The latest drop of base4

Well, I've started working with the latest drop of Base4.net (version 2.1) - in fact I've decided to take the plunge and actually start using it for the project I'm currently working on (a work project, rather then a home project) so I should be giving it a pretty good thrashing over the next 3-6 months.  I already had a rough schema in place, so I generated types from it using the bundled web admin... It seems pretty cool... though I've struck a few things:

  • nvarchar(256) fields seems to show up as being 512 characters in length within base4.net...  I'm not sure if this is a bug with the schema interrogation, guess we'll find out as a I dig further in ;o) - though it's only skin deep of course, as you can change them to 256 with a quick edit.

  • I managed to create an assembly that was invalid... not sure how ;o) I could look at in reflector .Net, but base4 wasn't having a bar of it (even though it had generated it) - I must have screwed up a relationship somewhere, but I wasn't really sure... ended up just dumping it and starting from scratch again.

  • It doesn't seem that easy to refactor existing schema's using the web designer... while I was learning it would have been good to generate a schema, play with the assembly, then start editing and generate again... instead I ended up having to save the schema to disk, delete, edit, and recreate.  Still, it's pretty slick :)


I think though it probably all comes down to:
  • My urge to make things happen without understanding the in's and outs of base4.net
  • Horses for courses, the web admin is great for getting off to ground with base4.net, but I think I'll have to get down and dirty with the xml pretty soon...


Though I can't discuss the project, I can discuss what I'll be building it in... so we have:
  • Base4.Net
  • SQL 2005
  • Castle (Monorail & IoC)
  • NLog (or log4net... though I'm loosing faith in the latter)
  • NUnit
  • RhinoMocks
  • Splicer.Net
  • Some COM & Unmanaged code for audio processing

This is also the first time I've used monorail for a commercial project (we already use a lot of the facilities and the windsor container in the Syzmk RMP product) so it's certainly some interesting times... as we progress I'll give some feedback on both the Monorail and Base4.Net "user experience" :)
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Sourcode for "Integrating NUnit & IronPython"

Back again!


Here is the source for the last post: ironpython-and-nunit.zip (23.01 KB) - it's pretty rough around the edges (and in the middle :P) because I was just trying to see if it was possible, rather then to produce some production-level toolkit for integrating IronPython and NUnit.



Edit 2008-03-14: An updated version of the code target
NUnit 2.4 and VS2008 can be found here.



Usage

First off... usage is pretty straightforward.
  • Test fixtures must have NUnitFixture as one of their base classes.
  • Test methods must be prefixed with "test".
  • The setup method, if you need one, must be called "setUp"
  • The teardown method, if you need one, must be called "tearDown"
  • Fixture setup and teardown methods are also supported, they are called "setUpFixture" and "tearDownFixture".
  • Assert.XXX methods are the same as NUnit, unless you want to assert an exception is thrown.
  • For asserting an exception is thrown, make a call to "self.failUnlessRaises(...)" - it has the same syntax as the PyUnit equivalent, however it's tweaked slightly so it will work with Python exceptions, or CLR exceptions.
  • Do not perform an "Import * from System" in your test fixtures - it will clober the native Python Exception type, and may give you odd behaviour.




Implementation

There are 8 classes and one python script which make up the implementation, the 8 classes are split between the "model"
(basically the python-side of the equation and the "NUnit extension" which are the necessary extensions to expose the model to NUnit...) - let's run through it quickly:



First we have the AbstractPythonSuite which hosts the python engine, and handles configuring and running the scripts containing the test fixtures.  Then we have a couple of classes which form the "model" consisting of a PythonFixture, representing a fixture, and PythonTestCase, representing a single test method in the fixture.



To build the model we have a PythonFixtureBuilder which is passed the python engine (from within the AbstractPythonSuite) and spits out all the fixture models, with the test cases assigned... this also assigns the various delegates for the setup/teardown/setupFixture/teardownFixture methods.



