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I've made a dictionary in Python that's composed of 4 lists as values, with a single letter key for each list. When I try to change a value in only one of the lists by referencing dict_name[key][list index], I'm finding that that same index is changing in the lists for every one of the keys in the dictionary.

Here's an example of the starting dictionary:

my_dict = {'A': [0, 0, 0, 0], 'C': [0, 0, 0, 0], 'G': [0, 0, 0, 0], 'T': [0, 0, 0, 0]}

And then I try to change a single value in one of the lists by iterating with a for-loop:

test_string = 'WXTZ'
k = len(test_string)

for i in range(k):
    if test_string[i] == 'T':
        my_dict['T'][i] += 1

But the output I'm getting doesn't make sense:

print(my_dict) = {'A': [0, 0, 1, 0], 'C': [0, 0, 1, 0], 'G': [0, 0, 1, 0], 'T': [0, 0, 1, 0]}

I would think that this should return the following instead:

print(my_dict) = {'A': [0, 0, 0, 0], 'C': [0, 0, 0, 0], 'G': [0, 0, 0, 0], 'T': [0, 0, 1, 0]}

Has anyone else experienced something like this before? How would I change only the 'T' list in this case with such a loop, if not this way?

I'm in Python 3 with Atom on Windows 10 if that's worth knowing. Thanks!

EDIT: A few of you asked about the code that I used to create my_dict. Including that below for reference:

holder = []
my_dict = {}
k = len(string)
for i in range(k):
    holder.append(0)

for char in 'ACGT':
    my_dict[char] = holder
charlie maxwell
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    You should show the code where you create `my_dict`. It looks like you used a single list and referenced it many times. For a discussion see: [How do I clone a list so that it doesn't change unexpectedly after assignment?](https://stackoverflow.com/questions/2612802/how-do-i-clone-a-list-so-that-it-doesnt-change-unexpectedly-after-assignment) – Mark Oct 11 '22 at 18:13
  • your code produces the correct output. I suspect the way you are creating `my_dict` actually results in *the same list* being used as a value for each of the keys – juanpa.arrivillaga Oct 11 '22 at 18:13
  • As an aside, this should probably just be `my_dict['T'][test_string.index('T')] += 1` if you know `'T'` is in your string – juanpa.arrivillaga Oct 11 '22 at 18:14
  • @juanpa.arrivillaga That works if T is in the string, but if it is only one time in it – PlainRavioli Oct 11 '22 at 18:17
  • @PlainRavioli yes, you are correct – juanpa.arrivillaga Oct 11 '22 at 19:02
  • @Mark thank you! That seemed to be exactly what was happening -- each of the 4 lists in the dictionary I had created were all referring to the same list that I used to create the dictionary. Great to know for future reference :) – charlie maxwell Oct 11 '22 at 20:15

1 Answers1

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Probably is what mark sugested in the comment, you are using the same list object in all the values of the dict.

I tried exactly what you exemplified:

my_dict = {'A': [0, 0, 0, 0], 'C': [0, 0, 0, 0], 'G': [0, 0, 0, 0], 'T': [0, 0, 0, 0]}

my_dict['T'][0] = 1

And that changed to:

{'A': [0, 0, 0, 0], 'C': [0, 0, 0, 0], 'G': [0, 0, 0, 0], 'T': [1, 0, 0, 0]}

Try initializing the dict as that:

my_dict = {'A': [0, 0, 0, 0], 'C': [0, 0, 0, 0], 'G': [0, 0, 0, 0], 'T': [0, 0, 0, 0]}

Or using different list objects like:

list1=[0,0,0,0]

list2=[0,0,0,0]

dict={'A': list1, 'B':list2}

Rodrigo Mosken
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