On Thu, May 14, 2020 at 2:58 AM Steven D'Aprano <st...@pearwood.info> wrote:
> On Mon, May 11, 2020 at 10:41:06AM -0700, Andrew Barnert via Python-ideas > wrote: > > > I think in general people will expect that a slice view on a sequence > > acts like “some kind of sequence”, not like the same kind they’re > > viewing—again, they won’t be surprised if you can’t insert into a > > slice of a list. > > o_O > > For nearly 30 years, We've been able to insert into a slice of a list. > I'm going to be *really* surprise if that stops working. > At this point, we're thinking a sequence view would be immutable anyway. Even for views on immutable objects. So That's a non-issue. In numpy, there really isn't a view object at all -- there are simply numpy arrays, and any array *may* share the data block with another array. But they are both "proper" arrays. In [20]: import numpy as np In [21]: A = np.ones((5,)) In [22]: B = A[:] In [23]: type(A) Out[23]: numpy.ndarray In [24]: type(B) Out[24]: numpy.ndarray There is a small distinction: in the above case, A "owns" the data block, but you can only tell if you poke into the flags: In [27]: A.flags Out[27]: C_CONTIGUOUS : True F_CONTIGUOUS : True OWNDATA : True WRITEABLE : True ALIGNED : True WRITEBACKIFCOPY : False UPDATEIFCOPY : False In [28]: B.flags Out[28]: C_CONTIGUOUS : True F_CONTIGUOUS : True OWNDATA : False WRITEABLE : True ALIGNED : True WRITEBACKIFCOPY : False UPDATEIFCOPY : False And user code very rarely needs to care about that. That flag is mostly used to manege the memory, and prevent dangerous operations: In [30]: A.resize((3,4)) --------------------------------------------------------------------------- ValueError Traceback (most recent call last) <ipython-input-30-cd4cee3d5a8d> in <module> ----> 1 A.resize((3,4)) ValueError: cannot resize an array that references or is referenced by another array in this way. There are good reasons for ndarrays being able to share data while still being mutable, but I don't think a "normal" Python seqence_view should be mutable -- it would lead to a lot of confusion. -CHB -- Christopher Barker, PhD Python Language Consulting - Teaching - Scientific Software Development - Desktop GUI and Web Development - wxPython, numpy, scipy, Cython
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