Adding a label or text to an ellipse object in kivy - user-interface

Question

I'd like to draw the x,y coordinates of a nested ellipse I have within the ellipse. However, I'm afraid I don't understand the object hierarchy enough for how to do that. I have tried adding a label event to be a child of the Ellipse, and I know it doesn't have a 'text' attribute that I can modify directly (as it's not a Button or a Label). Here's what I have:
MyWidget:
canvas.before:
Color:
rgba: 1,0,0,1
Ellipse:
size: min(self.size)*0.2, min(self.size)*0.2
pos: (self.x), (self.y)
Label:
text: unicode(self.x), unicode(self.y)
Error:
>> 40: Label:
41: text: unicode(self.x), unicode(self.y)
...
You can add only graphics Instruction in canvas.
So, I add it to a child of the Ellipse:
MyWidget:
canvas.before:
Color:
rgba: 1,0,0,1
Ellipse:
size: min(self.size)*0.2, min(self.size)*0.2
pos: (self.x), (self.y)
Label:
text: unicode(self.x), unicode(self.y)
>> 40: Label:
41: text: unicode(self.x), unicode(self.y)
...
You can add only graphics Instruction in canvas.
So, I try to put it in a canvas:
MyWidget:
canvas.before:
Color:
rgba: 1,0,0,1
Ellipse:
size: min(self.size)*0.2, min(self.size)*0.2
pos: (self.x), (self.y)
canvas:
Label:
text: unicode(self.x), unicode(self.y)
That doesn't have an error, but doesn't display any text.
Updating to address excellent answer below
MyWidget:
canvas.before:
Color:
rgba: 1,0,0,1
Ellipse:
size: min(self.size)*0.2, min(self.size)*0.2
pos: (self.x, self.y)
Label:
pos: (self.x,self.y)
size: min(self.size)*0.2, min(self.size)*0.2
text: "{},{}".format(str(self.x),str(self.y))
Gives me the following error:
44: Label:
>> 45: pos: (self.x, self.y)
46: size: min(self.size)*0.2, min(self.size)*0.2
47: text: "{},{}".format(str(self.x),str(self.y))
...
RuntimeError: maximum recursion depth exceeded while calling a Python object
I still can't get the text to appear /inside/ the Ellipse.

Answer 1

Your problem is that you cannot mix the canvas with widgets.
A way to think of it is that each widget has a canvas. You can draw what you like on that canvas, but the keyword there is draw - you can use kivy graphics instructions, like Ellipse or Rectangle or Line (or contextual instructions like Translate and Rotate). However, you can't use widgets. They don't work that way, they aren't things you draw on a canvas, they are widgets. When you add one widget to another with add_widget, one thing that happens is that its canvas is drawn on the canvas of the other, but this is kept to the canvases - it doesn't make sense to try and draw a whole widget on a canvas.
This is the root of all of your errors; Ellipse is not a widget. It doesn't have a canvas, it's only something you draw on a canvas. It can't have children because it isn't a widget. In kv language, the keyword canvas (and canvas.before etc.) doesn't mean there's a child widget called canvas, it's just a special syntax referring to the canvas of the parent widget.
So, to actually fix your problem, you need to take the Label out of the canvas. Something like the following should work:
MyWidget:
canvas.before:
Color:
rgba: 1,0,0,1
Ellipse:
size: min(self.size)*0.2, min(self.size)*0.2
pos: (self.x), (self.y)
Label:
pos: root.pos
size: min(self.size)*0.2, min(self.size)*0.2
text: unicode(self.x), unicode(self.y)
This is of course the normal syntax for making the Label just be a child of MyWidget. The important part is it's been given the same position and size as the ellipse so, since text is centered by default, the text will appear in the middle of the ellipse. If the text is long or something you might want to play with other things like setting text_size to automatically wrap, but that's a different problem.
Also, once you have multiple widgets with the same size description (like the size of both the Label and Ellipse in my example), it's often neater to move that calculation to a different place - for instance a NumericProperty of MyWidget. That way you can avoid code duplication, and also make it so that if you ever change the positioning requirements you can easily update both the Label and Ellipse simultaneously.