Appending to pre-populated list within a dictionary of lists alters all lists within dictionary

Question

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. In this case, every value in values gets appended to every list within value_groups. Non-functional code below:

values = ["a_one", "a_two", "b_three", "b_four", "c_five", "c_six"]
# ---START OF EDITED CODE---
initial_list = ["x", "y", "z"]
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 Stack Overflow 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…)

Asked By: ChefPaul92

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Answer 1

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 using some_list.copy() (or by copying with an empty slice some_list[:]):

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

Answered By: Steven Rumbalski

Appending to pre-populated list within a dictionary of lists alters all lists within dictionary

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