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Clarify the required semantics for broadcasting with __setitem__ #429

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Nov 17, 2022
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Clarify the required semantics for broadcasting with __setitem__ #429

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5ebe413
Clarify the required semantics for broadcasting with setitem
asmeurer May 10, 2022
59d8169
Add examples of when in-place operations are allowed to broadcast
asmeurer Nov 3, 2022
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15 changes: 14 additions & 1 deletion spec/API_specification/broadcasting.rst
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Expand Up @@ -112,4 +112,17 @@ The following examples demonstrate array shapes which do **not** broadcast.
In-place Semantics
------------------

As implied by the broadcasting algorithm, in-place element-wise operations must not change the shape of the in-place array as a result of broadcasting.
As implied by the broadcasting algorithm, in-place element-wise operations (including ``__setitem__``) must not change the shape of the in-place array as a result of broadcasting. Such operations should only be supported in the case where the right-hand operand can broadcast to the shape of the left-hand operand, after any indexing operations are performed.

For example:

::

x = empty((2, 3, 4))
a = empty((1, 3, 4))

# This is OK. The shape of a, (1, 3, 4), can broadcast to the shape of x[...], (2, 3, 4)
x[...] = a

# This is not allowed. The shape of a, (1, 3, 4), can NOT broadcast to the shape of x[1, ...], (3, 4)
x[1, ...] = a