At this point the model is complete, it's just a matter of integrating it with NUnit - for this we have some more classes.



First of we need the PythonSuiteExtension, which represents the overall test suite, this contains PythonFixtureExtension's, for the fixtures, and finally PythonTestMethod for the individual test methods - the python extensions take care of running the tests in the model and calling the setup and tear down methods at the right times, and recording the results of course.



Last of all we have the PythonSuiteExtensionBuilder as mentioned in the last post, which takes care of creating the PythonSuiteExtension's for each class derived from AbstractPythonSuite it discovers in the target assembly.



Last of all we have a small python script which provides the necessary implementation for the base NUnitFixture class that all test fixtures are derived from... and a helper method we use in the PythonFixtureBuilder to identify the test methods in a class.


class NUnitFixture:
def __init__(self):
pass
def setUp(self):
pass
def tearDown(self):
pass
def setUpFixture(self):
pass
def tearDownFixture(self):
pass
def failUnlessRaises(self, excClass, callableObj, *args, **kwargs):
if issubclass(excClass, System.Exception):
try:
callableObj(*args, **kwargs)

except Exception, e:

if hasattr(e, "clsException") and
(type(e.clsException) == excClass):
return
else:
try:
callableObj(*args, **kwargs)
except excClass:
return
if hasattr(excClass,'__name__'):
excName = excClass.__name__
else:

excName = str(excClass)

raise AssertionException("%s not raised" % excName)

def getTestCaseNames(testCaseClass, prefix):
def isTestMethod(attrname, testCaseClass=testCaseClass, prefix=prefix):
return attrname.startswith(prefix) and callable(getattr(testCaseClass, attrname))
testFnNames = filter(isTestMethod, dir(testCaseClass))
for baseclass in testCaseClass.__bases__:
for testFnName in getTestCaseNames(baseclass, prefix):
if testFnName not in testFnNames:
testFnNames.append(testFnName)
testFnNames.sort(cmp)
return testFnNames



And that's all there is to it...  If anyone actually finds this useful and wants to build on from it, please feel free, and if at all possible keep it open source (though you don't have to...)



Something to keep in mind is that at the moment if your test fixture can't actually be parsed and executed by the python engine, that exception will bubble up, eventually stopping the test assembly from being loaded at all... I think the nicer solution to this would be to add a custom NUnit test case for each script that failed to load, which in turn fails when executed as part of the assembly - dumping out the exception raised during construction.



At least then a tool like TestDriven.Net would quickly notify the developer of their mistake, instead of just falling over.



And very last of all, though I've been manually specifying the names of the script files.. using a class declaration like this:


public class MyPythonSuite : AbstractPythonSuite
{
public MyPythonSuite()
: base("MyPythonFixture.py")
{
}
}


You could make it a little more dynamic and do something like this:


public class DynamicPythonSuite : AbstractPythonSuite
{
public DynamicPythonSuite()
: base(FindSuitablePythonScripts())
{
}

private static string[] FindSuitablePythonScripts()
{
List scripts = new List();
Assembly assembly = typeof (DynamicPythonSuite).Assembly;
string indication = "#test";
foreach (string potentialScript in assembly.GetManifestResourceNames())
{
using (StreamReader reader =
new StreamReader(assembly.GetManifestResourceStream(potentialScript)))
{
if (reader.ReadLine().Trim().StartsWith(indication,
true, CultureInfo.InvariantCulture))
{
scripts.Add(potentialScript);
}
}
}
return scripts.ToArray();
}
}



Which will load all scripts with "#test" in the first line, which seem'is a little easier to me (not having to spell out the path to the script, and only having to create the test file to make things happen) - YMMV of course.
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Integrating NUnit & IronPython...

Breaking scripts

I've been thinking about the issues involved with using
IronPython in a product as a scripting language, internally we've
already struggled at Syzmk with my love of refactoring breaking
scripts in our deployed products left right and centre.

