Advanced Object Oriented Programming

Properties

Properties allow you to expose an attribute while controlling its modification. They reinforce the principle of encapsulation, which consists of hiding the implementation. Remember that a property is not limited to expose a private attribute.

Consider a property prop of a class MyClass instantiated in my_object.

Usage as decorators

The accessor (getter)

class MyClass:

@property
def prop(self):
    return value

Will be called as:

my_object = MyClass()
my_object.prop

The mutator (setter)

class MyClass:

@property
def prop(self):
    return value

@prop.setter
def prop(self, new_value)
    self.value = new_value

Will be called as:

my_object = MyClass()
my_object.prop = value

The suppressor (deletter)

class MyClass:

@property
def prop(self):
    return value

@prop.deletter
def prop(self)
    pass

The suppressor will be called by this king of expression:

my_object = MyClass()
del my_object.prop

Real implementation of a property

class MyClass:

def _get_value(self):
    return self.value

def _set_value(self, new_value)
    self.value = new_value

def _del_value(self):
    pass

prop = property(_get_value, _set_value, _del_value)

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Emulating container types

Size of the container

class MyClass:
    def __len__(self):
        return 42 # Doit retourner un entier

Usage:

len(my_object)

Membership test

class MyClass:
    def __contains__(self, item):
    return True

Usage:

value in my_object

subscripting

There are three methods for accessing, modifying, or deleting an item by selection.

class MyClass:
    def __getitem__(self, key):
        return value

    def __setitem__(self, key, new_value):
        self.value_from_key = new_value

    def __delitem__(self, key):
        pass

Usages :

my_object[element_id]
my_object[element_id] = value
del my_object[element_id]

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