是否有没有值的 Python 字典？
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Is there a Python dict without values?
提问by nucleartide
Instead of this:
取而代之的是：
a = {"foo": None, "bar": None}
Is there a way to write this?
有没有办法写这个？
b = {"foo", "bar"}
And still let bhave constant time access (i.e. not a Python set, which cannot be keyed into)?
并且仍然允许b持续时间访问（即不是 Python 集，无法键入）？
采纳答案by nneonneo
Actually, in Python 2.7 and 3.2+, this really does work:
实际上，在 Python 2.7 和 3.2+ 中，这确实有效：
>>> b = {"foo", "bar"}
>>> b
set(['foo', 'bar'])
You can't use []access on a set ("key into"), but you can test for inclusion:
您不能[]在集合上使用访问权限（“key into”），但您可以测试是否包含：
>>> 'x' in b
False
>>> 'foo' in b
True
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:
集合尽可能接近无值字典。它们具有一般情况下的恒定时间访问，需要可散列对象（即不存储列表或集合中的字典），甚至支持它们自己的理解语法：
{x**2 for x in xrange(100)}
回答by Ashwini Chaudhary
Yes, sets:
是的，sets：
set() -> new empty set object
set(iterable) -> new set object
Build an unordered collection of unique elements.
Related: How is set() implemented?
Time complexity : https://wiki.python.org/moin/TimeComplexity#set
回答by Claudiu
In order to "key" into a set in constant time use in:
为了在恒定时间内“键”成一个集合，请使用in：
>>> s = set(['foo', 'bar', 'baz'])
>>> 'foo' in s
True
>>> 'fork' in s
False
