kivy/doc/sources/gettingstarted/drawing.rst

Drawing
-------

Each widget has a canvas, i.e. a place to draw on. The canvas is a group of
instructions that should be executed whenever there is a change to the widget's
graphical representation.
You can add two types of instructions to the canvas: *context* instructions and
*vertex* instructions. You can add instructions either from Python or from the
kv file (the preferred way).
If you add them via the kv file, the advantage is that they are automatically
updated when any property they depend on changes. In Python, you need to do
this yourself.

.. image:: ../images/gs-drawing.png

In both cases, the canvas of `MyWidget` is re-drawn whenever the ``position``
or the ``size`` of the widget changes.

You can use **canvas.before** or **canvas.after**. This allows you to separate
your instructions based on when you want them to happen.

For an in-depth look at how Kivy's graphics are handled, look
:mod:`here <kivy.graphics>`.