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)?

What do you mean "cannot be keyed into"? If there are no values, the only way to "key in" is to see if a key is present, which sets can do with `obj in set`. — BrenBarn, Oct 18 '13 at 16:52
its unclear what you are asking for... specifically what kind of access you need to it, and what you mean by keyed-into — Joran Beasley, Oct 18 '13 at 16:54
https://wiki.python.org/moin/TimeComplexity ... yeah but avg case of O(1) — Joran Beasley, Oct 18 '13 at 16:55
@MateuszDymczyk: Then your response should be to OP, who stated that he wants "the same constant-time access that dicts have". Well, dicts don't technically have constant-time access. In any case, sets behave identically to dicts when it comes to checking whether a key is present. — John Y, Oct 18 '13 at 16:57
@JohnY: one could say that sets have the same constant-time access that dicts have in that both don't have it =P. — Claudiu, Oct 18 '13 at 19:26

score 31 · Accepted Answer · answered Oct 18 '13 at 16:54

31

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)}

answered Oct 18 '13 at 16:54

nneonneo

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how do you add a valueless key though like b += ... ? – Lost Crotchet Oct 15 '19 at 00:49
1

Use `b.add(v)` for single items or `b.update(v)` to copy elements from an iterable. – nneonneo Oct 18 '19 at 09:56

score 20 · Answer 2 · edited May 23 '17 at 11:47

20

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

edited May 23 '17 at 11:47

Community

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answered Oct 18 '13 at 16:51

Ashwini Chaudhary

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Note `set` literals only in 2.7 or 3.x – Kabie Oct 18 '13 at 16:55

Claudiu · Answer 3 · 2015-02-23T23:04:28.477

4

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

edited Feb 23 '15 at 23:04

answered Oct 18 '13 at 16:53

Claudiu

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This might slightly confusing. `"a"` is in the set because `set("abcd")` treated the string as an iterable of four characters. `set("abcd")` is the same as `set(['a', 'c', 'b', 'd'])`. So while `"fork"` definitely isn't in the set, neither is `"abcd"`, which this answer could imply. – poulter7 Feb 18 '15 at 16:23
1

`set(['foo', 'bar', 'baz')` should be `set(['foo', 'bar', 'baz'])`, this is clearer though :) – poulter7 Feb 23 '15 at 22:24

Is there a Python dict without values?

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