Make Dictionary with only keys?

Question

I need to make a dictionary containing only keys.

I cannot use d.append() as it is not a list, neither setdefault as it needs 2 arguments: a key and a value.

It should work as the following:

d = {}

add "a":

d = {"a"}

add "b":

d = {"a", "b"}

add "c"...

# Final result is:

d = {"a", "b", "c"}

What is the code I need to get this result? Or is it another solution? Such as making a list.

l = ["a", "b", "c"] # and transform it into a dictionary: d = {"a", "b", "c"} ?

`d = {"a", "b", "c"}` It's not a dictionary It's a `set` only — Rahul K P, Feb 20 '17 at 12:25
If you started with `d = set()` instead of `d = {}`, it would have worked. — khelwood, Feb 20 '17 at 12:26
`dict.fromkeys('abc')` will give you `{'a': None, 'c': None, 'b': None}`, however it sounds like what you want is a [`set`](https://docs.python.org/3/library/stdtypes.html#set). — martineau, Feb 20 '17 at 12:51

score 25 · Answer 1 · answered Feb 20 '17 at 12:27

25

A dict with only keys is called a set.

Start with an empty set instead of a dictionary.

d = set()
d.add('a')
d.add('b')
d.add('c')

You can also create a set via a {} expression:

d = { 'a', 'b', 'c' }

Or using a list:

d = set(['a', 'b', 'c'])

answered Feb 20 '17 at 12:27

khelwood

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and how I retrive value b? in a dictionary I don't need to iterate over it – Leonardo Rick Sep 25 '20 at 18:54
@LeonardoRick You can get the elements of a set by iterating it. In a dict if you already have a key you can look up the associated value, but in a set there is no value. So if you already have the key, there is nothing to look up. Or what are you asking? – khelwood Sep 25 '20 at 18:56
I already have the key, but wan't to check if this key is in the set. In a dict It will be fast as `if 'key' in dict`. Can I do that on a set? What's de difference of doing this on a set, dict or even a list – Leonardo Rick Sep 25 '20 at 19:23
1

@LeonardoRick Yes `if key in myset` is as fast as `if key in mydict`. Much faster than in a list. See [docs for set](https://docs.python.org/3/library/stdtypes.html#set) – khelwood Sep 25 '20 at 19:25
Important note: `set` will not preserve the insertion order. `print(d)` will print `{'b', 'c', 'a'}` (not {'a', 'b', 'c',}) – jerrymouse Sep 25 '21 at 19:32

Ma0 · Answer 2 · 2017-02-20T12:27:05.357

9

That should do it:

l = ["a", "b", "c"]

d = {k:None for k in l}

As @Rahul says in the comments, d = {"a", "b", "c"} is not a valid dictionary definition since it is lacking the values. You need to have values assigned to keys for a dictionary to exist and if you are lacking the values you can just assign None and update it later.

edited Feb 20 '17 at 12:27

answered Feb 20 '17 at 12:24

Ma0

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Thanks for your answer, but it's not what I want. This returns {'a': None, 'b': None, 'c': None} And I don't want the "None" – MartinM Feb 20 '17 at 12:26
@MartinM You have to have something. It does not have to be `None`. It might as well be an empty string (`''`). – Ma0 Feb 20 '17 at 12:28
Rahul K P and khelwood found the solution. Thanks anyway! – MartinM Feb 20 '17 at 12:54
Thats awesome - thanks :) – mnikley Oct 01 '21 at 11:31
@MartinM and the solution is ...? – user8395964 Apr 18 '23 at 09:28
@user8395964 you can see it in their answers; use a `set` – Ma0 Apr 20 '23 at 07:52

score 6 · Answer 3 · answered Feb 20 '17 at 12:28

6

You need a set not a dictionary,

l = ["a", "b", "c"]
d = set(l)

answered Feb 20 '17 at 12:28

Rahul K P

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Make Dictionary with only keys?

3 Answers3