Python for Android

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Think Java's the only game in town when it comes to programming Android apps? Think again.
Creating Scripts for Android

Your Android emulator now is ready to run your custom Python script, so let's create one. Before you do though, note that the published SL4A API is a subset of the full Android API, so certain features either are not available, in the process of being made available or fully supported (see Resources for a link to the current list). Don't let this put you off. What's there is more than enough to produce usable Android apps in Python.

A Sample Script

To get a feel for Python running on SL4A, let's port an existing script to the phone. The script in question is based on some code from Chapter 2 of O'Reilly Media's Head First Programming, which I cowrote with David Griffiths in 2009. This simple script connects to the Web site of a fictitious company called Beans'R'Us to grab the company's home page and extract the current price of coffee beans from the page's HTML. The code is straightforward, grabbing the HTML page from the server, searching for the pricing data, extracting it from the HTML page and then displaying it on screen:

from urllib import urlencode
from urllib2 import urlopen

pg = urlopen("http://www.beans-r-us.biz/prices.html")

text = pg.read().decode("utf8") 
where = text.find('>$') 
start_of_price = where + 2 
end_of_price = start_of_price + 4 
price = float(text[start_of_price:end_of_price])

print "The current price of coffee is:", price

This is Python 2 code, which is a deliberate choice, as the Python that comes with SL4A is the 2.6.2 release. To take this program for a spin, either load it into Python's IDLE tool or execute it from the command line:

$ python LJapp-cmd.py 
The current price of coffee is: 5.52

As you can see, this small script displays the currently published price of coffee beans.

Porting the Sample Script to Android

Turning this script into an Android app is just a matter of deciding on the Android UI elements you want to use, as the core functionality does not need to change. The Python on SL4A is fully functional, so the facilities you are used to with standard Python also are available on your smartphone.

To make this script more Android-like, let's display a friendly message on startup as well as one on exit. The makeToast() API call provides this functionality.

The dialogCreateSpinnerProgress() API call lets you display an Android spinner dialog, assuming you then remember to call the dialogShow() API call to make it visible. Let's display a spinner prior to requesting the Web page from the Beans'R'Us server, then dismiss the spinner dialog with the dialogDismiss() API call, once we have the data processed. And, let's vibrate the phone at this point too, just for the fun of it.

To conclude the script, use the dialogCreateAlert(), dialogSetItems() and dialogSetPositiveButtonText() API calls to display the price of beans within an Android dialog. To exit, simply tap the OK button.

Here's the Python code from earlier with the calls to the SL4A API added in:

import android

from urllib import urlencode
from urllib2 import urlopen

app = android.Android()

app.makeToast("Hello from LJapp")

appTitle = "LJapp"
appMsg = "Checking the price of coffee..."

app.dialogCreateSpinnerProgress(appTitle, appMsg)
app.dialogShow()

pg = urlopen("http://www.beans-r-us.biz/prices.html")
text = pg.read().decode("utf8") 
where = text.find('>$') 
start_of_price = where + 2 
end_of_price = start_of_price + 4 
price = float(text[start_of_price:end_of_price])

app.dialogDismiss()
app.vibrate()

appMsg = "The current price of coffee beans:"

app.dialogCreateAlert(appMsg)
app.dialogSetItems([price])
app.dialogSetPositiveButtonText('OK')
app.dialogShow()
resp = app.dialogGetResponse().result

app.makeToast("Bye!")

Other than the addition of the Android UI code, no other changes are required to the code from earlier, other than removing the earlier script's call to print (which is no longer required).

Transferring Your Script to the Emulator

To transfer your Python script to the emulator for testing, copy your code file into your Android directory, then use the adb utility within the tools directory to push your file to the SL4A scripts directory on the emulator:

$ tools/adb push LJapp.py /sdcard/sl4a/scripts
6 KB/s (748 bytes in 0.116s)

With the file transferred, check the list of scripts within SL4A and notice the addition of LJapp.py near the top of the list.

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Looks like Paul's been time

Anonymous's picture

Looks like Paul's been time travelling again. i like the article though. It covers exactly what i have spent the past hour searching for.

Well at least you got the

Jayson's picture

Well at least you got the info a week early! ;) This is a very comprehensive article about app programming. While I don't have the technical expertise to craft the SL4a technique without conquering a steep learning curve, it does open my eyes to a new way to do things. This is particularly helpful since I've been learning how mobile solutions are being tapped into by users. As one can imagine, a company that does mobile marketing(example: http://www.exacttarget.com) is going to have different needs then a company that specializes in manufacturing, or a user who wants to simplify his/her "mobile" life. Right now I fall into the latter category, but that doesn't mean I won't have a vastly different agenda in regards to app programming in the future.

Time will tell if Android development via Python is something I will tap into, but it's nice to have the programming app horizon broadened. Options are good!

Amazing!

DaNmarner's picture

this essay is posted on "Apr 30, 2011 By Paul Barry in Mobile". And today is Apr 23.

I noticed that to. I wonder

Justin C's picture

I noticed that to. I wonder what planet he was on when he published the article.

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