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Properties

The property function gives us a handy way to implement a simple descriptor without defining a separate class. Rather than create a complete class definition, we can write getter and setter method functions, and then bind these functions to an attribute name.

This allows us to create programs that look like the following example.

>>> 
oven= Temperature()

>>> 
oven.farenheit= 450

>>> 
oven.celsius

232.22222222222223
>>> 
oven.celsius= 175

>>> 
oven.farenheit

347.0

In this example, we set one attribute and the value of another attribute changes to mirror it precisely. We can do this by defining some method functions and binding them to attribute names.

Property Design Pattern. The Property design pattern has a number of method functions which are bound together with a single property name. The method functions can include any combination of a getter, a setter and a deleter.

To create a property, we define the instance variable and the method functions for some combination of getting, setting and deleting an attribute value. This is identical with the Getter and Setter design pattern. To make a property, we provide these method functions to the property function to bind the various methods to an attribute name.

Here's the definition of the property function.

property( fget , fset , fdel , doc ) → property attribute

Binds the given method functions into a property definition. This builds a descriptor object. Usually the result value is assigned to an attribute of a class.

Property Example. The following example shows a class definition with four method functions that are used to define two properties.

Example 25.2. property.py

class Temperature( object ):
    def fget( self ):
        return self.celsius * 9 / 5 + 32
    def fset( self, value ):
        self.celsius= (float(value)-32) * 5 / 9
    farenheit= property( fget, fset )
    def cset( self, value ):
        self.cTemp= float(value)
    def cget( self ):
        return self.cTemp
    celsius= property( cget, cset, doc="Celsius temperature" )
1

We create the farenheit property from the fget and fset method functions. When we use the farenheit attribute on the left side of an assignment statement, Python will use the setter method. When we use this attribute in an expression, Python will use the getter method. We don't show a deleter method; it would be used when the attribute is used in a del statement.

2

We create the celsius property from the cget and cset method functions. When we use the celsius attribute on the left side of an assignment statement, Python will use the setter method. When we use this attribute in an expression, Python will use the getter method.

The doc string provided for the celsius attribute is available as Temperature.celsius.__doc__.


 
 
  Published under the terms of the Open Publication License Design by Interspire