What is the key difference between `list.iadd` and `list.add` in Python?

Mary-Kate Olsen

Oct 30, 2024 am 03:11 AM

What is the key difference between `list.__iadd__` and `list.__add__` in Python?

Different Behaviors of list.__iadd__ and list.__add__

In Python, list objects provide two methods for list concatenation: iadd and __add__. Despite their similar purpose, these methods exhibit a significant difference in behavior.

iadd is the in-place list concatenation operator. When used with the = operator, it modifies the existing list in place. For example:

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

In this case, the call to x = [4] uses iadd to append the element 4 to the list x, resulting in the modified list [1, 2, 3, 4, 4].

On the other hand, add is the list addition operator. When used with the operator, it returns a new list rather than modifying the original. For example:

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

In this case, the call to x = x [4] uses add to create a new list that combines the elements of x and [4]. The original list x remains unchanged.

This difference in behavior is due to the concept of immutability in Python. Lists are mutable objects, meaning that in-place modifications can be made to them. iadd takes advantage of this mutability to directly modify the list, while add creates a new list to preserve the immutability of the original.

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