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A First Example
To demonstrate how this translates to working code, the following class uses a property to trace access to an attribute named name; the actual stored data is named _name so it does not clash with the property:
class Person: # Use (object) in 2.6
def __init__(self, name):
self._name = name
def getName(self):
print('fetch...')
return self._name
def setName(self, value):
print('change...')
self._name = value
def delName(self):
print('remove...')
del self._name
name = property(getName, setName, delName, "name property docs")
bob = Person('Bob Smith') # bob has a managed attribute
print(bob.name) # Runs getName
bob.name = 'Robert Smith' # Runs setName
print(bob.name)
del bob.name # Runs delName
print('-'*20)
sue = Person('Sue Jones') # sue inherits property too
print(sue.name)
print(Person.name.__doc__) # Or help(Person.name)
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Properties are available in both 2.6 and 3.0, but they require new-style object derivation in 2.6 to work correctly for assignments—add object as a superclass here to run this in 2.6 (you can the superclass in 3.0 too, but it’s implied and not required).
This particular property doesn’t do much—it simply intercepts and traces an attribute—but it serves to demonstrate the protocol. When this code is run, two instances inherit the property, just as they would any other attribute attached to their class. However, their attribute accesses are caught:
fetch...
Bob Smith
change...
fetch...
Robert Smith
remove...
--------------------
fetch...
Sue Jones
name property docs
Like all class attributes, properties are inherited by both instances and lower subclasses. If we change our example as follows, for example:
class Super:
...the original Person class code...
name = property(getName, setName, delName, 'name property docs')
class Person(Super):
pass # Properties are inherited
bob = Person('Bob Smith')
...rest unchanged...
the output is the same—the Person subclass inherits the name property from Super, and the bob instance gets it from Person. In terms of inheritance, properties work the same as normal methods; because they have access to the self instance argument, they can access instance state information like methods, as the next section demonstrates.
