On Mon, Jan 20, 2014 at 3:58 PM, Charles R Harris <charlesr.har...@gmail.com > wrote:
> > > > On Mon, Jan 20, 2014 at 3:35 PM, Nathaniel Smith <n...@pobox.com> wrote: > >> On Mon, Jan 20, 2014 at 10:28 PM, Charles R Harris >> <charlesr.har...@gmail.com> wrote: >> > >> > >> > >> > On Mon, Jan 20, 2014 at 2:27 PM, Oscar Benjamin < >> oscar.j.benja...@gmail.com> >> > wrote: >> >> >> >> >> >> On Jan 20, 2014 8:35 PM, "Charles R Harris" <charlesr.har...@gmail.com >> > >> >> wrote: >> >> > >> >> > I think we may want something like PEP 393. The S datatype may be the >> >> > wrong place to look, we might want a modification of U instead so as >> to >> >> > transparently get the benefit of python strings. >> >> >> >> The approach taken in PEP 393 (the FSR) makes more sense for str than >> it >> >> does for numpy arrays for two reasons: str is immutable and opaque. >> >> >> >> Since str is immutable the maximum code point in the string can be >> >> determined once when the string is created before anything else can >> get a >> >> pointer to the string buffer. >> >> >> >> Since it is opaque no one can rightly expect it to expose a particular >> >> binary format so it is free to choose without compromising any expected >> >> semantics. >> >> >> >> If someone can call buffer on an array then the FSR is a semantic >> change. >> >> >> >> If a numpy 'U' array used the FSR and consisted only of ASCII >> characters >> >> then it would have a one byte per char buffer. What then happens if >> you put >> >> a higher code point in? The buffer needs to be resized and the data >> copied >> >> over. But then what happens to any buffer objects or array views? They >> would >> >> be pointing at the old buffer from before the resize. Subsequent >> >> modifications to the resized array would not show up in other views >> and vice >> >> versa. >> >> >> >> I don't think that this can be done transparently since users of a >> numpy >> >> array need to know about the binary representation. That's why I >> suggest a >> >> dtype that has an encoding. Only in that way can it consistently have >> both a >> >> binary and a text interface. >> > >> > >> > I didn't say we should change the S type, but that we should have >> something, >> > say 's', that appeared to python as a string. I think if we want >> transparent >> > string interoperability with python together with a compressed >> > representation, and I think we need both, we are going to have to deal >> with >> > the difficulties of utf-8. That means raising errors if the string >> doesn't >> > fit in the allotted size, etc. Mind, this is a workaround for the mass >> of >> > ascii data that is already out there, not a substitute for 'U'. >> >> If we're going to be taking that much trouble, I'd suggest going ahead >> and adding a variable-length string type (where the array itself >> contains a pointer to a lookaside buffer, maybe with an optimization >> for stashing short strings directly). The fixed-length requirement is >> pretty onerous for lots of applications (e.g., pandas always uses >> dtype="O" for strings -- and that might be a good workaround for some >> people in this thread for now). The use of a lookaside buffer would >> also make it practical to resize the buffer when the maximum code >> point changed, for that matter... >> > The more I think about it, the more I think we may need to do that. Note that dynd has ragged arrays and I think they are implemented as pointers to buffers. The easy way for us to do that would be a specialization of object arrays to string types only as you suggest. <snip> Chuck
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