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A First Example
To see how this all comes together in more realistic code, let’s get started with the same first example we wrote for properties. The following defines a descriptor that intercepts access to an attribute named name in its clients. Its methods use their instance argument to access state information in the subject instance, where the name string is actually stored. Like properties, descriptors work properly only for new-style classes, so be sure to derive both classes in the following from object if you’re using 2.6:
class Name: # Use (object) in 2.6
"name descriptor docs"
def __get__(self, instance, owner):
print('fetch...')
return instance._name
def __set__(self, instance, value):
print('change...')
instance._name = value
def __delete__(self, instance):
print('remove...')
del instance._name
class Person: # Use (object) in 2.6
def __init__(self, name):
self._name = name
name = Name() # Assign descriptor to attr
bob = Person('Bob Smith') # bob has a managed attribute
print(bob.name) # Runs Name.__get__
bob.name = 'Robert Smith' # Runs Name.__set__
print(bob.name)
del bob.name # Runs Name.__delete__
print('-'*20)
sue = Person('Sue Jones') # sue inherits descriptor too
print(sue.name)
print(Name.__doc__) # Or help(Name)
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Notice in this code how we assign an instance of our descriptor class to a class attribute in the client class; because of this, it is inherited by all instances of the class, just like a class’s methods. Really, we must assign the descriptor to a class attribute like this—it won’t work if assigned to a self instance attribute instead. When the descriptor’s __get__ method is run, it is passed three objects to define its context:
- self is the Name class instance.
- instance is the Person class instance.
- owner is the Person class.
When this code is run the descriptor’s methods intercept accesses to the attribute, much like the property version. In fact, the output is the same again:
fetch...
Bob Smith
change...
fetch...
Robert Smith
remove...
--------------------
fetch...
Sue Jones
name descriptor docs
Also like in the property example, our descriptor class instance is a class attribute and thus is inherited by all instances of the client class and any subclasses. If we change the Person class in our example to the following, for instance, the output of our script is the same:
...
class Super:
def __init__(self, name):
self._name = name
name = Name()
class Person(Super): # Descriptors are inherited
pass
...
Also note that when a descriptor class is not useful outside the client class, it’s perfectly reasonable to embed the descriptor’s definition inside its client syntactically. Here’s what our example looks like if we use a nested class:
class Person:
def __init__(self, name):
self._name = name
class Name: # Using a nested class
"name descriptor docs"
def __get__(self, instance, owner):
print('fetch...')
return instance._name
def __set__(self, instance, value):
print('change...')
instance._name = value
def __delete__(self, instance):
print('remove...')
del instance._name
name = Name()
When coded this way, Name becomes a local variable in the scope of the Person class statement, such that it won’t clash with any names outside the class. This version works the same as the original—we’ve simply moved the descriptor class definition into the client class’s scope—but the last line of the testing code must change to fetch the docstring from its new location:
...
print(Person.Name.__doc__) # Differs: not Name.__doc__ outside class