I think the problems I need to solve first are:

  • Ensuring an understanding of the user experience when
    scripting with the application (read: dogfooding the scripting
    experience)

  • Making sure I identify where helper classes / wrappers etc.
    may be necessary to make the scripting experience more palatable
    to end users.
  • Ensuring any changes don't clobber the contracts between the
    scripting language and the client when refactoring the underlying
    .Net classes they rely on.

Regression tests for expected script usage is what it comes
down... but if you sit down and start trying to write these tests
in NUnit it all gets a bit unwieldy - there's a lot of time
wasted making sure your tabs are right, escaping characters
correctly in strings, and running scripts, evaluating the
results, and then asserting against them - it's frustrating, and
fails to emulate the experience of your users who are writing
scripts purely in Python.

PyUnit?

So my next port of call was to try the unit testing support that
comes with python (PyUnit) - it would definitely do the job - but
then I need to do a couple of things... first I need to start
bundling quite a few standard libraries with the scripting engine
in our product, and then I have to write additional functionality
to support capturing the results of running the PyUnit tests and
presenting them to the automated build process... It's all
sounding like a pain in the ass, and  the automation build
works nicely as it is, I'd rather not have to mess with it...

So I decided to do a little integration work, to see if I could
run a test suite written in python as part of a C# assembly.

NUnit meets (Iron)Python

First off, my solutions user experience is ok - not perfect - but
then I just wanted to make sure I could get it working, before I
put any effort into cleaning it up, and not being an expert in
extending NUnit, or coding python, it was probably doomed from
the start to be a mediocre solution ;o)

The steps to get it working in a test assembly are:

  1. Writing a fixture in python, and including it in the assembly
    as an embedded resource.
  2. Creating a suite which references the embedded python files -
    this is a class derived from the "AbstractPythonSuite".

  3. Creating a Suite builder (I had to do this in the test
    assembly itself, because it wasn't working when I bundled this in
    a support library).
  4. Run the suite ;o)

First off, we write the fixture, the only requirements are:

  • The fixture class is derived from "NUnitFixture".
  • The test methods are prefixed with "test" - as per PyUnit.
    And take only a "self" argument.

Here's an example fixture:

import System
from System import Console

class MyPythonFixture(NUnitFixture):

def testPass(self):
Console.WriteLine("--testPass--")
Assert.IsTrue(True)

def testFail(self):
Console.WriteLine("--testFail--")
Assert.IsTrue(False, "this will fail")

As you can see we are just using the existing NUnit Assert
methods... There is one caveat to this that I will cover in
another post (that of testing for expected exceptions).

Next we create a suite - which can wrap one or more python script
files (included as assembly resources) - this is all we need to
bridge the gap.

public class MyPythonSuite : AbstractPythonSuite
{
public MyPythonSuite()
: base("MyPythonFixture.py")
{
}
}

And last of all, we need to drop in a suite builder - in theory I
should be able to include this in a support assembly - but it
doesn't seem to work, here's the code:

[SuiteBuilder]
public class PythonSuiteExtensionBuilder : ISuiteBuilder
{
public TestSuite BuildFrom(Type type, int assemblyKey)
{
if (CanBuildFrom(type)) return new PythonSuiteExtension(type, assemblyKey);
return null;
}

public bool CanBuildFrom(Type type)
{
return (typeof(AbstractPythonSuite).IsAssignableFrom(type) && (type != typeof(AbstractPythonSuite)));
}
}

At this point when can test the assembly with NUnitGui,
and see our python fixtures appear under the "MyPythonSuite"...
pretty slick huh?


border="0" />



The big downsides of this approach are that you can't run the
tests individually using something like TestDriven.Net,
and Resharper's unit test window doesn't pick them up...
However NUnitGui and the corresponding NUnitConsole
have no problems with them, and running all the tests in your
assembly with TestDriven.Net does pick them up thankfully
- so as long you don't have many slow tests in your assembly it
should still be tolerable (you can run them while still
coding...).

