Understanding Python Properties

PROPERTIES:

Properties is a class for managing class attributes in Python. Property( )is a built-in function that creates and returns a property object

Syntax:

attribute_name =property(get_Attribute, set_Attribute, del_Attribute(Optional), doc_Attribue(Optional))

where, get_Attritube is function to get value of the attribute,

set_Attribute is function to set value of the attribute,

del_Attribute is function to delete the attribute and

doc_Attribute is a string (like a comment).

Example:

class Celsius:
    def __init__(self, temperature = 0):
        self.temperature = temperature
    def fahrenheit(self):
        return (self.temperature * 1.8) + 32
    def get_temperature(self):
        print("Getting value")
        return self._temperature
    def set_temperature(self, value):
        print("Setting value")
        self._temperature = value
temperature = property(get_temperature,set_temperature)

Here temperature is the property object. property attaches code (get_temperature and set_temperature) to the attribute (temperature). Any code that retrieves the value of temperature will automatically call get_temperature( ). Similarly, any code that assigns a value to temperature will automatically call set_temperature( ).

Example:

>>>c = Celsius( )

Getting value

Here set_temperature( ) was called even when we created an object. The reason is that when an object is created, __init__ method gets called. This method has the line self.temperature = temperature. This assignment called set_temperature( ).

>>>c.temperature
Getting value
0

c.temperature calls get_temperature()

The actual temperature value is stored in the private variable temperature. The attribute temperature is a property object which provides interface to this private variable.

We can use the property with decorators also, Here we are not defining names get_temperature and set_temperature to pollute the class namespace. For this, we reuse the name temperature while defining our getter and setter functions. we can re-write the above example:

class Celsius:
    def __init__(self, temperature = 0):
        self.temperature = temperature
    def fahrenheit(self):
        return (self.temperature * 1.8) + 32
    @property
    def temperature(self):
        print("Getting value")
        return self._temperature
    @temperature.setter
    def temperature(self, value):
        print("Setting value")
        self._temperature = value

Posted On 23 August 2011 By MicroPyramid


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