[Python-ideas] Adding iOS/Android support to Python

[Guido van Rossum]

This idea seems to be in the air. Just yesterday someone posted a set of
patches and suggestions to python-dev about making cross-compilation for
Android easier. I expect most people in the core Python team might not be
eager to jump on this bandwagon because they have no immediate need, but
these mobile platforms are only getting faster more popular, and I think it
would be a good idea to be prepared for when suddenly everybody wants to
develop mobile apps in Python.

On Fri, Oct 24, 2014 at 7:20 PM, Russell Keith-Magee <
russell at keith-magee.com> wrote:

> Hi all,
>
> tl;dr - I'd like to add iOS and Android to the list of platforms that
> Python supports out-of-the-box, and I'm looking for guidance on the way
> forward (and assistance, if anyone is keen).
>
> Longer version: For the last 9 months or so, I've been working on tools to
> enable developers to write native mobile apps using Python. My motivation
> is to get Toga (http://pybee.org/toga) to the point where it's a viable
> way to write mobile apps. However, I'm not alone in the "Python on mobile"
> goal - Kivy has been around for a few years, with similar goals (although
> they've got some different ideas and priorities).
>
> To date, I've been using the groundwork established by Kivy, which takes
> the Python 2.7.1 (for iOS) and Python 2.7.2 (for Android) source code,
> applies a bunch of patches, and wraps those in a set of build scripts to
> yield Python libraries that can be used on iOS and Android.
>
> I'd like to start supporting more recent versions of Python - most
> importantly, Python 3. While I could certainly continue maintaining patches
> externally (and I imagine I'll have to if I want to maintain Python 2.7
> support), I'd like to see that effort integrated into Python's repository.
>
> In particular, there are four areas where I can see changes required:
>
>  1) At least 2 new sys.platform definitions - "ios" and "android" (a third
> "iossimulator" platform might also be needed - the iOS simulator has enough
> differences that it can be helpful to be able to identify the platform)
>
>  2) Modifications to the build system to support cross-platform builds for
> mobile - AFAICT, cross platform builds currently only work on Linux and
> require readelf; iOS libraries can only be built on Mac, and building
> Android libraries don't require readelf (AFAICT).
>
>  3) Disabling certain modules on mobile platforms. Supporting modules like
> linuxaudiodev, ossaudiodev, readline, curses, idle and tkinter on mobile
> platforms doesn't make much sense; modules likes bsddb and bz2 are
> difficult to support due to library dependencies; and the need for modules
> like multiprocessing is arguable (and difficult to support on mobile). Even
> providing a Python executable/shell is arguable for these platforms.
>
>  4) Support for building fat libraries for iOS. Xcode uses a tool called
> lipo to mash multiple platform libraries into a single library file.
>
> I don't imagine 1 & 2 are especially controversial.
>
> However, I would anticipate that 3 might raise some concerns, as the
> general Python "batteries included" philosophy would be modified to "unless
> you're on one of these platforms". Any guidance on whether this approach
> would be palatable? Is there any precedent for "module not supported on
> platform X" that I'm not aware of? Any platforms where Python is *only*
> available as a library?
>
> I have no idea how to tackle 4. To create a complete iOS build, you have
> to do at least 4 complete Python builds - a native system build (so you can
> run Python scripts that are part of the build), an i386 build (for the
> simulator), an armv7 build, and an arm64 build - and then the build
> products of the last three need to be combined into a single framework
> directory.
>
> Given three independent platform-specific build directories, it's
> relatively easy to construct a "fat" framework, but it isn't clear to me
> how that would fit into Python's build system. I can see that the build
> system has support for "universal" builds, which looks like an artefact of
> the OS X PPC->i386 conversion, but the build approach appears to be
> different to that required for iOS.
>
> Is there any documentation of the build process itself? Or is there anyone
> who can point me in the right direction?
>
> I'm also not clear how to tackle the testing and CI problems. Since you
> can't run iOS code on a non-iOS machine, it isn't entirely clear to me what
> an acceptance test would look like for a mobile build.
>
> So - am I completely wasting my time? Are patches for mobile platforms
> likely to be accepted into Python's repository? Or should I stick with the
> "external patches" approach? Any pointers, hints, guidance or assistance
> would be gratefully accepted.
>
> Yours,
> Russ Magee %-)
>
>
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-- 
--Guido van Rossum (python.org/~guido)
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