Declaring properties within a class

Here we want to highlight an important difference between traditional Python coding and Kivy, and the usefulness of this change.

Getting ready

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.

How to do it…

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

1. Import Kivy and its properties
2. Define the class
3. Reference the Kivy property, in this case the numeric one:

   import kivy
   
   from kivy.event import EventDispatcher
   from kivy.properties import *
   
   class MyClass(EventDispatcher):
       numeric_var = NumericProperty(1.0)

How it works…

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).

There's more…

They help you to:

• Easily manipulate widgets defined in the Kv language
• Automatically observe any changes
• Check and validate values
• Optimize memory management

Kivy provides more properties as follows:

• NumericProperty
• StringProperty
• ListProperty
• ObjectProperty
• BooleanProperty
• BoundedNumericProperty
• OptionProperty
• ReferenceListProperty
• AliasProperty
• DictProperty

See also

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.