And of course, they will be picked up in your automated builds -
which is great news for me... I hate messing around with
CruiseControl.Net's configuration.

I'll tidy up the implementation a little and post the code
soon.

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help() works...

Well, Seo Sanghyeon asked me a pretty reasonable question of the last entry



"Why don't you use help(), instead of printing Overloads?"



Good question, I didn't realise that actually worked in IronPython for .Net types, I was very pleasantly surprised, I remember in the early betas i tried using help() on various managed types and it would just raise a NotImplementedException exception (or something like that, I forget now) - so what does that mean for my examples... well... let's take a system type like Guid and use help() on it.



IronPython 1.0.60816 on .NET 2.0.50727.42
Copyright (c) Microsoft Corporation. All rights reserved.

>>> import System

>>> from System import *

>>> help(Guid)

Help on Guid in module System in mscorlib, Version=2.0.0.0,
Culture=neutral, PublicKeyToken=b77a5c561934e089



 |      Guid(Array[Byte] b)

 |      Guid(UInt32 a, UInt16 b,
UInt16 c, Byte d, Byte e, Byte f, Byte g, Byteh, Byte i, Byte j,
Byte k)

 |      Guid(str g)

 |      Guid(int a, Int16 b, Int16
c, Array[Byte] d)

 |      Guid(int a, Int16 b, Int16
c, Byte d, Byte e, Byte f, Byte g, Byte h, Byte i, Byte j, Byte
k)

 |      Guid
CreateInstance[Guid]()

 |

 |  Data and other attributes defined here:

 |

 |      CompareTo(...)

 |             
int CompareTo(self, object value)

 |             
int CompareTo(self, Guid value)

 |      Equals(...)

 |             
bool Equals(self, object o)

 |             
bool Equals(self, Guid g)

 |      Finalize(...)

 |             
Finalize(self)

 |      GetHashCode(...)

 |             
int GetHashCode(self)

 |      GetType(...)

 |             
Type GetType(self)

 |      MemberwiseClone(...)

 |             
object MemberwiseClone(self)

 |      NewGuid(...)

 |             
Guid NewGuid()

 |      ToByteArray(...)

 |             
Array[Byte] ToByteArray(self)

 |      ToString(...)

 |             
str ToString(self)

 |             
str ToString(self, str format)

 |             
str ToString(self, str format, IFormatProvider provider)

 |      __eq__(...)

 |             
bool op_Equality(Guid a, Guid b)

 |      __init__(...)

 |             
x.__init__(...) initializes x; see x.__class__.__doc__ for
signature

 |             
x.__init__(...) initializes x; see x.__class__.__doc__ for
signature

 |      __ne__(...)

 |             
bool op_Inequality(Guid a, Guid b)

 |      __new__(...)

 |             
__new__(cls, Array[Byte] b)

 |             
__new__(cls, UInt32 a, UInt16 b, UInt16 c, Byte d, Byte e, Bytef,
Byte g, Byte h, Byte i, Byte j, Byte k)

 |             
__new__(cls, str g)

 |             
__new__(cls, int a, Int16 b, Int16 c, Array[Byte] d)

 |             
__new__(cls, int a, Int16 b, Int16 c, Byte d, Byte e, Byte f,
Byte g, Byte h, Byte i, Byte j, Byte k)

 |             
Guid CreateInstance[Guid]()

 |



>>>


Sweet, it works a treat - and will save a lot of digging time - though you might want to capture the results for big classes ;o)



Thanks Seo!



(btw, Seo is the man behind the Iron Python Community Edition & FePy projects - which can be found on sourceforge) and I would have to agree with this article that he does seem almost OmniPresent in the IronPython community...

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Instead of our planned presentation, we bring you rambling IronPython spelunking...

This entry was going to cover thinking about your .Net classes from the perspective of Python consumption... but actually it's not, we'll cover that another time.

What this post is really going to be about is how to spelunk a little into our managed classes via the IronPython interactive console - and to see how some common practices in .Net have interesting results in python.

