dict.viewkeys() 返回的数据类型是什么?[蟒蛇2.7] [英] What's the data type returned by dict.viewkeys()? [ python 2.7 ]

问题描述

我今天正在研究dict.viewkeys()，发现我的python 称它为dict_keys 对象.我可以将其作为可迭代对象处理，但它不是生成器，因为我可以对其进行多次迭代.

以我有限的知识，我只知道几种数据类型，例如 String、int、float、list、dict、tuple、set.

但是昨天我了解到 enumerate() 返回一对特殊的数据，dict() 只能使用该数据对一次，因此它是一个特殊的元组生成器，带有 <代码>(index_of_iteration, item) 值

这个 dict_keys 对象是另一种我不知道它到底是什么，但我知道如何使用它"类型的 Python 对象?

解决方案

它返回一个字典视图对象 (https://docs.python.org/2/library/stdtypes.html#dictionary-view-objects).

这是字典中元素的动态视图.即，如果您查看字典中的键，如果您从字典中删除一个键，它也会从视图中删除.请参阅下面的示例.

来自文档:

<预><代码>>>>菜肴 = {'鸡蛋':2，'香肠':1，'培根':1，'垃圾邮件':500}>>>键 = 菜肴.viewkeys()>>>值 = 菜肴.viewvalues()>>># 迭代>>>n = 0>>>对于 val 值:... n += val>>>打印(n)504>>># 键和值以相同的顺序迭代>>>列表(键)['鸡蛋'、'培根'、'香肠'、'垃圾邮件']>>>列表(值)[2, 1, 1, 500]>>># 视图对象是动态的并反映字典的变化>>>德尔菜肴['鸡蛋']>>>del 菜肴['香肠']>>>列表(键)['垃圾邮件'，'培根']>>># 设置操作>>>钥匙&{'鸡蛋'、'培根'、'沙拉'}{'培根'}

另见:什么是 Python 字典视图对象?

I am studying dict.viewkeys() today, I found that my python calls it dict_keys object. And I can deal it as an iterable, but it's not a generator, because I can iterate over it more than once.

By my limited knowledge, I know only a few data types such as String, int, float, list, dict, tuple, set.

But yesterday I learned that enumerate() returns a special pair of data which can be used by dict() only once, thus it's a special tuple generator with (index_of_iteration, item) value

Is this dict_keys object another "I don't know what it exactly is but I know how to use it" type of object in python, or something else?

解决方案

It returns a dictionary view object (https://docs.python.org/2/library/stdtypes.html#dictionary-view-objects).

This is a dynamic view of the elements in a dictionary. i.e. if you have a view to the keys in a dictionary, if you delete a key from the dictionary it will also be deleted from the view. See the examples below.

From the docs:

>>> dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500}
>>> keys = dishes.viewkeys()
>>> values = dishes.viewvalues()

>>> # iteration
>>> n = 0
>>> for val in values:
...     n += val
>>> print(n)
504

>>> # keys and values are iterated over in the same order
>>> list(keys)
['eggs', 'bacon', 'sausage', 'spam']
>>> list(values)
[2, 1, 1, 500]

>>> # view objects are dynamic and reflect dict changes
>>> del dishes['eggs']
>>> del dishes['sausage']
>>> list(keys)
['spam', 'bacon']

>>> # set operations
>>> keys & {'eggs', 'bacon', 'salad'}
{'bacon'}

Also see: What are Python dictionary view objects?

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