Is there a Python dict without values?

Question

Instead of this:

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

Is there a way to write this?

b = {"foo", "bar"} 

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

Answer 1

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

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

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

>>> '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:

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

Answer 2

Yes, 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