This entry is going to be a little slow paced, I'm making the assumption you're a .Net developer, and haven't cut Python code before, if you know your way around Python probably best to pass this one over.

Also worth noting at this point that I'm really just a Python hacker, and certainly no guru...

So first off, we have a class with a bunch of overloads for various methods, here is the interface... I've left the code for the class itself out, but the class is called MyFileStore.

public interface IMyFileStore
{
Guid AddFile(string fileName);
Guid AddFile(string fileName, string mimeType);
Guid AddFile(byte[] contents, string mimeType);
Guid[] AddFiles(params string[] fileNames);
Guid AddFile(byte[] contents);
byte[] ReadFile(Guid id, out string mimeType);
void ReadFile(Guid id, string fileName);
void ExtractFiles(ref IList myList);
}

Pretty simple, now lets look at using this class with Iron python, I'll be running with the ipy (the interactive interpreter) so we can investigate our types as we go along (this is an executable which comes with the Iron Python binaries). The ipy console has a prompt consisting of three greater then signs ">>>" so you'll know what I'm typing into the console by looking for those.

First off, we need to load our type... this is pretty trivial...

>>> import clr

>>>
clr.AddReferenceToFileAndPath("D:DemoSampleIronPythonLib.dll")

>>> from IronPythonLib import *

All done, lets make sure our class is in there, using the inbuilt function dir() - a list of built-in functions can be found here: http://www.python.net/quick-ref1_52.html#BuiltIn

>>> dir()

['IMyFileStore', 'MyFileStore', '_', '__builtins__', '__doc__',
'__name__', 'clr', 'results', 'site', 'store', 'sys']

MyFileStore, sweet.

First off, lets create an instance of my type (not yours ;o)

>>> store = new MyFileStore()

Traceback (most recent call last):

SyntaxError: unexpected token MyFileStore (, line 1)

>>>

Arse, python doesn't have a new operator... forgot that, lets try it again.

>>> store = MyFileStore

Right, looks good, interpreter didn't return any errors... but wait, lets make sure we got what we want, which is an instance of MyFileStore....

>>> store



>>>

Woops, that's not what we wanted, we want an instance, not the type... lets try again:

>>> store = MyFileStore()

>>> store



>>>

That's better, lets find out what our store is capable off, using the dir command.

>>> dir(store)

['AddFile', 'AddFiles', 'Equals', 'ExtractFiles', 'Finalize',
'GetHashCode', 'GetType', 'MakeDynamicType', 'MemberwiseClone',
'ReadFile', 'Reduce', 'ReferenceEquals', 'ToString', '__class__',
'__doc__', '__init__', '__module__', '__new__','__reduce__',
'__reduce_ex__', '__repr__']

>>>

We see our AddFile, AddFiles,ReadFile & ExtractFiles methods - everything looks to be in order... but what are the parameters for those methods?

>>> dir(store.AddFile)

['Call', 'CallInstance', 'Equals', 'Finalize', 'GetHashCode',
'GetTargetType', '

GetType', 'Make', 'MakeDelegate', 'MakeDynamicType',
'MemberwiseClone', 'Overloads', 'PrependInstance', 'Reduce',
'ReferenceEquals', 'ToString', '__call__', '__class__',
'__doc__', '__eq__', '__getitem__', '__hash__', '__init__',
'__module__', '__name__', '__new__', '__reduce__',
'__reduce_ex__', '__repr__', '__self__', '__str__']

>>>

Hmm... what's the "Overloads" thing, lets find out...

>>> dir(store.AddFile.Overloads)

['Equals', 'Finalize', 'Function', 'GetHashCode', 'GetOverload',
'GetTargetFunction', 'GetType', 'MakeDynamicType',
'MemberwiseClone', 'Reduce', 'ReferenceEquals', 'Targets',
'ToString', '__class__', '__doc__', '__getitem__', '__init__',
'__module__', '__new__', '__reduce__', '__reduce_ex__',
'__repr__', '__str__']

>>>

That doesn't seem very useful, though what if we check it's string representation?

