Python Kivy, need help to understanding a strange technique. self._object.clear() etc

Question

I feel guilty for asking question I can not properly name because I can not name the pattern used in the code.

There is a code on github I'm trying to understand and failing to do so. https://github.com/kivy-garden/speedmeter/blob/master/kivy_garden/speedmeter/init.py

Pattern I don't understand is in lines 128,129 and 181, 182 and many other places.

Big picture is. There a class

class MyClassName(Widget):

there are methods e.g.

    def _draw_this_and_that(self):
       self._someName.clear()
       add = self._someName.add
       add(Color(rgba=get_color_from_hex(color)))

This "_someName" found in whole code only in those 2 places as my code sample.

I understand that

   add = self._someName.add

creates function "add" but why that is needed? why not calling

   self._someName.add

instead?

I guess that

   self._someName.clear()

does erase whatever was added to "_someName", right?

I completely do not understand how

  add(Color(rgba=get_color_from_hex(color)))

does it job (but it does) and then whatever is drawn will be with that color.

Do I guess it right that if I need to change color (if some condition met) then I could just add different color?

  add(Color(rgba=get_color_from_hex(different_color)))

and don't stress that adding will cause memory leak because

   self._someName.clear()

will take care of it?

I never seen such pattern. I'd be very happy if someone could explain how it works and why. Thank you in advance!

Answer 1

The _somenameIG are canvas instruction groups that are created in the __init__() method:

    add = self.canvas.add
    for instructionGroupName in _ig:
        ig = InstructionGroup()
        setattr(self, '_%sIG' % instructionGroupName, ig)
        add(ig)

So, the self._someName.clear() is clearing a canvas instruction group, and the add() method adds instructions to the group.

Answer 2

In addition to the other answer I would like to address the following:

Pattern I don't understand is in lines 128,129 and...

According to its documentation the attr. sectors in line 130 is a list of alternating values and colors a copy of which is stored locally by l and then the color value(s) is(are) passed to the InstructionGroup. Since it is used as a redrawing mechanism, the instruction groups are removed from canvas by the method clear before drawing again.

creates function "add" but why that is needed? why not calling...

This is done mainly to prevent the re-evaluation process.