[IronPython] Magic methods on CLR types

Jeffrey Sax jeffrey at extremeoptimization.com
Wed Dec 2 21:19:13 PST 2009


#1: Ok. And I just found Converter.ConvertToIndex so it's easy enough to do.
Great! Thanks.

 

#2: I think something else is going on. An extension type should not remove
functionality, which is what is happening here. Consider this:

 

[assembly: ExtensionType(typeof(Sliceable), typeof(SliceableExtensions))]

namespace Compatibility.Python {

    public class Sliceable {

        public int this[int index] { get { return index; } }

    }

    public static class SliceableExtensions {

        public static ISlice __getitem__(Sliceable a, ISlice b) { return b;
}

    }

}

 

IronPython 2.6 (2.6.10920.0) on .NET 2.0.50727.4927

Type "help", "copyright", "credits" or "license" for more information.

>>> import clr

>>> clr.AddReferenceToFile("Compatibility.Python")

>>> from Compatibility.Python import *

>>> s = Sliceable()

>>> s[1]

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

TypeError: expected ISlice, got int

>>> s.Item[2]

2

>>> s.Item[10]

10

 

There is no IList<T> or anything enumerable, just the indexer method. And
it's basically hidden by the __getitem__ in the extension type. Item still
works, because that's a direct reflection of the CLR type.

 

Adding an extension type to enable standard Python functionality like
slicing for CLR objects seems like a common scenario that, IMHO, should be
properly supported, eventually. F# has something similar with type
augmentations. Yes, I can just create pass-through methods, but in my view
that shouldn't be necessary. (In F#, it isn't, but then there are other
limitations.)

 

Thanks,

 

Jeffrey

 

From: users-bounces at lists.ironpython.com
[mailto:users-bounces at lists.ironpython.com] On Behalf Of Dino Viehland
Sent: Wednesday, December 02, 2009 10:02 PM
To: Discussion of IronPython
Subject: Re: [IronPython] Magic methods on CLR types

 

#1 is definitely correct behavior, consider:

 

IronPython 2.6 (2.6.10920.0) on .NET 2.0.50727.4927

Type "help", "copyright", "credits" or "license" for more information.

>>> class c(object):

...     def __getitem__(self, index):

...             return index

...

>>> c()[0:1.2]

slice(0, 1.2, None)

 

It's Python's list implementation which is converting indexes to integers so
you'll need to do that as well.  You can do that via
IronPython.Runtime.Converter.ConvertToIndex though.

 

On #2 in theory we don't let you override existing members w/ extension
members.  But __getitem__ is taking precedence here because we pick up the
List<T> version later in resolution via a .NET mapping than looking up
members by name.  But you can define the integer overload for __getitem__
and forward it to the real implementation.  

 

We could also consider just adding slicing support for List<T> to IronPython
natively J We probably can't do that for all IList<T>'s though in case
there's one that accepts negative or out of range indexes.

 

From: users-bounces at lists.ironpython.com
[mailto:users-bounces at lists.ironpython.com] On Behalf Of Jeffrey Sax
Sent: Wednesday, December 02, 2009 5:39 PM
To: 'Discussion of IronPython'
Subject: Re: [IronPython] Magic methods on CLR types

 

I found 2 issues:

 

1. It looks like __index__ is not called for slice arguments of non-Python
objects (like List<T>'s). Reading PEP 357 (which introduced __index__), I
would expect __index__ to always be called, so that I can be sure that the
slice components are either int, long, or NULL. (I haven't verified this in
CPython.)

 

2. If I supply a  __getitem__ method in my extension type (in my case with a
strongly typed ISlice parameter), the underlying CLR indexers are no longer
picked up.

 

Extender assembly:

 

[assembly: ExtensionType(typeof(List<>), typeof(ListExtensions<>))]

namespace Compatibility.Python {

    public static class ListExtensions<T> {

        public static T __getitem__(List<T> a, ISlice slice) {

            return a[(int)slice.Start]; // Dummy

        }

    }

}

 

IP session 1:

IronPython 2.6 (2.6.10920.0) on .NET 2.0.50727.4927

Type "help", "copyright", "credits" or "license" for more information.

>>> from System.Collections.Generic import List

>>> a=List[int]()

>>> a.Add(55)

>>> a[0]

55

>>> a[0:1.2]

55

>>> b=[1,2,3,4]

>>> b[0:1.2]

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

TypeError: expected index value, got float

>>> 

 

IP session 2:

IronPython 2.6 (2.6.10920.0) on .NET 2.0.50727.4927

Type "help", "copyright", "credits" or "license" for more information.

>>> import clr

>>> clr.AddReferenceToFile("Compatibility.Python")

>>> from System.Collections.Generic import List

>>> a=List[int]()

>>> a.Add(55)

>>> a[0]

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

TypeError: expected ISlice, got int

>>> 

 

From: users-bounces at lists.ironpython.com
[mailto:users-bounces at lists.ironpython.com] On Behalf Of Dino Viehland
Sent: Tuesday, December 01, 2009 8:57 PM
To: Discussion of IronPython
Subject: Re: [IronPython] Magic methods on CLR types

 

If you want IronPython to recognize them you just need to create an assembly
w/ ExtensionTypeAttribute and have it point at the extended and extending
types.  Then the assembly needs to be loaded into the ScriptRuntime using
ScriptRuntime.LoadAssembly.

 

We still need to figure out our story for recognizing and importing normal
.NET extension methods that C# supports.

 

From: users-bounces at lists.ironpython.com
[mailto:users-bounces at lists.ironpython.com] On Behalf Of Jeffrey Sax
Sent: Tuesday, December 01, 2009 5:27 PM
To: 'Discussion of IronPython'
Subject: [IronPython] Magic methods on CLR types

 

Is there a way to add magic methods like __repr__, __call__, etc. to CLR
types?

 

1.       Can it be done in the external CLR assembly without polluting the
API for other languages? If so, how?

2.       Can it be done using F# style type augmentations by redefining such
methods on the Python type corresponding to the CLR type? If so, how?

3.       If neither of these alternatives is possible, is there a third way?

 

Thanks!

 

Jeffrey Sax

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