basic example outlining how to use descriptors

descriptorExample.py

# ***** "raw" descriptor not using "property" or property decorators.
class Descriptor(object):  # simply encapsulates the management of the attribute "_name"
    def __init__(self):
        self._name = ''

    def __get__(self, instance, owner):
        print "Getting: %s" % self._name
        return self._name

    def __set__(self, instance, name):
        print "Setting: %s" % name
        self._name = name.title() # title just makes the first letter in every word of the string upper-case

    def __delete__(self, instance):
        print "Deleting: %s" %self._name
        del self._name

class Person(object):
    name = Descriptor()

# ***** an equivalent descriptor, but this time using "property"
class PersonUsingDescriptorViaProperty(object):
    def __init__(self):
        self._name = ''

    def fget(self):
        print "Getting: %s" % self._name
        return self._name
    
    def fset(self, value):
        print "Setting: %s" % value
        self._name = value.title()

    def fdel(self):
        print "Deleting: %s" %self._name
        del self._name

    name = property(fget, fset, fdel, "Property docstring")

# ***** another equivalent descriptor, using property decorators
class PersonDescriptorViaPropertyDecorators(object):
    def __init__(self):
        self._name = ''

    @property
    def name(self):
        print "Getting: %s" % self._name
        return self._name

    @name.setter
    def name(self, value):
        print "Setting: %s" % value
        self._name = value.title()

    @name.deleter
    def name(self):
        print "Deleting: %s" % self._name
        del self._name

# Another approach is adding properties in runtime. This allows for not overriding getter, setter and deleter separately
# for every and each attribute. However I think it's generally what you want to do, so I don't think this pattern is very
# useful and this is apparent even in this basic example.
# I'd stick to the examples above.
class PersonRuntimeDescriptor(object):
    def addProperty(self, attribute):
        # create local getter and setter 
        getter = lambda self: self._getProperty(attribute)
        setter = lambda self, value: self._setProperty(attribute, value)

        # construct property attribute and add it to the class
        setattr(self.__class__, attribute, property(fget=getter, \
                                                    fset=setter, \
                                                    doc="Auto-generated property"))

    def _setProperty(self, attribute, value):
        print "Setting: %s = %s" %(attribute, value)
        setattr(self, '_' + attribute, value.title()) # note that applying title or any other specific function
            # to every property is not typically what we want to do.
            # This is partly why I don't think this runtime style is a very useful pattern.
            # Generally you'd want specific management to every attribute that isn't the same for every property.
            # This is achieved better and more clearly with the methods outlined above.

    def _getProperty(self, attribute):
        print "Getting: %s" %attribute
        return getattr(self, '_' + attribute)

if __name__ == "__main__":
    for user in (Person(), PersonUsingDescriptorViaProperty(), PersonDescriptorViaPropertyDecorators()): 
        # each iteration should work the same
        user.name = 'winston churchill'
        print user.name
        del user.name
        # trying to access user.name would give an AttributeError now because the attribute was removed rather than just its content
        print "\n-----\n"

# it's also possible to do it at runtime by using setattr and getattr 

    print "**** runtime descriptor ****"
    user = PersonRuntimeDescriptor()
    user.addProperty('name')
    user.addProperty('email')
    user.name = 'christopher columbus'
    user.email = '[email protected]'

    print user.name, user.email, user.__dict__

output.txt

> python descriptorExample.py

Setting: winston churchill
Getting: Winston Churchill
Winston Churchill
Deleting: Winston Churchill

-----

Setting: winston churchill
Getting: Winston Churchill
Winston Churchill
Deleting: Winston Churchill

-----

Setting: winston churchill
Getting: Winston Churchill
Winston Churchill
Deleting: Winston Churchill

-----

**** runtime descriptor ****
Setting: name = christopher columbus
Setting: email = [email protected]
Getting: name
Christopher Columbus Getting: email
[email protected] {'_email': '[email protected]', '_name': 'Christopher Columbus'}