Why Does Modifying One List Seem to Change Another in Python?

Why Changing One List Unexpectedly Alters Another

In Python, it's common to encounter situations where altering one list appears to impact another unanticipatedly. Let's examine why this occurs.

Consider the following code:

<code class="python">v = [0, 0, 0, 0, 0, 0, 0, 0, 0]
vec = v
vec[5] = 5</code>

After executing this code, both v and vec display the following modified list: [0, 0, 0, 0, 0, 5, 0, 0, 0].

Explanation:

vec and v are not separate lists but rather references to the same list in memory. When you assign vec = v, you're not creating a new list; instead, you're giving vec the same address as v. Therefore, any modifications made to vec directly affect the original list referred to by both v and vec.

Solution:

To create a copy of v rather than just a reference to it, you should use the following syntax:

<code class="python">vec = list(v)</code>

By using list(v), you create a new list with the same elements as v. Any changes made to vec will not affect v, and vice versa.

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