Python | Canvas in Kivy using .kv file

Kivy is a platform-independent GUI tool in Python. As it can be run on Android, IOS, Linux and Windows etc. It is basically used to develop the Android application, but it does not mean that it can not be used on Desktop applications.

Kivy Tutorial – Learn Kivy with Examples.

Canvas :

The Canvas is the root object used for drawing by a Widget. A kivy canvas is not the place where you paint. Each Widget in Kivy already has a Canvas by default. When you create a widget, you can create all the instructions needed for drawing. If self is your current widget. The instructions Color and Rectangle are automatically added to the canvas object and will be used when the window is drawn. To use Canvas you must have to import:

from kivy.graphics import Rectangle, Color

Basic Approach -
-> import kivy
-> import kivy App
-> import widget
-> import Canvas i.e.:
      from kivy.graphics import Rectangle, Color
-> set minimum version(optional)
-> Extend the Widget class
-> Create the App Class
-> create the .kv file:
    -> create the canvas
    -> Add action/callback if needed
-> return a Widget
-> Run an instance of the class

Implementation of the Approach: 1) Creating A simple canvas: main.py file

Python3

# import kivy module
import kivy

# this restrict the kivy version i.e
# below this kivy version you cannot
# use the app or software
kivy.require("1.9.1")

# base Class of your App inherits from the App class.
# app:always refers to the instance of your application
from kivy.app import App

# A Widget is the base building block
# of GUI interfaces in Kivy.
# It provides a Canvas that
# can be used to draw on screen.
from kivy.uix.widget import Widget

# From graphics module we are importing
# Rectangle and Color as they are
# basic building of canvas.
from kivy.graphics import Rectangle, Color


# class in which we are creating the canvas
class CanvasWidget(Widget):
    pass

# Create the App Class
class CanvasApp(App):
    def build(self):
        return CanvasWidget()

# run the App
CanvasApp().run()

Kivy looks for a Kv file with the same name as your App class in lowercase, minus “App” if it ends with ‘App’ e.g: CanvasApp –> canvas.kv

canvas.kv file:

Python3

# .kv file of canvas

<CanvasWidget@Widget>

    # creating canvas
    canvas:
        Color:
            rgba: 0, 0, 1, 1    # Blue

        # size and position of Canvas
        Rectangle:
            pos: self.pos
            size: self.size

Output:

Now, How can we change the color of canvas by any action so below is an example in which clicking the color of the screen changes.

main.py file:

Python3

# import kivy module
import kivy

# this restrict the kivy version i.e
# below this kivy version you cannot
# use the app or software
kivy.require("1.9.1")

# base Class of your App inherits from the App class.
# app:always refers to the instance of your application
from kivy.app import App

# From graphics module we are importing
# Rectangle and Color as they are
# basic building of canvas.
from kivy.graphics import Rectangle, Color

# The ButtonBehavior mixin class provides Button behavior.
from kivy.uix.button import ButtonBehavior

# The Label widget is for rendering text.
from kivy.uix.label import Label

# class in which we are creating the canvas
class CanvasWidget(ButtonBehavior, Label):
    pass

# Create the App Class
class CanvasApp(App):
    def build(self):
        return CanvasWidget()

# run the App
CanvasApp().run()

.kv file:

Python3

# .kv file of canvas

<CanvasWidget>

    # Creating Canvas
    canvas:

        # Color is blue if button is pressed,
        # otherwise color is red
        Color:
            rgb: (1, 0, 0, 1) if self.state == 'normal' else (0, 0, 1, 1)

        # Rounded rectangle canvas
        RoundedRectangle:
            size: self.size
            pos: self.pos

            # Play with these if you want smooth corners for your button
            radius: 100, 100, 100, 100


    # Print the text when touched or button pressed
    on_release:
        print("I have been clicked")

Output:

Note: Kivy drawing instructions are not automatically relative to the position or size of the widget. Therefore you need to consider these factors when drawing. In order to make your drawing instructions relative to the widget, the instructions need either to be declared in the KvLang or bound to pos and size changes.