Inside a class, you can define methods, properties, descriptors, and other attributes. Methods can be regular, static, or class methods defined with decorators. Properties allow getting and setting attribute values through getter and setter methods. Descriptors are customized object attributes that implement certain descriptor methods like __get__ and __set__.

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)