[Suggestion] A special function to create JS classes

[denis-migdal]

Hi,

Currently, when a Python class extends a JS class, it doesn't really inherit from it.

This may lead to some issues when such class is used in JS code, e.g. trying to extend an existing JS WebComponent.

We can dev some workaround with JS/Py helper functions. But this can be quite complex, playing with internal functions ($call, $getattr), etc.

I suggest to add a JSKlass() function that would allow to create real JS classes in Brython, and use it as a normal JS class (so requiring internally to $B.jsobj2pyobj() it to then use it in the Brython code).

It could be used like that:

class MyClass(javascript.JSKlass(window.HTMLElement)):
    def foo(self, a):
         pass

    prop = 3

That would be written as something like:

MyClass = $B.jsobj2pyobj( class MyClass extends HTMLElement {

     $BRY_DEF = {}; // the brython "implementation" of the class (mainly the methods ?).

     prop = $Bpyobj2jsobj(3);

     foo(...args) {
         return $B.call( this.$BRY_DEF.foo, [0,0,0])( ...args ); // I think this needs some pyobj2jsobj and jsobj2pyobj
     }
});

Ofc, there would be some restrictions on what we are allowed to do in such class.
But i think this would be quite helpful.

Cordially,

[PierreQuentel]

You can already inherit a Javascript class. What would be different if MySquare inherited from javascript.JSClass(window.Square) ?

<script type="text/python" debug="3">
from browser import window

class MySquare(window.Square):
    def perimeter(self):
         return self.side * 4

    prop = 3

square = MySquare(5)
assert square.area() == 25 # from the JS class
assert square.prop == 3 # from the Python class
assert square.perimeter() == 20 # also from the Python class
</script>

<script>
class Square {
  constructor(side){
    this.side = side
  }
  area(){
    return this.side ** 2
  }
}
window.Square = Square
</script>

[denis-migdal]

square instanceof Square would be true on the JS side.
We can also encounter some tricky nuances with the JS constructor, getter, overloads, etc.

I think the current way have issues when trying to inherit native classes, to then give it back to JS.
In such case duck typing may not enough, and we might really need to truly inherit from the native class.

If inheriting from javascript.JSClass(...) could be viewed by Brython as a signal to produce a clean JS class interface, I think it would help a lot when creating classes from Brython, we then intent to give to a JS function/library.

Such class would then be treated by Brython as a normal JS class (as if it was imported from window).

I think this would reduce the likelyhood of issues, and unexpected incompatibilities.

Currently I use a quite dirty workaround, with my Brython class' instances possessing an instance of a JS class, and then, on the JS side, creating a JS class inheriting from the native class, and transferring method calls to the Brython class. The instance possessed by the Brython class is then an instance of my JS class inheriting the native class... but in some situations, I can have loops during method calls. I'm not sure I call the Brython class methods correctly from my JS class...
... this is quite a mess xD.

[denis-migdal]

I may have a new way to represent Python classes that I am currently testing in SBrython.

I'll update here once I pass the classes unit test.

[denis-migdal]

I'm cleaning my issues.

One work-around was presented here: #2518

An issue that occurs when inheriting from JS classes (old issue, dunno if it is still here) ?
#2393

Another possible issue : #2253