Saturday, June 18, 2016

Unit Testing and Code Coverage in Python

Unit Test is one of the most important part of development artifacts, which helps to automatically test whatever code we write. Coverage report measures quality of Unit Tests and points out unused/untested code. This tutorial will help to get started with Unit Testing and Coverage report in Python.
For this tutorial we will use Eclipse PyDEV as Python IDE and its integrated Coverage plugin to generate Code Coverage report.

Environment Setup:
  1. Install PyDEV Eclipse Plug-in. PyDEV comes with Code Coverage feature (Show View - Code Coverage) which integrates with python module "coverage"
  2. Install Coverage module:  
# python -m pip install "coverage < 4.0.0"
Coverage module higher than version 3.x doesn't work with PyDEV. Coverage report gets generated as binary file under-
Sample Code: 
Download sample code from GitHub. This includes a function (under /src) to get ODD and EVEN numbers for a supplied Count. If supplied count is ODD, it returns all ODD numbers and vice versa. For this implementation, we have written Tests (under /test) which validates count of numbers returned. Count of numbers returned will be half of Supplied Count e.g. Supplied Count 100, it'll return 50 EVEN numbers.

Project Structure:
Create Python Project ("UnitTest") and then create 2 Source Folders under this Project- "src" and "test". Source codes are kept under "src" and respective Unit Test scripts under "tests" Folder. Unit Test scripts are created using same Package structure as of the actual script but Module name prefixed with "test_".

Unit Tests:
Python (since 2.1) includes Unit Test package (unittest). It helps to test scenarios of functions/classes and assert conditions. We'll cover more about unittest functionality in upcoming tutorials. In this session, I'll cover very simple assertion (assertEqual), which validates equality.

import unittest
import as data

class Test(unittest.TestCase):

    def testData(self):
        count = 9
        expectedCount = count/2
        nums = data.getNumbers( count )
        self.assertEqual( len( nums ), expectedCount, 'Count mismatch...')

if __name__ == "__main__":

Above code validates functionality of function. If we run this Test (Right click on - Run As - Python unit-test)

Coverage Report:
Once we execute above unit test, internally it generates Coverage Report (.coverage File). Now lets open "Code Coverage"view (Window - Show View - Code Coverage).
Drag and Drop our Project "UnitTest" at the bottom area of "Code Coverage" view.

If you have already run the Unit Tests, it'll show report on right-hand side of "Code Coverage" view. As you can see, we are passing only EVEN count as a part of Unit Test, so it has marked else section and _odd() functions as untested.

Isn't it Cool ? It points out any untested code and measures accuracy of your Unit Tests.

Wednesday, September 18, 2013

Android Looper and Toast from WorkerThread

Have you ever tried to launch Android Toast message from worker thread ? Probably you are wondering why the heck it is giving this error-

java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

In this article we are going to explore reason behind the above exception and try to understand how Looper works in Android. At the end, I am going to explain one approach to run Toast from a worker thread, but before that we need to understand how Looper works. If you already know in/out of Looper, you can skip below section and directly move to the solution part.

Looper is a very basic wrapper class which attach a MessageQueue to a Thread and manage this queue. MessageQueue is a structure to sequentialize simultaneous processing requests of a Thread.  In Android, message/request processing classes like Handler uses Looper to manage their respective MessageQueue.
Looper = Thread + MessageQueue

Android Looper Life Cycle:
As you can see in the above figure, Looper Life cycle starts when we call prepare(), this static method creates instance of Looper class and store this in a ThreadLocal variable. Below code snippet from
    private static void prepare(boolean quitAllowed) {
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        sThreadLocal.set(new Looper(quitAllowed));

    private Looper(boolean quitAllowed) {
        mQueue = new MessageQueue(quitAllowed);
        mRun = true;
        mThread = Thread.currentThread();
Once Looper instance is created we have to call loop() which starts an infinite Loop and process requests accumulated in the Message Queue. Below code from
    public static void loop() {

    // Looper instance and Thread verification

        final MessageQueue queue = me.mQueue;

        for (;;) {
            Message msg =; // might block
            if (msg == null) {
                // No message indicates that the message queue is quitting.
            // Process Messages

Due to above Infinite Loop, Looper blocks the current Thread execution until we call Quit. To get a hook to know when Looper is done with Message processing or when Message Queue is empty, we need to register MessageQueue.IdleHandler listener to the MessageQueue associated to the respective Looper. This can be achieved as- 
// Prepare looper
// Register Queue listener hook
MessageQueue queue = Looper.myQueue();
queue.addIdleHandler(new IdleHandler() {
     public boolean queueIdle() {
          // TODO Auto-generated method stub
          return false;
// Start looping Message Queue

Running Toast from Worker Thread
Now, lets explain how we can run Toast in a thread other than UI or Main Thread. If we look inside of, when we call makeText() internally it initialize Handler on the Calling thread. So, if the calling thread has not yet been attached to a MessageQueue Toast fails to create its instance. The only way to attach MessageQueue to a Thread is to use Looper, as a result throws below exception-
 java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare() -> ->

public Toast(Context context) {
        mContext = context;
        mTN = new TN();
        mTN.mY = context.getResources().getDimensionPixelSize(

public Handler() {
        // .......
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        mQueue = mLooper.mQueue;
        mCallback = null;

So, I hope we now understand the real meaning of this famous error message. Lets now try an alternate approach to execute Toast in a separate Thread.

We need to initialize Looper and attach MessageQueue to our calling Thread prior to launch Toast. When we call show() method of Toast, it sends Handler message to process display logic of Toast, then it waits for defined time (LONG/SHORT) and then calls hide() which again sends Handler message to remove Toast View. Thus Looper associated to Toast is requesting for MessageQueue twice, so we'll keep a counter to track number of MessageQueue requests and when it is 2, we'll quit the Looper. It is really important to quit the Looper, as if we don't do so, it will block all further operations, since Looper.loop() is a blocking call.

Here is the code snippet to run Toast in a separate Thread-

     public void aboutLooper() {
         Thread th = new Thread() {
              public void run() {
                  System.out.println("Start Looper...");
                  // Prepare looper

                  // Register Queue listener hook
                  MessageQueue queue = Looper.myQueue();
                  queue.addIdleHandler(new IdleHandler() {
                       int mReqCount = 0;

                       public boolean queueIdle() {
                           if (++mReqCount == 2) {
                                // Quit looper
                                return false;
                           } else
                                return true;

                  // Show Toast- will be called when Looper.loop() starts
                  Toast.makeText(MainActivity.this, "Hey there!!",
                  // Start looping Message Queue- Blocking call
                  System.out.println("It appears after Looper.myLooper().quit()");