[Numpy-discussion] Add guaranteed no-copy to array creation and reshape?

[Sebastian Berg]

Hi all,

In https://github.com/numpy/numpy/pull/11897 I am looking into the
addition of a `copy=np.never_copy` argument to:
  * np.array
  * arr.reshape/np.reshape
  * arr.astype

Which would cause an error to be raised when numpy cannot guarantee
that the returned array is a view of the input array.
The motivation is to easier avoid accidental copies of large data, or
ensure that in-place manipulation will be meaningful.

The copy flag API would be:
  * `copy=True` forces a copy
  * `copy=False` allows numpy to copy if necessary
  * `copy=np.never_copy` will error if a copy would be necessary
  * (almost) all other input will be deprecated.

Unfortunately using `copy="never"` is tricky, because currently
`np.array(..., copy="never")` behaves exactly the same as
`np.array(..., copy=bool("never"))`. So that the wrong result would be
given on old numpy versions and it would be a behaviour change. 

Some things that are a not so nice maybe:
 * adding/using `np.never_copy` is not very nice
 * Scalars need a copy and so will not be allowed
 * For rare array-likes numpy may not be able to guarantee no-copy,
   although it could happen (but should not).


The history is that a long while ago I considered adding a copy flag to
`reshape` so that it is possible to do `copy=np.never_copy` (or
similar) to ensure that no copy is made. In these, you may want
something like an assertion:

```
new_arr = arr.reshape(new_shape)
assert np.may_share_memory(arr, new_arr)

# Which is sometimes -- but should not be -- written as:
arr.shape = new_shape  # unnecessary container modification

# Or:
view = np.array(arr, order="F")
assert np.may_share_memory(arr, new_arr)
```

but is more readable and will not cause an intermediate copy on error.


So what do you think? Other variants would be to not expose this for
`np.array` and probably limit `copy="never"` to the reshape method. Or
just to not do it at all. Or to also accept "never" for `reshape`,
although I think I would prefer to keep it in sync and wait for a few
years to consider that.

Best,

Sebastian

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