What is the difference between `list.__iadd__` and `list.__add__` and why does it lead to different results when modifying lists in Python?

Understanding the Differences Between list.__iadd__ and list.__add__

In Python, objects can define special "magic" methods to support their interaction with certain operators. Two such methods are iadd and __add__, used for in-place and regular addition operations, respectively.

Consider the following two code snippets:

<code class="python"># Example 1
x = y = [1, 2, 3, 4]
x += [4]
print(x)  # [1, 2, 3, 4, 4]
print(y)  # [1, 2, 3, 4, 4]

# Example 2
x = y = [1, 2, 3, 4]
x = x + [4]
print(x)  # [1, 2, 3, 4, 4]
print(y)  # [1, 2, 3, 4]</code>

Why do these two examples behave differently?

In both cases, we start with two identical lists, x and y, and we perform an addition operation using either x = [4] or x = x [4].

When using x = [4], Python's __iadd__ method is called, which performs an in-place addition. This means that the original list x is mutated and its contents are updated with the elements in the list [4]. As a result, both x and y now contain the same modified list.

In contrast, when using x = x [4], Python's __add__ method is called, which returns a new list containing the elements from both x and [4]. The original list x remains unchanged, and y still points to the initial unmodified list.

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