我将WeakValueDictionary与setdefault一起使用，并且希望不创建默认事物的实例
import weakref
my_dict = weakref.WeakValueDictionary()
# ...
# Will create an instance of MyBigFancyClass even if it's not used
value = my_dict.setdefault('my-key',default=MyBigFancyClass())
在不使用MyBigFancyClass的情况下，如何避免创建它？
您可以使用setdefault块来代替使用try/except，或者我个人更希望直接覆盖__getitem__：
import weakref
class MyBigFancyClass:
pass
class MyWeakValueDictionary(weakref.WeakValueDictionary):
def __getitem__(self,item):
try:
return super().__getitem__(item)
except KeyError:
value = self[item] = MyBigFancyClass()
return value
my_dict = MyWeakValueDictionary()
value = my_dict['my-key']
您可以避免setdefault，并使用KeyError
import weakref
my_dict = weakref.WeakValueDictionary()
# ...
try:
value = my_dict['my-key']
except KeyError:
value = MyBigFancyClass()
my_dict['my-key'] = value