I think it depends on how you think about it. As mentioned elsewhere, "=" is a special statement syntax rather than an operator in Python, but if you were to think of it like an operator, it would be evaluated in three phases: (1) evaluate one side, (2) evaluate the other side, (3) perform operation. The __setitem__ method defines how the left side participates in step 3, but IMO there's a separate LHS evaluation step that's independent of the assignment itself.

You might call this code snippet analogous:

    class ShoutyNumber:
        def __init__(self, n):
            self.n = n
            print(f'Created {self.n}')
    
        def __add__(self, other):
            print(f'Added with {self.n}')
            return self.n + other.n
    
    ShoutyNumber(1) + ShoutyNumber(2)

Yields:

    Created 1
    Created 2
    Added with 1

But I understand that in some mental models and some implementations, it's cleaner to have the entire RHS value ready before starting any evaluation of the LHS. I think that avoids the need to have "assignable reference" as a special type of expression result.