Why Does Python's = Operator Behave Differently with Lists?

Unusual Behavior of = Operator for Lists

Within Python, the = operator might exhibit unexpected behavior when utilized with lists. This peculiar behavior stems from a distinction between the iadd and add special methods.

The iadd method allows for in-place addition, modifying the object it operates on. On the other hand, add typically returns a new object and is employed by the operator.

For mutable objects like lists, = invokes __iadd__, resulting in modification of the object itself. However, for immutable types such as tuples, strings, and integers, a new object is generated (effectively, a = b translates to a = a b).

Consequently, the choice between iadd and add is crucial. Using a = b leads to iadd being called and subsequent modification of a, while a = a b creates a new object and assigns it to a. These represent distinct operations.

For types supporting both iadd and add__, careful consideration is required when choosing the appropriate method. a = b will trigger __iadd and thus modify a, whereas a = a b will yield a new object and assign it to a.

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