是否有没有值的Python字典？ [英] Is there a Python dict without values?

问题描述

代替此：

a = {"foo": None, "bar": None}

有没有办法写这个？

b = {"foo", "bar"}

b 具有恒定的访问时间（即没有Python集，无法键入）？

And still let b have constant time access (i.e. not a Python set, which cannot be keyed into)?

推荐答案

实际上，在Python 2.7和3.2+中，这确实有效：

Actually, in Python 2.7 and 3.2+, this really does work:

>>> b = {"foo", "bar"}
>>> b
set(['foo', 'bar'])

你不能使用 [] 访问集合（键入），但是您可以测试是否包含：

You can't use [] access on a set ("key into"), but you can test for inclusion:

>>> 'x' in b
False
>>> 'foo' in b
True

集合与无价值字典一样接近得到。它们具有平均情况下的恒定时间访问权限，需要可散列的对象（即，没有存储列表或集合中的字典），甚至还支持它们自己的理解语法：

Sets are as close to value-less dictionaries as it gets. They have average-case constant-time access, require hashable objects (i.e. no storing lists or dicts in sets), and even support their own comprehension syntax:

{x**2 for x in xrange(100)}

这篇关于是否有没有值的Python字典？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！