How to init dictionary only with keys in python

Question

I want to initialize a dictionary in python when I only have the keys and not the values - The values will come later in the code.

I thought about to do something like this:

dict = {}
for key in key_list:
    dict[key] = None

The question is if this is a good practice for this problem or do you have a better recommendation?

Because the keys are initialized from one file and the values come from another file. And each line in the inputs file has another value. I want to have the dictionary before I read the values — Shaked Eyal, Apr 10 '19 at 14:35
just store the "keys" in a list or something? or read both files at once and make the dict as you go — SuperStew, Apr 10 '19 at 14:36
I suspect something like `keys = read_data(file_one); values = read_data(file_two); d = dict(zip(keys, values))` might be better — Sean, Apr 10 '19 at 14:37
@ShakedEyal sure you can https://stackoverflow.com/questions/11295171/reading-two-text-files-line-by-line-simultaneously — SuperStew, Apr 10 '19 at 14:42
@SuperStew I know you can do it, but it is not in the requirements of the program. I need to read them separately — Shaked Eyal, Apr 10 '19 at 14:44

C.Nivs · Accepted Answer · 2019-04-10T14:39:21.380

4

You can do this with the fromkeys method:

# As noted in comments, fromkeys is a staticmethod on the dict class and will return a dict instance
d = dict.fromkeys(list(range(10)))

d
{0: None, 1: None, 2: None, 3: None, 4: None, 5: None, 6: None, 7: None, 8: None, 9: None}

To more directly address the point:

list_of_keys = ['o1', 'o2']

d = dict.fromkeys(list_of_keys)
{'o1': None, 'o2': None}

edited Apr 10 '19 at 14:39

answered Apr 10 '19 at 14:30

C.Nivs

12,353
2
19
44

3

No need to create the list, `fromkeys` will iterate on its own, and there is no need to create `d` since `fromkeys` is a staticmethod. `dict.fromkeys(range(10))` – DeepSpace Apr 10 '19 at 14:32
@DeepSpace I left it as a `list` to be directly applicable to what OP is asking for, *can I create a dict from a list of keys*, rather than could it be created from any iterable – C.Nivs Apr 10 '19 at 14:36
And if I have specific keys and not numbers? for example, I have something like this: `{'o1': None, 'o2': None}` – Shaked Eyal Apr 10 '19 at 14:37
@ShakedEyal it would take the same structure: `dict.fromkeys(['o1', 'o2', ...])` – C.Nivs Apr 10 '19 at 14:38
2

@C.Nivs But OP already has a list, so a better example (if we follow your argument) would be `dict.fromkeys(key_list)` – DeepSpace Apr 10 '19 at 14:38
@DeepSpace true, I've added in a more direct example – C.Nivs Apr 10 '19 at 14:40
Thank you for your help – Shaked Eyal Apr 10 '19 at 14:41

score 1 · Answer 2 · answered Apr 10 '19 at 14:38

1

If your keys are already in memory, assuming an iterable object, then:

d = {key:None for key in keys}

It is the same as your posted code, just condensed.

answered Apr 10 '19 at 14:38

SEpapoulis

96
3

score 0 · Answer 3 · answered Apr 10 '19 at 14:42

0

dict.fromkeys([1, 2, 3, 4])

You can try that class method.

answered Apr 10 '19 at 14:42

Mr.Robot

333
1
2
13

How to init dictionary only with keys in python

3 Answers3