Descriptor Protocol
1 like370 views
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__.
![“Normal” attribute access
class Foo(object): pass
f = Foo()
f.bla
# implemented in object.__getattribute__:
== type(f).__dict__[‘bla’].__get__(f, type(f))](https://image.slidesharecdn.com/20110707-110708025455-phpapp01/85/Descriptor-Protocol-7-320.jpg)
Descriptor Protocol
- 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)