什么是映射对象,根据dict类型? [英] What is a mapping object, according to dict type?
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问题描述
文档列出了创建dict实例的3种方法:
The documentation lists 3 ways for creating a dict instance:
class dict(**kwarg)
class dict(mapping, **kwarg)
class dict(iterable, **kwarg)
这里是一个映射究竟是什么? dict(mapping)
要工作的最小接口是什么?
What exactly is a mapping here? What's the minimal interface required for dict(mapping)
to work?
推荐答案
从 CPython的源代码,此评论:
/* We accept for the argument either a concrete dictionary object,
* or an abstract "mapping" object. For the former, we can do
* things quite efficiently. For the latter, we only require that
* PyMapping_Keys() and PyObject_GetItem() be supported.
*/
所以,dict(映射)工作所需的最小接口似乎是 .keys()
和 .__ getitem __()
。
So, "the minimal interface required for dict(mapping) to work" appears to be .keys()
and .__getitem__()
.
示例程序:
class M:
def keys(self):
return [1,2,3]
def __getitem__(self, x):
return x*2
m = M()
d = dict(m)
assert d == {1:2, 2:4, 3:6}
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