Beginning IronPython

Okay, so you’ve read the chapter on IronPython syntax. Brilliant, learning a new programming language is easy! You feel like such a polyglot. What’s all the fuss about?  All that remains now is a few formalities. Install the framework and knock out a quick app to demonstrate how easy it is. The only problem is that nothing works and the error messages make no sense.

Back to the world of the programming beginner

You need to learn a few practical lessons before you can say that you are a competent Python programmer. It is important to be able to interpret the common error messages and structure your project.  Normally you will find that the official documents will gloss over this stuff because they like to leave it to your discretion. As a result you will find yourself searching the internet for a blog to help!

  • You can execute from the command line with ipy.exe, use the IronPython Console or the VS Tools.

Exercise calling C# code from IronPython

Use IronPython to create some tests/specifications; one static .NET CLR class and one normal .NET CLR class. The classes will be contained in an assembly project called Polyglot, as we want to use multiple programming languages in this example.

  • Greet static class. This will have one method and one property. The method accepts a parameter, a name, and the returns a greeting for that name, e.g. “Hello Jon”. The property simply returns a string “Hello World!”.
  • Math class. This class deals with simple arithmetic, does not handle negative numbers maintains a list of answers. It has Add and Subtract methods and the Subtract method throws an exception if it cannot return a positive number. It also has an array containing all the results with the most recent result at the beginning of the list.

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These unit tests will demonstration how to use a unit testing framework, import and run CLR code and the difference between an instance and a type variable in Python.

Points to Note

  1. line 2: import unittest
    Use the default unit test frameworks which is similar to NUnit in .NET.
  2. line 4: sys.path.Add('C:\\sut')
    We have added a directory to the system paths. This is where I have stored to Polyglot.dll will be tested.
  3. line 5: clr.AddReferenceToFile('Polyglot.dll')
    Add a reference to Polyglot assemble so it can be called via the CLR.
  4. line 7: from Polyglot import Greet
    Specify that the Greet class in the Polygot namespace will be used. There is no equivalent command in C# because C# intrinically uses all the types in a namespace, i.e. using namespace.
  5. line 9: class GreetTest(unittest.TestCase):
    Brackets are used to inherit from a class
  6. line 10: softwareUnderTest = Greet
    A reference to the C# Greet class type has been created. This is a static class so we can call methods on the class by call the methods directly from the type reference.
  7. line 19: softwareUnderTest = Math()
    An instance of the C# Math class has been created. The new keyword is not required like in C# however we must remember to use brackets.
  8. Method definitions & self. All class level variables, methods and types are referenced with self.
  9. Indentation and white-space are important. Python uses these instead of the {} in C based languages.
  10. line 46: unittest.main() Used to execute the unit tests.

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Example IronPython Test Script

.NET Dynamic Language Runtime

import sys
import unittest
import clr
sys.path.Add('C:\\sut') # folder containing software to be tested

from Polyglot import Greet

class GreetTest(unittest.TestCase):
    softwareUnderTest = Greet
    def testGreet_ShowCsGreeting_GreetWithName(self):     
        self.assertEquals('Hello Brian', self.softwareUnderTest.GreetMe('Brian'), 'Greet Me test')        
    def testGreet_ShowCsGreeting_GreetHello(self):  
        self.assertEquals('Hello World!', self.softwareUnderTest.Hello, 'Greet Hello test')

from Polyglot import Math

class MathTest(unittest.TestCase):
    softwareUnderTest = Math()
    def testMath_Add_Simple(self):
        self.assertEquals(3, self.softwareUnderTest.Add(1,2), 'Simple Add')
    def testMath_Subtract_Simple(self):
        self.assertEqual(3, self.softwareUnderTest.Subtract(5,2), 'Simple Subtract')
    def testMath_Subtract_NegativeResultFails(self):
        with self.assertRaises(ValueError):
            self.softwareUnderTest.Subtract(3, 4)
    def testMath_Results_IsSetAndOrdered(self):
        result1 = 2
        result2 = 3
        result3 = 4
        self.softwareUnderTest = Math()
        self.assertEqual(0, len(self.softwareUnderTest.Results)) # to begin with there are no results
        self.assertEqual(1, len(self.softwareUnderTest.Results)) # after one operation there will be one result
        self.assertEqual(result1, self.softwareUnderTest.Results[0]) # check the result

        # run a couple more operations and check they are stored propery
        self.assertEqual(result3, self.softwareUnderTest.Results[0])
        self.assertEqual(result2, self.softwareUnderTest.Results[1])
        self.assertEqual(result1, self.softwareUnderTest.Results[2])

    if __name__ == '__main__':
    e = sys.exc_info()[0]
    print( "Error: %s" % e )
    clr.ClearProfilerData() # unload the .dll file so the file lock is released

a=raw_input('The end!')

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