Python scripts for Android (Shallow Thoughts)

Akkana's Musings on Open Source Computing, Science, and Nature.

Wed, 28 Aug 2013

Python scripts for Android

Python on Android. Wouldn't that make so many things so much easier?

I've known for a long time about SL4A, but when I read, a year or two ago, that Google officially disclaimed support for languages other than Java and C and didn't want their employees working on projects like SL4A, I decided it wasn't a good bet.

But recently I heard from someone who had just discovered SL4A and its Python support and talked about it like a going thing. I had an Android scripting problem I really wanted to solve, and decided it was time to take another look.

It turns out SL4A and its Python interpreter are still being maintained, and indeed, I was able to solve my problem that way. But the documentation was scanty at best. So here are some shortcuts.

Getting Python running on Android

How do you install it in the first place? Took me three or four tries: it turns out it's extremely picky about the order in which you do things, and the documentation doesn't warn you about that. Follow these steps:

  1. Enable "Unknown Sources" under Application settings if you haven't already.
  2. Download both sl4a_r6.apk and PythonForAndroid_r4.apk
  3. Install sl4a from the apk. Do not install Python yet.
  4. Find SL4A in Applications and run it. It will say "no matches found" (i.e. no scripts) but that's okay: the important thing is that it creates the directory where the scripts will live, /sdcard/sl4a/scripts, without which PythonForAndroid would fail to install.
  5. Install PythonForAndroid from the apk.
  6. Find Python for Android in Applications and run it. Tap Install. This will install the sample scripts, and you'll be ready to go.

Make a shortcut on the home screen:

You've written a script and it does what you want. But to run it, you have to run SL4A, choose the Python interpreter, scroll around to find the script, tap on it, and indicate whether or not you want to see the console. Way too many steps!

Turns out you can make a shortcut on the home screen to an SL4A script, like this: (thanks to this tip):

This will give you the familiar twin-snake Python icon on your home screen. There doesn't seem to be any way to change this to a different icon.

Wait, what about UI?

Well, that still seems to be a big hole in the whole SL4A model. You can write great scripts that print to the console. You can even do a few specialized things, like popup menus, messages (what the Python Android module calls makeToast()) and notifications. The test.py sample script is a great illustration of how to use all those features, plus a lot more.

But what if you want to show a window, put a few buttons in it, let the user control things? Nobody seems to have thought about that possibility. I mean, it's not "sorry, we haven't had time to implement this", it isn't even mentioned as something someone would want to do on an Android device. Boggle.

The only possibility I've found is that there is apparently a way to use Android's WebView class from Python. I have not tried this yet; when I do, I'll write it up separately.

WebView may not be the best way to do UI. I've spent many hours tearing my hair out over its limitations even when called from Java. But still, it's something. And one very interesting thing about it is that it provides an easy way to call up an HTML page, either local or remote, from an Android home screen icon. So that may be the best reason yet to check out SL4A.

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[ 21:31 Aug 28, 2013    More programming | permalink to this entry | comments ]
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