[Numpy-discussion] Support of '@='?

[Marten van Kerkwijk]

>
> Just if you are curious why it is an error at the moment. We can't have
> it be filled in by python to be not in-place (`M = M @ P` meaning), but
> copying over the result is a bit annoying and nobody was quite sure
> about it, so it was delayed.


The problem with using out in-place is clear from trying `np.matmul(a, a,
out=a)`:
```
In [487]: a
array([[ 1.       ,  0.       ,  0.       ],
       [ 0.       ,  0.8660254,  0.5      ],
       [ 0.       , -0.5      ,  0.8660254]])

In [488]: np.matmul(a, a)
Out[488]:
array([[ 1.       ,  0.       ,  0.       ],
       [ 0.       ,  0.5      ,  0.8660254],
       [ 0.       , -0.8660254,  0.5      ]])

In [489]: np.matmul(a, a, out=a)
Out[489]:
array([[ 0.,  0.,  0.],
       [ 0.,  0.,  0.],
       [ 0.,  0.,  0.]])
```

It would seem hard to avoid doing the copying (though obviously one should
iterate over higher dimensiones, ie., temp.shape = M.shape[-2:]). Not
dissimilar from cumsum etc which are also not true ufuncs (but where things
can be made to work by ensuring operations are doing in the right order).

-- Marten
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