Inheriting and extending Python classes in C# #862

Activity

1. filmor commented on May 15, 2019

   

   filmor

   on May 15, 2019

   Member

   More actions

   I'm not sure what you mean by obj is Class here, but isinstance can indeed be overridden, and we do so already: https://github.com/pythonnet/pythonnet/blob/master/src/runtime/metatype.cs#L259

   In metatype, we register __instancecheck__ (the one I linked) and __subclasscheck__ for this purpose, you can try to extend them for your use case.

   🚀React with 🚀1Reacted by Victor
2. lostmsu commented on May 15, 2019

   

   lostmsu

   on May 15, 2019

   Last edited by lostmsu

   MemberAuthor

   More actions

   @filmor is it possible to use MetaType to represent my C# class as being derived from a specific Python type? Or do I have to simply go with __instancecheck__?

   Can MetaType be used for A (C#) derives from B (C#) deriving from C (Python)?

   obj is Class is an "old" way of doing isinstance(obj, Class). I am not sure if they have the same semantics.
3. filmor commented on May 15, 2019

   

   filmor

   on May 15, 2019

   Member

   More actions

   obj is Class is not the old way of doing isinstance, it's an entirely different thing (checking that obj and Class are the exact same objects).

   The issue here is that B does not derive from the Python class. In __instancecheck__ we solve this by checking whether the object we are looking at is of our metaclass (https://github.com/pythonnet/pythonnet/blob/master/src/runtime/metatype.cs#L287) and if that's the case, checking if it's "assignable from" (https://github.com/pythonnet/pythonnet/blob/master/src/runtime/metatype.cs#L300).

   What you could do is introducing an attribute that is checked in DoInstanceCheck:

    [Python.DerivesFromPython("package.class.bla")]
    class MyClass { ... }
   

   Alternatively, you could try to override __instancecheck__ directly on your (base) class.

   👍React with 👍1Reacted by Victor
4. lostmsu commented on May 15, 2019

   

   lostmsu

   on May 15, 2019

   MemberAuthor

   More actions

   @filmor thanks for your help. I will try that approach tomorrow.

   Also, excuse my lack of knowledge about Python. Using Python.NET is actually the way I am trying to avoid it as much as possible :)
5. lostmsu commented on May 16, 2019

   

   lostmsu

   on May 16, 2019

   Last edited by lostmsu

   MemberAuthor

   More actions

   @filmor looked at some docs about isinstance and its relationship to __instancecheck__, and can't figure how to apply it to my problem.

   Suppose I have this 3rd party code (so I can't modify it):

   class Base:
     pass
   
   def accept(base_obj):
     if not isinstance(base_obj, Base):
       raise TypeError()
     print("OK!")

   And I want to define a derived class in C# like this:

   class Derived {
     ...
   }
   
   accept(new Derived().ToPython());

   Problem is, according to my understanding from SO, I have to override __instancecheck__ in Base.__class__ for it to recognize my instance of Derived to be an instance of Base. But I can't! I can only change what is in Derived.

   Is there a different way to tell Python, that Derived is actually "derived" from Base?
6. lostmsu commented on May 16, 2019

   

   lostmsu

   on May 16, 2019

   MemberAuthor

   More actions

   @filmor I looked at the Python source code, and this check eventually goes down to walking MRO in PyType_IsSubtype: https://hg.python.org/cpython/file/v3.5.3/Objects/typeobject.c#l1345
7. lostmsu

   added 6 commits that reference this issue on Jul 7, 2021

   allow substituting base types for CLR types (as seen from Python)

   ...

   b5c9b58

   allow substituting base types for CLR types (as seen from Python)

   ...

   a55d2be

   allow substituting base types for CLR types (as seen from Python)

   ...

   43a5e70

   allow substituting base types for CLR types (as seen from Python)

   ...

   9898b3c

   allow substituting base types for CLR types (as seen from Python)

   ...

   2f546e0

   allow substituting base types for CLR types (as seen from Python)

   ...

   ab2fde2
8. lostmsu

   added 2 commits that reference this issue on Aug 4, 2021

   allow substituting base types for CLR types (as seen from Python)

   ...

   92e6f8c

   allow substituting base types for CLR types (as seen from Python)

   ...

   d3c6942