[Numpy-discussion] Unpleasant behavior with poly1d and numpy scalar multiplication

Fernando Perez fperez.net@gmail....
Sat Feb 13 02:11:20 CST 2010


Mmh, today I got bitten by this again.  It took me a while to figure
out what was going on while trying to construct a pedagogical example
manipulating numpy poly1d objects, and after searching for 'poly1d
multiplication float' in my gmail inbox, the *only* post I found was
this old one of mine, so I guess I'll just resuscitate it:

On Tue, Jul 31, 2007 at 2:54 PM, Fernando Perez <fperez.net@gmail.com> wrote:
> Hi all,
>
> consider this little script:
>
> from numpy import poly1d, float, float32
> p=poly1d([1.,2.])
> three=float(3)
> three32=float32(3)
>
> print 'three*p:',three*p
> print 'three32*p:',three32*p
> print 'p*three32:',p*three32
>
>
> which produces when run:
>
> In [3]: run pol1d.py
> three*p:
> 3 x + 6
> three32*p: [ 3.  6.]
> p*three32:
> 3 x + 6
>
>
> The fact that multiplication between poly1d objects and numbers is:
>
> - non-commutative when the numbers are numpy scalars
> - different for the same number if it is a python float vs a numpy scalar
>
> is rather unpleasant, and I can see this causing hard to find bugs,
> depending on whether your code gets a parameter that came as a python
> float or a numpy one.
>
> This was found today by a colleague on numpy 1.0.4.dev3937.   It feels
> like a bug to me, do others agree? Or is it consistent with a part of
> the zen of numpy I've missed thus far?

Tim H. mentioned how it might be tricky to fix. I'm wondering if there
are any new ideas since on this front, because it's really awkward to
explain to new students that poly1d objects have this kind of odd
behavior regarding operations with scalars.

The same underlying problem happens for addition, but in this case the
answer (depending on the order of operations) changes even more:

In [560]: p
Out[560]: poly1d([ 1.,  2.])

In [561]: print(p)

1 x + 2

In [562]: p+3
Out[562]: poly1d([ 1.,  5.])

In [563]: p+three32
Out[563]: poly1d([ 1.,  5.])

In [564]: three32+p
Out[564]: array([ 4.,  5.])  # !!!

I'm ok with teaching students that in floating point, basic algebraic
operations may not be exactly associative and that ignoring this fact
can lead to nasty surprises.  But explaining that a+b and b+a give
completely different *types* of answer is kind of defeating my 'python
is the simple language you want to learn' :)

Is this really unfixable, or does one of our resident gurus have some
ideas on how to approach the problem?

Thanks!

f


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