Why Does `x = [4]` Modify `x` but `x = x [4]` Create a New List in Python?

Disparate Behavior: list.__iadd__ vs list.__add__

Consider the following enigmatic Python code:

x = y = [1, 2, 3, 4]
x += [4]
print(x)  # [1, 2, 3, 4, 4]
print(y)  # [1, 2, 3, 4, 4]

Now, juxtapose it with this code snippet:

x = y = [1, 2, 3, 4]
x = x + [4]
print(x)  # [1, 2, 3, 4, 4]
print(y)  # [1, 2, 3, 4]

What perplexes us is the stark difference in their behavior. Why does x = [4] alter the original list x, while x = x [4] results in a new list?

The answer lies in Python's object methods. list.__iadd__ mutates the existing list by appending the elements of the second list onto it. Conversely, list.__add__ creates a new list comprising the elements of both input lists.

An expression like x = y first attempts to invoke __iadd__. If __iadd__ is available, it will execute this mutation magic. Otherwise, it will default to __add__, which creates a new list and assigns it to x. Since list class defines __iadd__, the first code snippet mutates the original x list, while the latter code snippet creates a new list and assigns it to x, leaving y unmodified.

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