*SOLVED* Python: Appending to pre-populated list within a dictionary of lists alters all lists within dictionary

Thanks @StevenRumbalski for correcting the code I posted and figuring out the problem. I was using a pre-defined list to populate the lists within the dictionary, so when I appended to any list within the dictionary, the referenced dictionary was updated, causing all of the lists in the dictionary to change at once.

I have encountered a puzzling error when I’m attempting to sort some values into a dictionary of lists using string prefixes. Here is the basic code I am running:

values = ["a_one", "a_two", "b_three", "b_four", "c_five", "c_six"]
value_groups = {"a": [],
                "b": [],
                "c": []}

for value in values:
    for prefix in value_groups.keys():
        if value.startswith(prefix):
            value_groups[prefix].append(value)
            break

The desired, and actual output of this code is:

value_groups = {"a": ["a_one", "a_two"],
                "b": ["b_three", "b_four"],
                "c": ["c_five", "c_six"]}

So the code above works as-is.

My problem arises when I initialize the lists within value_groups with one or more strings. The code below is updated with my mistake. In this case, every value in values gets appended to every list within value_groups. Non-functional code below:

initial_list = ["x", "y", "z"]
values = ["a_one", "a_two", "b_three", "b_four", "c_five", "c_six"]
# ---START OF EDITED CODE---
value_groups = {"a": initial_list,
                "b": initial_list,
                "c": initial_list}
# ---END OF EDITED CODE---

for value in values:
    for prefix in value_groups.keys():
        if value.startswith(prefix):
            value_groups[prefix].append(value)
            break

The desired output of this code would be:

value_groups = {"a": ["x", "y", "z", "a_one", "a_two"],
                "b": ["x", "y", "z", "b_three", "b_four"],
                "c": ["x", "y", "z", "c_five", "c_six"]}

But instead, I am getting three identical lists within value_groups:

value_groups = {"a": ["x", "y", "z", "a_one", "a_two",
                      "b_three", "b_four", "c_five", "c_six"], 
                "b": ["x", "y", "z", "a_one", "a_two",
                      "b_three", "b_four", "c_five", "c_six"], 
                "c": ["x", "y", "z", "a_one", "a_two",
                      "b_three", "b_four", "c_five", "c_six"]}

My intuition (and preliminary stackoverflow research) is that there is some sort of memory referencing mechanism that I am running afoul of, but I am not quite sure what it is. Anyone out there able to explain it to me? (if that is, indeed, the problem…)

>Solution :

value_groups = {"a": ["x", "y", "z"],
                "b": ["x", "y", "z"],
                "c": ["x", "y", "z"]}

I’m pretty certain that’s not how you’re defining value_groups. What you’ve probably done is something like this (where some_list was previously defined):

some_list = ["x", "y", "z"]
value_groups = {"a": some_list, 
                "b": some_list, 
                "c": some_list}`   

If that’s the case, every key in the dict refers to the same list. A fix would be to make copies of some_list with an empty slice (some_list[:] or some_list.copy()):

some_list = ["x", "y", "z"]
value_groups = {"a": some_list[:], 
                "b": some_list[:], 
                "c": some_list[:]}