List comprehension being passed to Ordered Dict creation is being created as a reference

Question

I am trying to create a ordered dic for a given set of keys. I structure i am going for is `

     OrderedDict([('A', [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]), 
                  ('B', [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]), 
                  ('C', [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]), 
                  ('D', [0, 0, 0, 0, 0, 0, 0, 0, 0, 0])])`

This is being created by the following function.

d = OrderedDict.fromkeys(keys, [0 for _ in range(10)])

I am successfully getting the required structure.

The problem is the values of the dict all are being created as references. Meaning when i try to do the following d['A'][1] = "11111" it changed values of every key given. i get the result as the following.

    OrderedDict([('A', [0, '11111', 0, 0, 0, 0, 0, 0, 0, 0]), 
                 ('B', [0, '11111', 0, 0, 0, 0, 0, 0, 0, 0]), 
                 ('C', [0, '11111', 0, 0, 0, 0, 0, 0, 0, 0]), 
                 ('D', [0, '11111', 0, 0, 0, 0, 0, 0, 0, 0])])

i did try deepcopy but didnt really help. the work around i currently have is

d['A'] = d['A'][:]

i dont really like the above soln because i have about 6mil keys and values.. so doing this is a bit of a hassle.

I would like to know a better way of doing this.. and also why this is happening.

I believe if you can iterate over each list and call `list()` on it, you'll get a list which is not made up of linked references. — clubby789, Oct 14 '19 at 21:03

score 2 · Accepted Answer · answered Oct 14 '19 at 21:00

2

The classmethod fromkeys uses the same default value for all dictionary keys.

To avoid that, specify a new instance for each key:

d = OrderedDict((k, [0]*10) for k in keys)

answered Oct 14 '19 at 21:00

wim

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List comprehension being passed to Ordered Dict creation is being created as a reference

1 Answers1