Andrzej on Test Driven Development

Test Driven Development helps me creating better software. TDD is not only about testing. It's more about designing and managing scope. I'll try to show what I mean by using a simple example based on tabbedimages application.

Tabbedimages is a simple image viewer. We are going to add drag and drop feature to it.
I start with a requirement, which I analyze and spike for a working solution. Based on the analysis I create a user story and an automated acceptance test. The acceptance test "drives" me when I'll add unit tests and the production code.

• Requirement
• tabbedimages is a simple image viewer.
• We want to add a new feature to tabbedimages.
• Title.
• 'Drag&drop support'.
• TIP: It's good to have a short title for a user story.
• Analysis: let's list things to worry about:
• Single image files
• Multiple image files
• Directories
• Non-image files
• Already open image.
  
• Spiking
• TIP: This part should give us better understanding of the problem
• If we're sure how to implement the new feature we can skip this step.
  
• tabbedimages is implemented using IronPython and Windows Forms.
• The code is availalable here.
  
• Google for 'drag and drop windowsforms' and see some code examples.
• Check out the fresh version of tabbedimages.
  
• Try to add the required feature to our code base (without tests).
• Discover that Windows Forms has support for DragDropEffect.
  
• Which displays a 'plus' sign if the thing that we're trying to drag is acceptable.
• Add the DragDropEffect to the list of things to worry about.
• User story:
• Marten wants to drag and drop his images from his desktop to tabbedimages.
• He starts tabbedimages.
• He then drags the 'Faye001.jpg' file over the application.
• The plus sign appears.
• He drops it.
• A new tab is created with a label saying 'Faye001.jpg'
• 'She's cute' he thinks
• Marten realizes that there are more Faye's pictures.
• He drags 'Faye001.jpg' (again) and 'Faye002.jpg'.
• He drops them.
• Two new tabs are created.
• He drags and drops readme.txt file.
• The message box appears saying 'readme.txt doesn't appear to be a valid image file'
• He quits tabbedimages.
• Functional test:
• Write the ideal code (DSL-like) that follows the user story steps:
  

marten.starts()
he.asserts_number_of_tabs(0)
he.drags('Faye001.jpg')
assert shows_plus()
he.drops('Faye001.jpg')
he.asserts_number_of_tabs(1)
he.asserts_tab_labels(['Faye001.jpg'])
fayes_pictures = ['Faye001.jpg', 'Faye002.jpg']
marten.drags_and_drops(fayes_pictures)
marten.drops(fayes_pictures)
he.asserts_number_of_tabs(3)
he.asserts_tab_labels(['Faye001.jpg', Faye001.jpg', Faye002.jpg'])
he.drags_and_drops('readme.txt')
he.sees_message_box("readme.txt doesn't appear to be a valid image file")
he.quits()

• Implementation
  
• Run the Functional Test (FT).
• Whenever FT fails or you can think of any edge cases not covered by FT:
  
• Write appropriate Unit Test that reflects the problem.
• Add the implementation to fix the problem.

After the last phase the drag & drop feature should be ready to use. Of course, this example presented a very simple problem. Hopefully, the Test Driven Development as shown here should be easy to understand. In the near future, I'll try to write more about the implementation phase, including some refactoring techniques, so stay tuned!

Doctests for showing snippets of code

From Public

• djangosnippets.org shows many snippets of Python/Django code along with doctests.
• Have a look at partitioning lists example.
• This is what I think a good usage of doctests.
  
• I'm not sure about using them *instead* of unittests.
• Michael is also not sure
• BTW, I find many of those Python code samples really pretty.
• I couldn't find any with the string 'self' inside.
• Maybe that's why I like them ;-)
  

def partition(thelist, n):
"""
Break a list into ``n`` pieces. The last list may be larger than the rest if
the list doesn't break cleanly. That is::

   >>> l = range(10)

   >>> partition(l, 2)
   [[0, 1, 2, 3, 4], [5, 6, 7, 8, 9]]

   >>> partition(l, 3)
   [[0, 1, 2], [3, 4, 5], [6, 7, 8, 9]]

   >>> partition(l, 4)
   [[0, 1], [2, 3], [4, 5], [6, 7, 8, 9]]

   >>> partition(l, 5)
   [[0, 1], [2, 3], [4, 5], [6, 7], [8, 9]]

"""
try:
   n = int(n)
   thelist = list(thelist)
except (ValueError, TypeError):
   return [thelist]
p = len(thelist) / n
return [thelist[p*i:p*(i+1)] for i in range(n - 1)] + [thelist[p*(i+1):]]