How to implement __repr__ for a C# class to be used in python.net?

[AlexArcPy]

Environment

• Pythonnet version: 2.3.0
• Python version: 2.7
• Operating System: Windows 10

Details

I have a C# class library which I use in Python. I have a Point class (a code snippet):

public Point(double x, double y)
{
	this.x = x;
	this.y = y;
}

public override string ToString()
{
	return $"Point({x}, {y})";
}

When I work with this class library in Python (in Python shell):

>>> p1 = Point(10,15)
>>> p1
<Geometry.Point object at 0x000000000558F828>
>>> str(p1)
'Point(10, 15)'

Because I override the ToString(), when I run str(p1) I see my custom view of the object. However, when I just access a variable p1, then it goes to the __repr__() method under the hood which gives me ugly <Geometry.Point object at 0x000000000558F828>.

Is there a method in C# class that is possible to override so that pythonnet would use that method when getting the __repr__?

[den-run-ai]

@AlexArcPy have you tried implementing Repr() on your C# class, it should be available here:

pythonnet/src/runtime/pyobject.cs

Line 851 in 06ded7a

public string Repr()

pythonnet/src/runtime/pyobject.cs

Line 867 in 06ded7a

public override string ToString()

[AlexArcPy]

@denfromufa , thanks for the comment. I've just tried, but to no effect.

	public string Repr()
	{
		return $"Point({x}, {y})";
	}

In a Python IDE shell:

>>> p1
<Geometry.Point object at 0x00000000056A1550>
>>> p1.Repr()
u'Point(10, 15)'

In a cmd-based Python interpreter:

>>> p1 = Point(10,15)
>>> p1
<Geometry.Point object at 0x0000000005C44400>
>>> p1.Repr()
u'Point(10, 15)'
>>> p1.__repr__()
'<Geometry.Point object at 0x0000000005C44400>'

Should I be implementing this method in any kind of special way?

[den-run-ai]

@AlexArcPy looks like i was wrong, i'm not sure how this is possible to implement.

[den-run-ai]

@AlexArcPy ok, i took a second look and the first step is to find out which tp_repr method is called in your case using VS C# debugger:

https://github.com/pythonnet/pythonnet/search?p=1&q=tp_repr&unscoped_q=tp_repr

After that we can tell if it is possible to implement __repr__ on managed types without modifying pythonnet source code.

[den-run-ai]

https://stackoverflow.com/questions/50664853/how-to-implement-repr-for-a-c-sharp-class-to-be-used-in-python-net

[AlexArcPy]

@denfromufa , thanks a lot for digging into. This is definitely not a top priority feature to work on, so no stress! Should you find something more - please post! Thanks!

[koubaa]

I've got a working implementation (it uses reflection to call __repr__ if one is defined) but I don't know how to fallback to the default if a __repr__ is not defined.

In standard CPython its done via object.__repr__(inst) but I'm not familiar enough with pythonnet to know how to get a reference to "class object"

Also, the behavior according to python documentation is this:
The return value must be a string object. If a class defines __repr__() but not __str__(), then __repr__() is also used when an “informal” string representation of instances of that class is required.

Do we want this in pythonnet too?

[koubaa]

I was able to figure it out. I'll make a pull request with my changes (including the behavior from the python doc I mentioned above).

[koubaa]

#808

[den-run-ai]

@koubaa thanks for this PR, let's get the CI tests passed.