What's the Key Difference Between `i = x` and `i = i x` in Python?

Understanding the Differences Between "i = x" and "i = i x" in Python

The = operator, which performs the operation "i = x," has been known to cause confusion as it may have different effects compared to the standard "i = i x" notation. However, the distinction between these operators lies not in their syntax but in their underlying implementation.

iadd vs. add Methods

= invokes the iadd method if it exists, while calls the add method. The iadd method is intended for modifying mutable objects in place, returning the modified object, whereas add typically returns a new instance.

Immutability vs. Mutability

For immutable objects, both methods create a new instance. However, with mutable objects, iadd modifies the existing object without creating a new one.

Practical Example

Consider the following code:

a = [1, 2, 3]
b = a
b += [1, 2, 3]

Here, both a and b initially point to the same list. However, when we use = on b, it modifies the list in place, and since a references the same object, it sees the change as well.

In contrast, if we use b = b [1, 2, 3], a new list is created and assigned to b. a is unaffected, as it still points to the original list.

Conclusion

The difference between = and lies in their underlying method implementation and how they interact with mutable and immutable objects. = is intended for modifying mutable objects in place, while generally creates a new instance for both mutable and immutable objects. This understanding is crucial for correctly manipulating objects in Python and avoiding potential pitfalls.

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