When Do `i = x` and `i = i x` Produce Different Results in Python?

When Assignment and Augmentation Differ: Exploring "i = x" vs. "i = i x"

The use of the = operator in Python can lead to unexpected behavior in certain scenarios. Let's examine the differences between i = x and i = i x to understand when they diverge.

Understanding the Operator Overload

The = operator calls the iadd method if it exists, or the add method if iadd is not available. In contrast, the operator primarily calls the add method.

Mutable vs. Immutable Objects

The behavior of = depends on whether the object being assigned is mutable (can be changed) or immutable (cannot be changed). For immutable objects, both = and create a new instance. However, iadd modifies the original object and assigns it back to the variable name, overwriting the previous reference.

Example: Lists

To illustrate the difference, consider the following code:

a = [1, 2, 3]
b = a
b += [1, 2, 3]
print(a)  # [1, 2, 3, 1, 2, 3]
print(b)  # [1, 2, 3, 1, 2, 3]

Since lists are mutable, = modifies b in place, which affects a because both variables reference the same list.

Now, consider:

a = [1, 2, 3]
b = a
b = b + [1, 2, 3]
print(a)  # [1, 2, 3]
print(b)  # [1, 2, 3, 1, 2, 3]

In this case, b = b [1, 2, 3] creates a new list, leaving a unchanged. This is because calls the add method, which returns a new instance.

Exception Handling for ' ='

In the case where x.__add__ is not implemented or returns NotImplemented, and x and y have different types, the operator falls back on y.__radd__ if it exists. Therefore, the following is equivalent:

foo_instance += bar_instance

foo_instance = bar_instance.__radd__(bar_instance, foo_instance)

Subclass Overriding

When foo_instance and bar_instance are of different types, and bar_instance is a subclass of foo_instance, bar_instance.__radd__ will be attempted before foo_instance.__add__. This allows subclasses to override the behavior of their superclasses.

Conclusion

Understanding the differences between i = x and i = i x is crucial for avoiding unexpected results in Python programming. By knowing when and how these operators behave differently, you can effectively manipulate both immutable and mutable objects.

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