Why Does Modifying One List Affect Another in Python?

Unexpectedly Linked Lists in Python

When working with lists in Python, unexpected behaviors may arise. Let's understand why changing one list unexpectedly affects another.

Problem:

Consider the following Python code:

<code class="python">v = [0, 0, 0, 0, 0, 0, 0, 0, 0]
vec = v  # Assignment by reference
vec[5] = 5</code>

After this code executes, both v and vec show the modified value at index 5:

>>> print(vec)
[0, 0, 0, 0, 0, 5, 0, 0, 0]
>>> print(v)
[0, 0, 0, 0, 0, 5, 0, 0, 0]

Why does v change even though it wasn't explicitly modified?

Understanding the Issue:

In Python, when you assign a list to a new variable, you are not creating a copy; instead, you are creating a reference. This means that both v and vec point to the same underlying list object in memory.

Solution:

To have two separate lists with identical values, you need to create a copy using the list() constructor:

<code class="python">vec = list(v)  # Creates a copy</code>

Now, when you make changes to vec, v will remain unaffected.

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