Question

I’d like to use object database to persist some classes created in IronPython. The database is db4o for .NET 2.0 (downloaded today). The code looks like this:

import clr
clr.AddReferenceToFileAndPath(r"C:\dev\Db4objects\db4o-7.12-net20\bin\net-2.0\Db4objects.Db4o.dll")
from Db4objects.Db4o import *
db = Db4oFactory.OpenFile(r'G:\IronPython\test\db4o\database.db')

class Person(object):  
  def __init__(self, name, age):
    self.Name = name
    self.Age = age

  def __str__(self):
    return 'Person: ' + self.Name + ' ' + str(self.Age)

p1 = Person('testp', 34)
db.Store(p1)

I get exception at db.Store(p1)

Unexpected char '$'
ThrowUncheckedException at offset 4 in file:line:column <filename unknown>:0:0
FatalShutdown at offset 136 in file:line:column <filename unknown>:0:0
AsTopLevelCall at offset 112 in file:line:column <filename unknown>:0:0
AsTopLevelStore at offset 34 in file:line:column <filename unknown>:0:0
StoreInternal at offset 69 in file:line:column <filename unknown>:0:0
Store at offset 66 in file:line:column <filename unknown>:0:0
Store at offset 12 in file:line:column <filename unknown>:0:0
Store at offset 15 in file:line:column <filename unknown>:0:0
   v Microsoft.Scripting.Actions.Calls.MethodCandidate.Caller.CallWithInstance(Object[] args, Boolean& shouldOptimize)
   v IronPython.Runtime.Types.BuiltinFunction.BuiltinMethodCaller`2.Call1(CallSite site, CodeContext context, TFuncType
func, T0 arg0)
   v System.Dynamic.UpdateDelegates.UpdateAndExecute3[T0,T1,T2,TRet](CallSite site, T0 arg0, T1 arg1, T2 arg2)
   v Microsoft.Scripting.Interpreter.DynamicInstruction`4.Run(InterpretedFrame frame)
   v Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)
   v Microsoft.Scripting.Interpreter.LightLambda.Run2[T0,T1,TRet](T0 arg0, T1 arg1)
   v IronPython.Compiler.PythonScriptCode.RunWorker(CodeContext ctx)
   v IronPython.Compiler.PythonScriptCode.Run(Scope scope)
   v IronPython.Hosting.PythonCommandLine.<>c__DisplayClass1.<RunOneInteraction>b__0()
Exception: Unexpected char '$'
CLR Exception:
    Exception
:
Unexpected char '$'

I suspect that the problem is with IronPython and its type system, because the class Person is not standard .NET class. I tried to persist System.IO.FileInfo and it worked well. How can I store an object that is instance of a class in IronPython?

Answer 1

Db4o uses the CLR-reflection mechanisms to retrieve meta-information of the objects, in order to store it. This works fine for C#, VB.NET etc. Languages such as IronPython and IronRuby used an additional layer on top the CLR to allow all the wonderful dynamics features.

Now when db4o uses the CLR-reflection is sees add the additional stuff which is used to implement the dynamic features. In the attempt to store this information is fails. There not a simple an elegant solution available today =(

1. As already stated in the comments, you could define you data-object in C# or VB.NET, so that you have plain old CLR objects. I think this is the quickest solution and should work.
2. You could ‘teach’ db4o about the object-model of python by writing you own reflector. But that’s certainly not easy.
3. Db4o has a ‘SelfReflector’ which is intended for environments without reflection. There basically add manually the required meta-info for each object. However I cannot find any documentation on this.

Another possibility would be to use a ODBMS which is created specially for Python, like ZODB instead of db4o. But I’m not sure if ZOBR runs on IronPython.