Python: Why does the dict.fromkeys method not produce a working dicitonary

Question

I had the following dictionary:

ref_range = range(0,100)
aas = list("ACDEFGHIKLMNPQRSTVWXY*")
new_dict = {}
new_dict = new_dict.fromkeys(ref_range,{k:0 for k in aas})

Then I added a 1 to a specific key

new_dict[30]['G'] += 1

>>>new_dict[30]['G']
1

but

>>>new_dict[31]['G']
1

What is going on here? I only incremented the nested key 30, 'G' by one.

Note: If I generate the dictionary this way:

new_dict = {}
for i in ref_range:
   new_dict[i] = {a:0 for a in aas}

Everything behaves fine. I think this is a similar question here, but I wanted to know a bit about why this happening rather than how to solve it.

Answer 1

fromkeys(S, v) sets all of the keys in S to the same value v. Meaning that all of the keys in your dictionary new_dict refer to the same dictionary object, not to their own copies of that dictionary.

To set each to a different dict object you cannot use fromkeys. You need to just set each key to a new dict in a loop.

Besides what you have you could also do

{i: {a: 0 for a in aas} for i in ref_range}