Why does my command add the new value to every list in my dictionary?

Question

Why does my command add the new value "test" to every list in my dictionary and not only to the one corresponding to key "key1"?

In:

# Create list of entries
list_f = ["key1", "key2"]

# Create dictionary out of list
dic_f = dict.fromkeys(list_f, [])

# Only append "test" as value to the key "key1"
dic_f["key1"].append("test")
dic_f

Out:

{'key1': ['test'],
 'key2': ['test']}

Desired output:

{'key1': ['test'],
 'key2': []}

Note: >95% of the time, you don't want to preinitialize a `dict` with a specified set of keys. You just want to use `collections.defaultdict`. In this case, `collections.defaultdict(list)` would do the trick; it won't have a `'key2'` until you look it up, but as soon as you do, it will autovivify it with a fresh new `list`. — ShadowRanger, Jun 07 '22 at 14:41

Ohad Sharet · Answer 1 · 2022-06-07T14:38:51.363

I believe that happens sinse all of the the values in the dictionary are the same list,

What I mean by that is , every value in you dictionary points to the same empty list in the memory , so when you change one list all of the change since all of them are the same list

list_f = ["key1", "key2"]

# Create dictionary out of list

dic_f = {}
for key in list_f:
    dic_f[key]=[]

# Only append "test" as value to the key "key1"
dic_f["key1"].append("test")
print(dic_f)

This should do the trick :)

Why does my command add the new value to every list in my dictionary?

1 Answers1