Declaring properties within a class

We need to remember the traditional form to declare properties in Python. Usually, if we want to declare a property in Python, we do something such as:

class MyClass(object):
    def __init__(self):
        super(MyClass,self).__init__()
        self._numeric_var = 1
@property
    def numeric_var(self): 
        return self._numeric_var

We are declaring a numeric one, whereas if we use MyClass().numeric_var in the Python shell, we get 1 in return.

Now, to declare this property in Kivy we follow these steps:

The idea behind this is that you inherit the declaration from Kivy's properties, which reduces the number of code lines.

To use them, you have to declare them at a class level. That is, directly in the class, not in any method for the class. A property is a class attribute that will automatically create instance attributes. Each property, by default, provides an on_<propertyname> event that is called whenever the property's state/value changes.

Something additional to point out is that NumericProperty accepts all the Python numeric values: ints, floats, and longs.

In general, Kivy properties can be overridden easily when creating the instance of the class, using keyword arguments such as ClassName(property=newvalue).

They help you to:

Kivy provides more properties as follows:

These properties actually implement the Observer pattern; if you want to learn more about patterns, you can find information online at http://www.oodesign.com/observer-pattern.html.