How do numpy’s in-place operations (e.g. `+=`) work?

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The first thing you need to realise is that a += x doesn’t map exactly to a.__iadd__(x), instead it maps to a = a.__iadd__(x). Notice that the documentation specifically says that in-place operators return their result, and this doesn’t have to be self (although in practice, it usually is). This means a[i] += x trivially maps to:

a.__setitem__(i, a.__getitem__(i).__iadd__(x))

So, the addition technically happens in-place, but only on a temporary object. There is still potentially one less temporary object created than if it called __add__, though.

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