On Wed, Dec 26, 2018, 18:29 Sebastian Berg <sebast...@sipsolutions.net wrote:
> 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 > Could this approach be used to deprecate `ravel` and let us just use `flatten`? >
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