Initializing a dictionary in python with a key value and no corresponding values

Question

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

User · Answer

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

User · Answer

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

User · Answer

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    try      my data k  data k    except KeyError      pass 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

User · Answer

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

User · Answer

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

User · Answer

Comprehension could be also convenient in this case    from a list keys     quot k1 quot    quot k2 quot   d    k None for k in keys     or from another dict d1     quot k1 quot    1   quot k2 quot    2  d2    k None for k in d1 keys     d2     k1   None   k2   None

User · Answer

You could initialize them to None

User · Answer

q   input  Apple   w   input  Ball   Definition     apple   q   ball   w

[python] Initializing a dictionary in python with a key value and no corresponding values

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