>>> store.AddFile.Overloads


at 0x0000000000000036 [{'Guid AddFile(self, str fileName)':
, 'Guid AddFile(self, str
fileName, str mimeType)': ,
'Guid AddFile(self, Array[Byte] contents, str mimeType)':
, 'Guid AddFile(self,
Array[Byte] contents)':
AddFile>}]>

>>>

Look at that, there are our 4 overloads - but it's a bit jumbled, what am I looking at it....

Well at first glance it looks a bit like a python dictionary... notice the colon separating the signature for the method and ... what if we try to grab one for use later, maybe for some functional programming, why don't we get the one which takes the fileName and mimeType parameters eh?



>>> myFunc =
store.AddFile.Overloads[str,str]

Traceback (most recent call last):

File , line 0, in ##100

TypeError: __getitem__() takes exactly 1 argument (1 given)

>>>

Bugger, that didn't work... still, not a bad guess - one parameter eh? Lets have another stab, maybe it just wants us to group the types together somehow?

>>> myFunc =
store.AddFile.Overloads[(str,str)]

>>> myFunc


0x0000000000000035>

>>>

Looks better, of course if we wanted to we could use our function right now... let's give it a whirl:

>>> myFunc("d:testinput.3gp",
"video/3gpp")


[7b6d6dc5-bbe6-4831-bff7-21984031684f]>

>>>

Looks to have worked, the method has successfully returned us a Guid identifying the new file... shame we didn't save it into a variable for future reference, cest la vie.

Luckily I can just copy and paste it - lets see if I can get the file back using this overload...

byte[] ReadFile(Guid id, out string mimeType);

Simple as.. lets try...

>>> id =
Guid("7b6d6dc5-bbe6-4831-bff7-21984031684f")

Traceback (most recent call last):

File , line 0, in ##116

NameError: name 'Guid' not defined

>>>

Woops, we need to import the System namespace...

>>> import System

Now lets try again...

>>> id =
Guid("7b6d6dc5-bbe6-4831-bff7-21984031684f")

Traceback (most recent call last):

File , line 0, in ##116

NameError: name 'Guid' not defined

>>>

Huh... oh wait, let's check our local symbol table:

>>> dir()

['MyFileStore', 'System', '_', '__builtins__', '__doc__',
'__name__', 'clr', 'myArray', 'myFunc', 'results', 'site',
'store', 'sys']

System eh? Lets dig into it...

>>> dir(System)

['AccessViolationException', 'Action', 'ActivationContext',
'Activator', 'AppDom

ain', 'AppDomainInitializer', 'AppDomainManager',
'AppDomainManagerInitializatio

nOptions', 'AppDomainSetup', 'AppDomainUnloadedException',
'ApplicationException... and all the rest.

>>

Well, what can we do about that... we could import them all using "from System import *" - but we only need Guid, so why don't we just grab that eh?

>>> from System import Guid

>>> dir()

['Guid', 'MyFileStore', 'System', '_', '__builtins__', '__doc__',
'__name__', 'clr', 'myArray', 'myFunc', 'results', 'site',
'store', 'sys']

>>>

Sweet, now lets try it again... finally!

>>> id =
Guid("7b6d6dc5-bbe6-4831-bff7-21984031684f")

>>> mimeType = ""

>>> contents = store.ReadFile(id, mimeType)

Traceback (most recent call last):

File , line 0, in ##136

File , line 0, in ReadFile##66

File D:devProjectsIronPythonDemoIronPythonLibMyClass.cs,
line 70, in Read

File

File mscorlib, line unknown, in WriteAllBytes

File mscorlib, line unknown, in .ctor

File mscorlib, line unknown, in Init

ValueError: Empty path name is not legal.

>>>

Oh no! what went wrong??... what's this about an empty path name, I was expecting to get some bytes back.... Hmmm, don't panic, lets just do some digging...

