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

The Subtle Difference Between "i = x" and "i = i x" in Python

In Python, the assignment operator "=" does what you would expect: it assigns a value to a variable. However, the augmented assignment operator " =" has a slightly different behavior, depending on the type of object being modified.

The key to understanding the difference lies in the magic methods "__iadd__" and "__add__". "__iadd__" is called when the augmented assignment operator is used, while "__add__" is called when the regular " " operator is used.

Mutable vs. Immutable Objects

The main distinction between " =" and " " becomes apparent when dealing with mutable and immutable objects. Immutable objects cannot be changed, so their magic methods will typically return a new instance. On the other hand, mutable objects can be modified, and their magic methods will typically modify the object itself.

Case of Immutable Objects (e.g., Integers)

For immutable objects like integers, both " =" and " " have the same effect. They will create a new integer and assign it to the variable. For example:

i = 1
i += 1  # Same as i = i + 1

Case of Mutable Objects (e.g., Lists)

For mutable objects like lists, the behavior of " =" differs from " ". Consider the following example:

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]

In this case, using " =" modifies the list "b" in place. Both "a" and "b" now point to the same modified list.

Contrast this with:

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

Here, using " " creates a new list, which is assigned to "b". "a" is unaffected because it still points to the original list.

Conclusion

In summary, the choice between " =" and " " depends on the type of object being assigned. For immutable objects, both operators are equivalent. For mutable objects, " =" modifies the object in place, while " " creates a copy.

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