[Numpy-discussion] Ransom Proposals
Charles R Harris
charlesr.harris at gmail.com
Mon Mar 27 15:47:05 CST 2006
On 3/27/06, Tim Hochberg <tim.hochberg at cox.net> wrote:
>
> Charles R Harris wrote:
>
> >
> >
> > On 3/27/06, *Tim Hochberg* <tim.hochberg at cox.net
> > <mailto:tim.hochberg at cox.net>> wrote:
> >
> > Charles R Harris wrote:
> > [CHOP]
> >
> > >
> > > How about functions always return a copy unless they are carefully
> > > documented interface helper functions like asarray. That's how I
> > > generally think of them anyway.
> >
> > I don't really care as long as they (a) stop being evil and (b)
> > there's
> > some way to do stuff efficiently. Functions that always return
> copies
> > seem sort of useless to me, but I'd be happy to move to methods if
> > that
> > was the case. However, I suspect you won't make much headway with
> this
> > since my impression is that many of the people who do care about
> > functions also care passionately about efficiency as well.
> >
> > > Of course, the main virtue of asarray is that it helps write
> > functions
> > > that do precisely what Tim is arguing against: mixing list and
> array
> > > types and returning copies in the first case, views in the
> > second. So
> > > if we throw out functions we should throw out asarray also and
> > make a
> > > rule that numpy functions don't take lists.
> >
> > This isn't right. I think it's perfectly fine, desirable in fact,
> for
> > functions to operate on both list and array types. The problem only
> > arises when you return ether a view or copy of one of the original
> > inputs depending on what it was. In my experience, this is
> relatively
> > hard to do in all but the most trivial of functions since most
> > operations return a new array. However, it is true that I think
> people
> > should be strongly discouraged from doing this. For example:
> >
> > def add(a, b):
> > a = asarray(a)
> > b = asarray(b)
> > return a + b
> >
> > is fine, but:
> >
> > def iadd(a, b):
> > a = asarray(a)
> > b = asarray(b)
> > a += b
> > return a
> >
> > is not and should be rewritten. Possibly like:
> >
> > def iadd(a, b):
> > if not isinstance(a, ndarray): raise TypeError("'a' must be
> an
> > array")
> > b = asarray(b)
> > a += b
> > return a
> >
> > since += is a bit squirley. (Try somelist += somearray, to see
> > what I mean).
> >
> >
> > It's a syntax error, which I believe is the right thing.
Really? If so, this is a version thing. This is what I get running on
> Python 2.4.1 and current numpy SVN (0.9.7.2286):
>
> >>> l = list(a)
> >>> l
> [999, 1, 2, 3, 4, 5, 6, 7, 8]
> >>> a
> array([999, 1, 2, 3, 4, 5, 6, 7, 8])
> >>> l += a
> >>> l
> array([1998, 2, 4, 6, 8, 10, 12, 14, 16])
> >>> a
> array([999, 1, 2, 3, 4, 5, 6, 7, 8])
Ugh. IMHO, this should throw an error. For these sort of assignment
operators I think the right hand side should be cast to the type of the
left, which I think of as some sort of fancy register variable. If anything,
the list should be appended to. I'll have to try the latest SVN just to
appreciate this.
> -tim
>
> Chuck
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