Python provides several ways to test your code. One such method is using the doctest module. This Python module will inspect your source code and execute test programs found in your comments. It is perhaps the simplest and easiest testing software to use with Python.

The code below shows a simple output of a function, doubleit(), being tested. Note that the three asterisks are for multi-line comments. The three greater-than signs (>>>) mimic Python's interactive prompt. You type in how you would execute that function interactively. Note the space between the >>> and the text. This space is required. Then on the next line you type in the expected answer. Do not place a space after this answer. If you do, Python issues an error and believes that 4 followed by a space is not the same as 4.

The function doubleit() doubles the value passed to it. The programmer specified the test in the comments by showing it being called with the value of 3. Mistakenly, the programmer expects the double of 3 to be 4, instead of 6. Fortunately, this will trigger the output of doctest so you can see what the output looks like. If there are errors, then doctest returns the messages to stdout. If there are no errors, nothing is returned to stdout.

	  def doubleit(n):
		"""
		This is example test
		>>> doubleit(3)      #  <---- call the function with this value
		4                    #  <---- what you expect to see (WRONG so you can see results)
		"""
		return 2 * n
		
	  def _test(): 
		import doctest
		doctest.testmod()
		
	  if __name__ == "__main__":
 		_test()			#<--- toggle on/off your test by commenting out this line
			  
      

This program is run from the Windows command line. The errors are echoed out to stdout. If you redirect the output into a file, there may be trouble using long filenames with spaces inside quotes (as required by Windows). The easiest way is to just specify an output filename with no path and the doctest will create the file in the same directory as the Python executable.

The program would be called with the following command at the DOS command line:

python "c:\joy of\joy of python programming\test_doctest1.py" > test_results.txt

The text below shows the output of doctest(). Notice it shows the expected and the actual values.

**********************************************************************
File "c:\joy of\joy of python programming\test_doctest1.py", line 4, in __main__.doubleit
Failed example:
    doubleit(3)
Expected:
    4
Got:
    6
**********************************************************************
1 items had failures:
   1 of   1 in __main__.doubleit
***Test Failed*** 1 failures.

The beautiful concept of doctest is that the programmer can sprinkle these unit tests in each function. In fact, the programmer can specify multiple tests of the same function by just repeating the interactive prompt (>>>) call sequence with the next line specifying the answer.

There are many more options including an API which the programmer can call directly. Please see the documentation for further details:

http://www.python.org/doc/lib/module-doctest.html for documentation on doctest.