The doctest and unittest modules of Python allow you to provide test harnesses for your classes / packages. Designing applications from the bottom up, you'll want to ensure that each of your code levels works and works well. You'll want to provide an example of what it should do for the next level or programmer up, and you may want to be able to run the code time and time again - perhaps to test some device or algorithm you're plugging in underneath - and get a test report out from that.

With the doctest module, you initially run the python class from the interactive command line and the cut and paste the interaction into a documentation string. When you then run the code to test it, the doctest module checks that the test reproduces the same output that's in the docstring. There's an example here.

The unittest module requires you to write a series of test methods, each starting with the word "test" in their name and including code that asserts what conditions they should find after completion - perhaps an equality, or a certain value exists somewhere in a list. And at the end of all the tests, the module produces a report of which tests passed and failed. There are examples of unittest code here and here

Here's an example of unittest - I have added another test to the source code examples above (which test the system's random module!) and in this first case it all worked.

92:1 grahamellis$ python utter
testchoice (__main__.TestSequenceFunctions) ... ok
testflop (__main__.TestSequenceFunctions) ... ok
testsample (__main__.TestSequenceFunctions) ... ok
testshuffle (__main__.TestSequenceFunctions) ... ok
 
----------------------------------------------------------------------
Ran 4 tests in 0.007s
 
OK
92:1 grahamellis$

Now altering the code to assert that 42 is a number that can be found in a list of suffled numbers from 0 to 10 (which clearly it is not!) I got the following result:

92:1 grahamellis$ python utter
testchoice (__main__.TestSequenceFunctions) ... ok
testflop (__main__.TestSequenceFunctions) ... FAIL
testsample (__main__.TestSequenceFunctions) ... ok
testshuffle (__main__.TestSequenceFunctions) ... ok
 
======================================================================
FAIL: testflop (__main__.TestSequenceFunctions)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "utter", line 21, in testflop
    self.assert_(element in self.seq)
AssertionError
 
----------------------------------------------------------------------
Ran 4 tests in 0.100s
 
FAILED (failures=1)
92:1 grahamellis$

Python also has a profiler module that allows you to see your code coverage, and you should always remember to add a short test harness onto the end of your modules to ensure that the module, when loaded stand alone, is selfvalidating. There's an example here.
(written 2009-09-16, updated 2009-09-17)

 Y116 - Python - Applying OO design techniques and best practise
  [340] Code and code maintainance efficiency - (2005-06-08)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [668] Python - block insets help with documentation - (2006-04-04)
  [836] Build on what you already have with OO - (2006-08-17)
  [945] Code quality counts - (2006-11-26)
  [1181] Good Programming practise - where to initialise variables - (2007-05-09)
  [2363] Alpaca Case or Camel Case - (2009-08-16)
  [2485] How do I set up a constant in Python? - (2009-10-31)
  [2523] Plan your application before you start - (2009-12-02)
  [2604] Tips for writing a test program (Ruby / Python / Java) - (2010-01-29)
  [3887] Inheritance, Composition and Associated objects - when to use which - Python example - (2012-10-10)
  [4028] Really Simple Class and Inheritance example in Python - (2013-03-04)
  [4118] We not only teach PHP and Python - we teach good PHP and Python Practice! - (2013-06-18)
  [4359] How to avoid too many recalculations within an object - (2014-12-21)
  [4718] Defining an object that is a modified standard type in Python - (2016-11-02)

Y115 - Additional Python Facilities
  [183] The elegance of Python - (2005-01-19)
  [208] Examples - Gadfly, NI Number, and Tcl to C interface - (2005-02-10)
  [239] What and why for the epoch - (2005-03-08)
  [463] Splitting the difference - (2005-10-13)
  [663] Python to MySQL - (2006-03-31)
  [672] Keeping your regular expressions simple - (2006-04-05)
  [753] Python 3000 - the next generation - (2006-06-09)
  [901] Python - listing out the contents of all variables - (2006-10-21)
  [1043] Sending an email from Python - (2007-01-18)
  [1136] Buffering output - why it is done and issues raised in Tcl, Perl, Python and PHP - (2007-04-06)
  [1149] Turning objects into something you can store - Pickling (Python) - (2007-04-15)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1337] A series of tyre damages - (2007-09-08)
  [1876] Python Regular Expressions - (2008-11-08)
  [2435] Serialization - storing and reloading objects - (2009-10-04)
  [2462] Python - how it saves on compile time - (2009-10-20)
  [2655] Python - what is going on around me? - (2010-02-28)
  [2721] Regular Expressions in Python - (2010-04-14)
  [2745] Connecting Python to sqlite and MySQL databases - (2010-04-28)
  [2746] Model - View - Controller demo, Sqlite - Python 3 - Qt4 - (2010-04-29)
  [2764] Python decorators - your own, staticmethod and classmethod - (2010-05-14)
  [2765] Running operating system commands from your Python program - (2010-05-14)
  [2786] Factory methods and SqLite in use in a Python teaching example - (2010-05-29)
  [2790] Joining a MySQL table from within a Python program - (2010-06-02)
  [3089] Python regular expressions - repeating, splitting, lookahead and lookbehind - (2010-12-17)
  [3442] A demonstration of how many Python facilities work together - (2011-09-16)
  [3469] Teaching dilemma - old tricks and techniques, or recent enhancements? - (2011-10-08)
  [4085] JSON from Python - first principles, easy example - (2013-05-13)
  [4211] Handling JSON in Python (and a csv, marshall and pickle comparison) - (2013-11-16)
  [4298] Python - an interesting application - (2014-09-18)
  [4439] Json is the new marshall, pickle and cPickle / Python - (2015-02-22)
  [4451] Running an operating system command from your Python program - the new way with the subprocess module - (2015-03-06)
  [4536] Json load from URL, recursive display, Python 3.4 - (2015-10-14)
  [4593] Command line parameter handling in Python via the argparse module - (2015-12-08)
  [4709] Some gems from Intermediate Python - (2016-10-30)

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