Hi!

Broadcasting almost always returns a repeated output 
(except when all arrays are the same shape), that’s the entire point. I suspect this function is in fairly widespread use and will therefore cause a lot of downstream issues when repeating, so I’m -0.5 on a DeprecationWarning. A FutureWarning might be more appropriate, in which case I’m +0.2.

As for making the output read-only, that might break code, but most likely the code was erroneous anyway. But breaking backward-compatibility without a grace period is unheard of in this community. I’m +0.5 on it anyway. 🤷🏻‍♂️

Overall, a kind of hairy problem with no clear solution.

Best Regards,
Hameer Abbasi

On Tuesday, Dec 25, 2018 at 12:13 PM, Matti Picus <matti.picus@gmail.com> wrote:
In PR 12609 https://github.com/numpy/numpy/pull/12609 I added code to
emit a DepricationWarning when broadcast_arrays returns an array where
the output is repeated. While this is a minimal fix to the problem,
perhaps we should consider making the output readonly immediately instead?


- A deprecation cycle requires two changes to downstream user's code:
one to filter the deprecation warning, and another when we actually make
the change

- Writing to the repeated data will cause errors now.


What do you think, should we change the behaviour at all, and if so
should we depricate it over two releases or change it immediately?


The original issue is here https://github.com/numpy/numpy/issues/2705


Matti

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