1. Descriptors
jss 2011-07-07

2. What can you do
inside a class suite?

3. @c_decorator1
@c_decorator0(foo=23)
class Foo(object):
__metaclass__ = MetaClass
wenglo = 0
garply, corge = mktuple(qux)
@staticmethod
def fred(): pass
def __new__(klass): pass
All In One: @classmethod
def wilma(klass): pass
def __init__(self, *_): pass
@f_decorator
def foo(self, *_): pass
@property
def bar(self): pass
@bar.setter
def set_bar(self, v): pass
helga = Descriptor()
print locals()

4. @c_decorator1
@c_decorator0(foo=23)
class Foo(object):
__metaclass__ = MetaClass
wenglo = 0
garply, corge = mktuple(qux)
@staticmethod
def fred(): pass
def __new__(klass): pass
All In One: @classmethod
def wilma(klass): pass
def __init__(self, *_): pass
@f_decorator
def foo(self, *_): pass
@property
def bar(self): pass
@bar.setter
def set_bar(self, v): pass
helga = Descriptor()
print locals()

5. Descriptor Protocol
• Descriptors are customized Object Attributes
• An object with any of these defined is a descriptor:
__get__(self, obj, type=None)
__set__(self, obj, val)
__delete__(self, obj)

6. Generic Descriptor Examples
class Foo(object):
def wenglo(self): f = Foo()
return self.qux f.wenglo()
@classmethod Foo.foo()
def foo(klass): Foo.wenglo()
return klass.bar
@staticmethod
def buz():
return 42

7. “Normal” attribute access
class Foo(object): pass
f = Foo()
f.bla
# implemented in object.__getattribute__:
== type(f).__dict__[‘bla’].__get__(f, type(f))

8. Demo
show some code:
check out ListProperty in postamt.py

9. Implementing property()
# by Raymond Hettinger: http://users.rcn.com/python/download/Descriptor.htm
class property(object):
def __init__(self, fget=None, fset=None, fdel=None, doc=None):
self.fget = fget
self.fset = fset
self.fdel = fdel
self.__doc__ = doc
def __get__(self, obj, objtype=None):
if obj is None:
return self
if self.fget is None:
raise AttributeError, "unreadable attribute"
return self.fget(obj)
def __set__(self, obj, value):
if self.fset is None:
raise AttributeError, "can't set attribute"
self.fset(obj, value)
def __delete__(self, obj):
if self.fdel is None:
raise AttributeError, "can't delete attribute"
self.fdel(obj)