[SciPy-dev] design of a physical quantities package: seeking comments

[Darren Dale]

Hi Charles,

Since udunits2 is not officially released, and since udunits is working out so 
well, I think I am happy with things as they are now.

Thanks,
Darren

On Tuesday 05 August 2008 12:15:47 Charles Doutriaux wrote:
> Hi Darren,
>
> Obviously I didn't have time to look into the udunits2 pbm yet. But I
> see you far along already.
>
> Do you still want me to look into that or are you happy with udunits?
>
> C.
>
> Darren Dale wrote:
> > On Sunday 03 August 2008 12:54:12 pm Darren Dale wrote:
> >> On Sunday 03 August 2008 11:35:47 am Jonathan Guyer wrote:
> >>> On Aug 3, 2008, at 9:22 AM, Darren Dale wrote:
> >>>>> More, what happens when you have a quantity in Newton-metres (say)
> >>>>> and
> >>>>> you ask it to convert to feet? Do you get Newton-feet, or do all the
> >>>>> occurrences of "feet" in the Newtons get converted?
> >>>>
> >>>> They all get converted.
> >>>
> >>> I'm with Anne. I'm having a really hard time seeing when I would want
> >>> this as a default behavior, whereas I frequently want to be able
> >>> automatically convert BTUs to Joules and atmospheres to Pascals, but I
> >>> want an exception if somebody tries to give me a pressure when I ask
> >>> for an energy. Having it possible to do what you suggest seems like it
> >>> might be useful sometimes, but I don't think it should be the default
> >>> behavior.
> >>
> >> [...]
> >>
> >>> Using this scheme:
> >>>  >>> from fipy import PhysicalField
> >>>  >>> PhysicalField("12 lyr/cm**3")
> >>>
> >>> PhysicalField(12.0,'lyr/cm**3')
> >>>
> >>>  >>> PhysicalField("1 lyr") / "7.3 cm**2"
> >>>
> >>> PhysicalField(0.13698630136986301,'lyr/cm**2')
> >>>
> >>>  >>> PhysicalField("12 lyr/cm**3").inUnitsOf("m**-2")
> >>>
> >>> PhysicalField(1.1352876567096959e+23,'1/m**2')
> >>>
> >>> so the compound unit that Anne wants is supported and conversion to
> >>> canonical units happens only when requested.
> >>
> >> But in the meantime, you aggregate units like
> >> "dynes kg ft m^3 s / lbs Km ps^4 watts ohms". Quick, what units should I
> >> ask for that won't yield an error? It seems like it is more helpful to
> >> reduce as you go, and later when you want your value expressed with a
> >> compound unit, you ask for it. Let me try it my way and see what you
> >> think, and if people are not sold, it should be simple to reimplement so
> >> all units are aggregated, and reduced only on request.
> >
> > I think maybe we can have it both ways. I borrowed (and slightly
> > modified) the parser enthought put together to interpret unit strings.
> > Perhaps it could be modified further such that 'kg*(pc/cm^3)*K/s^4'
> > dictates that pc/cm^3 is a compound unit and should not to be
> > automatically decomposed. A method could be added to decompose compound
> > units on request, in case that is desired.
> >
> > I put a mercurial repo up, you can check it out with
> > "hg clone \
> > http:// dale.chess.cornell.edu/~darren/cgi-bin/hgwebdir.cgi/quantities \
> > quantities"
> >
> > There is no documentation yet, and I'm a little unhappy with some of the
> > organization, but I was really just blitzing through trying to get a demo
> > together. It installs in the usual way. There is no documentation, so
> > here is a crash course:
> >
> > In [1]: from quantities import NDQuantity
> > udunits(3): Already initialized from
> > file "/usr/lib/python2.5/site-packages/quan
> > tities/quantities-data/udunits.dat"
> >
> > In [2]: a=NDQuantity([1,2,3.0],'J')
> >
> > In [3]: a
> > Out[3]: NDQuantity([ 1.,  2.,  3.]), kg * m^2 / s^2
> >
> > In [4]: b=NDQuantity([1,2,3.0],'BTU')
> >
> > In [5]: b
> > Out[5]: NDQuantity([ 1055.05585262,  2110.11170524,  3165.16755786]), kg
> > * m^2 / s^2
> >
> > In [6]: c=NDQuantity([1,2,3.0],'m**-2')
> >
> > In [7]: c
> > Out[7]: NDQuantity([ 1.,  2.,  3.]), 1 / m^2
> >
> > In [8]: c.inUnitsOf('parsec/cm^3')
> > Out[8]: NDQuantity([  3.24077885e-23,   6.48155770e-23,  
> > 9.72233655e-23]), parsec/cm^3
> >
> > In [9]: d=c.inUnitsOf('parsec/cm^3')
> >
> > In [10]: c
> > Out[10]: NDQuantity([ 1.,  2.,  3.]), 1 / m^2
> >
> > In [11]: d/a
> > Out[11]: NDQuantity([  3.24077885e-23,   3.24077885e-23,  
> > 3.24077885e-23]), s^2 * parsec/cm^3 / kg m^2
> >
> > In [12]: b/a
> > Out[12]: NDQuantity([ 1055.05585262,  1055.05585262,  1055.05585262]),
> > (dimensionless)
> >
> >
> > There are some temporary limitations. Enthoughts parser has certain
> > expectations, which places some limitations on the units string passed to
> > the constructor. udunits, which is used for all unit conversions, has a
> > different set of expectations and limitations. This can be improved, I
> > just havent gotten around to it yet. The available units are defined in
> > _units.py. Quantities cannot yet be created by multiplying a _unit times
> > another number or numpy array. It would be better for the unit
> > definitions to be first class quantities, then you could do 10*J and get
> > a quantitiy, but there is a circular import issue that I need to work
> > through.
> >
> > I kept this in for now:
> >
> > In [3]: a
> > Out[3]: NDQuantity([ 1.,  2.,  3.]), kg * m^2 / s^2
> >
> > In [21]: a.units='ft'
> >
> > In [22]: a
> > Out[22]: NDQuantity([ 10.76391042,  21.52782083,  32.29173125]), kg *
> > ft^2 / s^2
> >
> > I ran into the concatenate issue that came up a while back, it drops the
> > units:
> >
> > In [26]: numpy.concatenate([a,a])
> > Out[26]:
> > array([ 10.76391042,  21.52782083,  32.29173125,  10.76391042,
> >         21.52782083,  32.29173125])
> >
> > sqrt doesnt seem to work:
> >
> > In [28]: numpy.sqrt(a**2+a**2)
> > Out[28]: NDQuantity([ 15.2224681 ,  30.44493619,  45.66740429]), kg^2 *
> > ft^4 / s^4
> >
> > but this does:
> >
> > In [30]: (a**2+a**2)**0.5
> > Out[30]: NDQuantity([ 15.2224681 ,  30.44493619,  45.66740429]), kg *
> > ft^2 / s^2
> >
> > Anyway, feel free to kick the tires and send feedback.
> >
> >
> > Darren
> > _______________________________________________
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>
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