add a value for a list of specific keys in dictionary

Question

I am trying to add a specific value to a specific key in a dictionary, and I got a list of special keys. I am using a for loop to do this, but the result is adding everything to all the keys in the dictionary

keyList = ['a','b','c']
testdict = dict.fromkeys(keyList,[])
specKey = ['a','c']

for i in specKey:
    t = testdict.get(i)
    t.append(i)
print(testdict)

The result is this

{'a': ['a', 'c'], 'b': ['a', 'c'], 'c': ['a', 'c']}

But I expect the result should be like this:

{'a': ['a'], 'b': [], 'c': ['c']}

Can someone tell me what I did wrong? Thanks!

To avoid this issue, use a dict comprehension instead: `testdict = {k:[] for k in keyList}` — Green Cloak Guy, Sep 07 '20 at 14:19
`fromkeys` is usually only useful if you use an *immutable* value to initialize each key. — chepner, Sep 07 '20 at 14:19
Oh, so I made a mistake when I create the dictionary... Thank you guys! — cwl6, Sep 07 '20 at 14:24

Aviv Yaniv · Accepted Answer · 2020-09-07T14:25:50.623

1

The line:

dict.fromkeys(keyList, [])

creates the same list which is shared by all keys.

Instead, explicitly create list to each key:

keyList = ['a','b','c']
testdict = { k : list() for k in keyList}
specKey = ['a','c']

for i in specKey:
    testdict[i].append(i)

# {'a': ['a'], 'b': [], 'c': ['c']}
print(testdict)

edited Sep 07 '20 at 14:25

answered Sep 07 '20 at 14:20

Aviv Yaniv

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add a value for a list of specific keys in dictionary

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