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\"}

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

In order to "key" into a set in constant time use in:

>>> s = set(['foo', 'bar', 'baz'])
>>> 'foo' in s
True
>>> 'fork' in s
False

Answer 3

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