[Numpy-discussion] numpy.polynomial.Polynomial

Charles R Harris charlesr.harris@gmail....
Fri Oct 22 11:26:55 CDT 2010


On Fri, Oct 22, 2010 at 9:51 AM, <josef.pktd@gmail.com> wrote:

> I'm subclassing numpy.polynomial.Polynomial. So far it works well.
>
> One question on inplace changes
>
> Is it safe to change coef directly without creating a new instance?
> I'm not trying to change anything else in the polynomial, just for
> example pad, truncate or invert the coef inplace, e.g
>
> def pad(self, maxlag):
>    self.coef = np.r_[self.coef, np.zeros(maxlag - len(self.coef))]
>
> Currently, I have rewritten this to return a new instance.
>
>
You can (currently) modify the coef and it should work, but I think it best
to regard the Polynomial class as immutable. I'm even contemplating making
the coef attribute read only just to avoid such things. Another tip is to
use // instead of / for division, polynomials are rather like integers that
way and don't have a true divide so plain old / will fail for python 3.x

Note that most operations will trim trailing zeros off the result.

In [6]: P((1,1,1,0,0,0))
Out[6]: Polynomial([ 1.,  1.,  1.,  0.,  0.,  0.], [-1.,  1.])

In [7]: P((1,1,1,0,0,0)) + 1
Out[7]: Polynomial([ 2.,  1.,  1.], [-1.,  1.])

The reason the constructor doesn't was because trailing zeros can be of
interest in least squares fits. Is there a particular use case for which
trailing zeros are important for you? The polynomial modules aren't finished
products yet, I can still add some functionality if you think it useful.

Chuck
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