p4a is a packaging tool for bundling Python itself, plus any extra
Python modules or other dependencies (including those with compiled
components such as numpy or Kivy itself), into an Android project that
can be compiled into a normal APK and run on any Android device. On a
technical level it's concerned specifically with compiling these
dependencies for the Android architecture and environment, so that
Python itself runs just the same as on the desktop. This comes with all
the standard advantages and disadvantages: on the bad side, Python can
be a little slow to start and there's no practical way to build a GUI
with native Java widgets (though small numbers of Java widgets can be
instantiated and manipulated via JNI). On the other hand, if these
aren't a concern (e.g. with games, certain kinds of apps, or just
personal preference) the big advantage is that most Python modules work
exactly as on the desktop, and even C extensions often require little or
no modification.

In the past, p4a only supported a basic Pygame backend for Kivy's own
use, but we noticed that a number of other projects were forking the
build tools and having to hack out the Kivy-specific bits (duplicating a
lot of work) to build other kinds of application. p4a now is a much more
general packager supporting multiple bootstrap types, each holding the
Java and Android project code to start Python and create a specific kind
of GUI. These currently include SDL2 (works with Kivy or other wrappers
like PySDL2 or Vispy), a webview bootstrap for use with e.g. a Flask
backend, and Kivy's old Pygame backend. It's also possible to build a
gui-less Python script, and there's some preliminary support for
exporting the Python build for use in mainly-Java apps. Other projects
can now use p4a by just adding or adapting bootstrap code to their
purposes, and nothing depends on Kivy (or much else).

The rewrite has also removed some long-standing limitations and allowed
adding some important new features. These include experimental support
for Python 3 apps via the CrystaX NDK [2] (whose additions to Google's
NDK allow Python to compile without patching even on older Android
versions), support for multiple architectures, and a much improved
internal infrastructure to make supporting new compiled dependencies
much easier.

Looking to the future, there are still plenty of things to fix or
improve about running Python on Android this way. Building Python (2 or
3) is made much easier with CrystaX even without the recent improvements
in Android's libc or in Python itself (I'll admit I'm not too familiar
with the current state of Python patching for Android builds), but these
parts of the toolchain need work to be as stable as the old patched
Python 2.7. Using platform-specific APIs can be pretty easy with JNI
wrappers like pyjnius or rubicon-java, but it can also be annoying and
it would probably be preferable to support more of them with Python API
wrappers like Plyer [3] instead. And of course, python-for-android is
still based around compiling everything locally, contributing to the
fact that it doesn't work on Windows and meaning that even on Linux or
OS X the process is more laborious than really necessary for most users.
We've focused on this method because it has to be done somewhere, and
binary distribution gets complicated if trying to be clever about
dependency options, but a major improvement would be to support
precompiled distributions so that users can build APKs without needing
to compile anything. I had this in mind when thinking about the new
toolchain, partly thanks to Russell setting an excellent example with
his BeeWare mobile distribution, so it shouldn't be too hard to start
supporting. It may even be possible to be clever about it, using wheels
built for Android.

So, that's the current state of python-for-android. Any comments,
questions or criticisms are very welcome!

Best,

Alexander


[1] https://github.com/kivy/python-for-android

[2] https://www.crystax.net/

[3] https://github.com/kivy/plyer

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