Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Teams

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

I was wondering if there was a way to initialize a dictionary in python with keys but no corresponding values until I set them. Such as:

Definition = {'apple': , 'ball': }
and then later i can set them:
Definition[key] = something
I only want to initialize keys but I don't know the corresponding values until I have to set them later. Basically I know what keys I want to add the values as they are found. Thanks.
                There is no such thing as a key without a value in a dict.  You can just set the value to None, though.
– BrenBarn
                Nov 19, 2013 at 18:54
                What does it mean to define a key with no value?  If you try to access that item, would it generate an error (as if you had never defined that key), or yield something (in which case that something is its value)?
– Scott Hunter
                Nov 19, 2013 at 19:01
                Since you know you're going to replace the values later, it really doesn't matter what you set them to.
– Mark Ransom
                Nov 19, 2013 at 19:02
                I was creating a dictionary that goes through another dictionary to only grab the values of the keys I wanted so I wanted to be able to compare keys and then set them if they are equal. Might of been a bad way but it works! Thanks again!
– user2989027
                Nov 19, 2013 at 19:12
Use the fromkeys function to initialize a dictionary with any default value. In your case, you will initialize with None since you don't have a default value in mind.
empty_dict = dict.fromkeys(['apple','ball'])
this will initialize empty_dict as:
empty_dict = {'apple': None, 'ball': None}
As an alternative, if you wanted to initialize the dictionary with some default value other than None, you can do:
default_value = 'xyz'
nonempty_dict = dict.fromkeys(['apple','ball'],default_value)
                When using dict.fromkeys(..., default_value), make sure default_value is not of a type that requires a constructor (like list), else you'll get references to the same object in all your buckets.
– Teodor Anton
                Nov 4, 2016 at 7:52
                Or if None is a legitimate value for one or more of your key-value tuples, you can use something like "SENTINEL = object()" and use SENTINEL as your sentinel value, testing with "is" instead of "==".
– dstromberg
                Nov 19, 2013 at 19:00
                Yep agreed, Look at your overall scope of data and determine what value should never be available ever. -1, false, none, random int, random string and so on.. Old question but top result still.
– Angry 84
                Nov 9, 2015 at 1:53
                @Markus: Such an error would occur when trying to do something like Definition[key] = something when Definition is None (which is not what is being advocated here), not when Definition[key] is None (which is).
– Scott Hunter
                Jun 30, 2019 at 20:04
you could use a defaultdict.  It will let you set dictionary values without worrying if the key already exists.  If you access a key that has not been initialized yet it will return a value you specify (in the below example it will return None)
from collections import defaultdict
your_dict = defaultdict(lambda : None)
It would be good to know what your purpose is, why you want to initialize the keys in the first place. I am not sure you need to do that at all.
1) If you want to count the number of occurrences of keys, you can just do:
Definition = {}
# ...
Definition[key] = Definition.get(key, 0) + 1
2) If you want to get None (or some other value) later for keys that you did not encounter, again you can just use the get() method:
Definition.get(key)  # returns None if key not stored
Definition.get(key, default_other_than_none)
3) For all other purposes, you can just use a list of the expected keys, and check if the keys found later match those.
For example, if you only want to store values for those keys:
expected_keys = ['apple', 'banana']
# ...
if key_found in expected_keys:
    Definition[key_found] = value
Or if you want to make sure all expected keys were found:
assert(all(key in Definition for key in expected_keys))
You can initialize the values as empty strings and fill them in later as they are found.
dictionary = {'one':'','two':''}
dictionary['one']=1
dictionary['two']=2
Based on the clarifying comment by @user2989027, I think a good solution is the following:
definition = ['apple', 'ball']
data = {'orange':1, 'pear':2, 'apple':3, 'ball':4}
my_data = {}
for k in definition:
    my_data[k]=data[k]
  except KeyError:
print my_data
I tried not to do anything fancy here.  I setup my data and an empty dictionary.  I then loop through a list of strings that represent potential keys in my data dictionary.  I copy each value from data to my_data, but consider the case where data may not have the key that I want.
        
Thanks for contributing an answer to Stack Overflow!
Please be sure to answer the question. Provide details and share your research!
But avoid …
Asking for help, clarification, or responding to other answers.
Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.