访问工厂的关键字defaultdict [英] Accessing key in factory of defaultdict
问题描述
我正在尝试类似的操作:
从集合导入defaultdict
import hashlib
def factory():
key ='aaa'
return {'key-md5':hashlib.md5('%s'%(key))hexdigest()}
a = defaultdict(factory)
打印a ['aaa']
(实际上,为什么我需要访问工厂的密钥的原因不是计算一个 md5
,而是出于其他原因,这只是一个例子) p>
正如你所看到的,在工厂我无法访问密钥:我只是强迫它,这没有任何意义。
可以使用 defaultdict
,以便我可以在工厂中访问密钥?
__缺少
不通过 defaultdict
的__ 键
到工厂功能。
如果
default_factory
不是无
,它被称为无参数到
提供给定键的默认值,此值插入
键入的字典,并返回。
使用自定义的 __缺少__
方法。
>>> class myDict(dict):
... def __init __(self,factory):
... self.factory = factory
... def __missing __(self,key):
... self [key] = self.factory(key)
... return self [key]
...
>>> d = MyDict(lambda x:-x)
>>>> d [1]
-1
>>> d
{1:-1}
I am trying to do something similar to this:
from collections import defaultdict
import hashlib
def factory():
key = 'aaa'
return { 'key-md5' : hashlib.md5('%s' % (key)).hexdigest() }
a = defaultdict(factory)
print a['aaa']
(actually, the reason why I need access to the key in the factory is not to compute an md5
, but for other reasons; this is just an example)
As you can see, in the factory I have no access to the key: I am just forcing it, which makes no sense whatsoever.
Is it possible to use defaultdict
in a way that I can access the key in the factory?
__missing__
of defaultdict
does not pass key
to factory function.
If
default_factory
is notNone
, it is called without arguments to provide a default value for the given key, this value is inserted in the dictionary for the key, and returned.
Make your own dictionary class with custom __missing__
method.
>>> class MyDict(dict):
... def __init__(self, factory):
... self.factory = factory
... def __missing__(self, key):
... self[key] = self.factory(key)
... return self[key]
...
>>> d = MyDict(lambda x: -x)
>>> d[1]
-1
>>> d
{1: -1}
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