Adding `writeable` to `flags`

NumPy arrays have the writeable option in flags. This can be useful for checking to see if one can write to an array or not. Also it can be useful to ensure an array is not written to (by setting this flag to False). Would be useful to have this flag on CuPy arrays as well.

[leofang]

I think what @kmaehashi meant is that it is hard to respect that flag in almost all basic CuPy operations except in ElementwiseKernel/ReductionKernel, where the input/output array attributes are put into inspection before processing.

I think this puts an extra layer of burden in general across all Python GPU libraries. Note that Numba does not respect that flag either. @jakirkham, could you point us to examples where (and how) this is implemented?

[jakirkham]

Right the interest here would be to mark arrays as read-only. 🙂

Yeah NumPy honors this at the C level as well. It does bit masking against the flags (an int) to check. Only operations that are manipulating the data itself.

I’m not sure. Would think __setitem__ and the inplace math operations would be the main issues. Are there other cases outside of these that would manipulate data in CuPy? You know the code here better than I. 😉

That seems like a useful feature to include. Though agree it is a different concern.