CODEWITHSUNDEEP
pythonkivy
topher217
topher217

Reputation: 1347

Python / Kivy Initial Variable Assignment

I'm relatively new to Python and OOP, and going through the Kivy documentation and found some syntax I was not used to seeing. I am not really sure how to search for the answer since I don't know what to call this type of syntax, so I'll illustrate with the code itself.

Here is the code from the kivy docs:

import kivy
kivy.require('1.0.5')

from kivy.uix.floatlayout import FloatLayout
from kivy.app import App
from kivy.properties import ObjectProperty, StringProperty


class Controller(FloatLayout):
    '''Create a controller that receives a custom widget from the kv lang file.

    Add an action to be called from the kv lang file.
    '''
    label_wid = ObjectProperty()
    info = StringProperty()

    def do_action(self):
        self.label_wid.text = 'My label after button press'
        self.info = 'New info text'


class ControllerApp(App):

    def build(self):
        return Controller(info='Hello world')


if __name__ == '__main__':
    ControllerApp().run()

When the above code runs 

return Controller(info='Hello world')

I understand that it is setting a value for Controller().info (or controller_object_instance.info within the kivy.app.build() code), but I don't understand how this string assignment to the variable 'info' works, since I would think you would need an init() function at the top of the class to accept this assignment. Since there is no init() within the Controller() class, why doesn't 

Controller(info='Hello world')

throw an error? How does the above line of code know to properly assign the value 'Hello world' to its own attribute 'info'? Is there some init() function inherited from App and this can someone be used or does it have something to do with these two lines of code?

label_wid = ObjectProperty()
info = StringProperty()

Any help appreciated. Just looking for a way to search for this more on my own, and I'm stuck on what topic to search for.

Upvotes: 1

Views: 671

Answers (1)

el3ien
el3ien

Reputation: 5405

When you created the class Controller, you wanted to inherit the class FloatLayout.
This means, whenever you create a Controller object, you create a FloatLayout which you extended. So the __init__ function from FloatLayout at executes.

Look at this example:

class Animal:

    def __init__(self,**kwargs):
        self.info = kwargs.get("info")


class Dog(Animal):

    pass


obj = Dog(info="Hello world")

print(obj.info)

Now you want to create a Dog, but a dog is also an Animal, so you want to inherit all it's methods and attributes.
So if you pass info as an argument when creating an instance of Dog, it will be passed to Animal's __init__ function.

Upvotes: 1

Related Questions