>>> store.ReadFile.Overloads


at 0x0000000000000039 [{'(Array[Byte], str) ReadFile(self, Guid
id)': , 'ReadFile(self, Guid
id, str fileName)':
ReadFile>}]>

>>>

Well there's the problem, I've called the wrong version, but... that's odd, there's a ReadFile there I haven't defined in my class... and one that's missing... hmmm, time to file a bug report.

Or not, this is just where the Python and .Net world don't see eye to eye - Python doesn't know about out parameters as such, so it simulates the effect by altering the method's signature - lets have a go at playing by IronPython's rules:

>>> contentsAndMimeType =
store.ReadFile(id)

>>> len(contentsAndMimeType)

2

>>> type(contentsAndMimeType[0])



>>> type(contentsAndMimeType[1])



>>> contentsAndMimeType[1]

'video/3gpp'

>>>

So our "out string mimeType" parameter was returned by the method instead, as the second item in an array... hmm... interesting.

Well if python does this to out parameters, what does it do to ref parameters - lets try the extract files method to see what happens... this method is implemented to create a new list if we don't supply a valid instance, but lets pass in a valid one first - I wont be caught out this time, so I'll import the types I need first (List):

>>> from System.Collections.Generic import
*

>>> bytes = List()

Traceback (most recent call last):

SyntaxError: unexpected token ] (, line 1)

>>>

Oh dear, looks like Python doesn't use the syntax... lets start digging again...

>>> dir(List)

['Add', 'AddRange', 'AsReadOnly', 'BinarySearch', 'Capacity',
'Clear', 'Contains

', 'ConvertAll', 'CopyTo', 'Count', 'Enumerator', 'Equals',
'Exists', 'Finalize'

, 'Find', 'FindAll', 'FindIndex', 'FindLast', 'FindLastIndex',
'ForEach', 'GetEn

umerator', 'GetHashCode', 'GetRange', 'GetType', 'IndexOf',
'Insert', 'InsertRange', 'LastIndexOf', 'MakeDynamicType',
'MemberwiseClone', 'Reduce', 'ReferenceEquals', 'Remove',
'RemoveAll', 'RemoveAt', 'RemoveRange', 'Reverse', 'Sort',
'ToArray', 'ToString', 'TrimExcess', 'TrueForAll', '__class__',
'__doc__', '__getitem__', '__init__', '__module__', '__new__',
'__reduce__', '__reduce_ex__', '__repr__', '__setitem__']

>>> List



>>>

Hmm... nothing obvious there, but we could hazard a guess...

>>> List[str]



>>>

Hmmm... so lets give our List of byte array another go...

>>> bytes = List[byte[]]()

Traceback (most recent call last):

SyntaxError: unexpected token ] (, line 1)

Hmm... it doesn't like byte, lets have a look...

>>> byte

Traceback (most recent call last):

File , line 0, in ##56

NameError: name 'byte' not defined

>>> Byte



>>>

Ahhh... I forgot that "byte" (lowercase) isn't actually what the Clr calls it... hmm... we could try it again, but it wont work - simply because Byte[] doesn't make sense (if we're using square brackets to index into a generic type... ) so having another dig in the IronPython docs we see that we need to index to the type "Array" with the type of our array, given this knowledge we can get what we want...

>>> bytes = List[Array[Byte]]()

>>> bytes



>>>

Sweet, now to call that ExtractFiles method...

>>> store.ExtractFiles(bytes)



>>> bytes



>>> len(bytes)

8

Well it returned a list... but it did accept our reference parameter, and it has updated the collection we passed to it... but what if we had passed it a null instead?

