descriptors

Question 1

descriptor example

Answer 1

from weakref import WeakKeyDictionary

class Positive():
	def \_\_init(self):
		self._instance_data = WeakKeyDictionary()

	def \_\_get\_\_(self, instance, owner_class):
		return self._instance_data[instance]

	def \_\_set\_\_(self, instance, value):
		if value <=0:
			raise ValueError()
		self._instance_data[instance] = value

	def \_\_delete\_\_(self, instance):
		raise AttributeError()

Question 2

if descriptor called from a class

Answer 2

instance is None

Question 3

non-data descriptor

Answer 3

only __get__ method

Question 4

attribute lookup precedence

Answer 4

data descriptor,__dict__, non-data descriptor