>>> bytes = None

>>> store.ExtractFiles(bytes)



>>> len(bytes)

Traceback (most recent call last):

File , line 0, in ##83

File , line 0, in Length##75

TypeError: len() of unsized object of type <>
'NoneType'>

>>>

Hmmm... it returned a list, but it didn't set the value in our variable "bytes" - this isn't really ref like behavior... yet, if you think about it we can get almost the same thing by doing this:

>>> bytes = store.ExtractFiles(bytes)

>>> len(bytes)

8

>>>

Right, our journey has almost come to an end... looking at the interface we have a method declared like so:

Guid[] AddFiles(params string[] fileNames);

params is handy in .Net code - anything to save tedious typing of array declarations, lets see if we can do the same thing in python?

>>> store.AddFiles("d:testinput1.3gp",
"d:testinput2.3gp")

System.Guid[](
[d3e8f099-3005-461c-a63a-fdc69b2091ee]>, <>
object at 0x0000000000000032
[d9d1fb25-fc7a-47a2-89ac-c11264bfa47f]>)

>>>

Sweet, that's a pleasant surprise after that whole out and ref debacle, I was beginning to loosing hope!

But wait, why not revive our faith in Python a little more by looking at a few tricks...

>>> paramsDict = { "fileName" :
"d:testinput.3gp", "mimeType" : "video/3gpp"}

>>> store.AddFile(**paramsDict)


[e07680d9-f544-4847-9a1e-d04a0ef137f7]>

>>>

What did we just do? Well... given a dictionary, where the keys are the parameter names, we used them as the parameters for one of our .Net methods using the double asterisk syntax - think of what you'd have to do to code this in .Net... reflection city
;o)

It doesn't just have to be dictionaries... we can use arrays too...

>>> paramsArray = [ "d:testinput.3gp",
"video/3gpp" ]

>>> store.AddFile(*paramsArray)


[85006c43-e6f5-4e90-b837-0868788cf453]>

>>>

Neat, of course, being a dynamic language and all, the selection of the appropriate overload is based on the types in the array or dictionary... it would be a nightmare to do both of these in .Net with reflection yourself.

Faith still not restored eh, what about creating a python class that wraps an existing (possibly sealed) .Net class, that forwards calls on unless we want to override the behavior...

>>> class MyFileStoreWrapper:

... def __init__(self, realStore):

... self.realStore = realStore

... def ExtractFiles(self, myList):

... raise Exception, "this method isn't allowed"

... def __getattr__(self, name):

... if name == "ExtractFiles": return ExtractFiles

... return eval("self.realStore."+name)

...

>>> store = MyFileStoreWrapper(MyFileStore())

>>> store.AddFile("d:testinput.3gp")


[a2bf8747-cc71-4b79-8c02-0b61a87ff67f]

>

>>> store.ExtractFiles(None)

Traceback (most recent call last):

File , line 0, in ##52

File , line 0, in ExtractFiles

Exception: this method isn't allowed

>>>

ExtractFiles can't be invoked on our wrapper, however our wrapper automagically responds for other methods like AddFile - sadly I've used an eval (which, rhymes with Evil - coincidence?) here because MyFileStore doesn't expose a __getattr__ method (because it's a .Net classes, not a native Python one).

The nice thing now, is that given a native python class, we can do some things we aren't allowed to do to the underlying .Net class - like adding new methods at run time.

>>> def ExtractMoreFiles(self, myList):

... raise Exception, 'not implemented yet'

...

>>> MyFileStoreWrapper.ExtractMoreFiles =
ExtractMoreFiles

>>> store.ExtractMoreFiles(None)

Traceback (most recent call last):

File , line 0, in ##60

File , line 0, in ExtractMoreFiles

Exception: not implemented yet

>>>

I think it's pretty cool, and though python's meta-programming model isn't quite up to ruby standards, it's still pretty easy to dig down and create some pleasantly surprising results :) and if you have your thinking caps on you can probably see how this stuff would really help to bring a DSL to life for your applications special needs.

And that's where I'm going to conclude this rambling post... ;o) Hopefully it'll spark some thoughts about what's possible with IronPython and allow you avoid some simple mistakes when using .Net classes in iron python.

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