[SciPy-dev] design of a physical quantities package: seeking comments
Mon Aug 4 09:31:51 CDT 2008
I'm new to the scipy-dev list, but we're just wrapping up a project
here at Enthought where I've used the current Enthought units
infrastructure. If I have time today I'll add my comments based on
our experience here with using physical quantities in a real-world
My main comment here is the following:
On Aug 3, 2008, at 2:46 AM, Anne Archibald wrote:
> 2008/8/2 Darren Dale <firstname.lastname@example.org>:
>> On Saturday 02 August 2008 4:23:43 pm Anne Archibald wrote:
>>> 2008/8/2 Darren Dale <email@example.com>:
>>> What about user-defined functions? It's worth having some kind of
>>> decorator that enforces that a particular function acts on something
>>> with no units, and maybe enforces particular units.
>> Could you give an example? I don't follow.
> Well, suppose I have a function that, like "exp", only accepts
> unitless quantities. Suppose in fact it's already written. It would be
> nice to have a decorator so all I have to write is
> def myfunc(x):
> Similarly, if I want to implement the Planck black-body function, it
> would be nice to be able to write
> def B(T,nu):
For an example of this sort of idea already implemented (although not
with this particular interface), have a look at the @has_units
decorator in enthought.numerical_modelling.units. It is designed to
parse docstrings to extract unit information for the input and output
Other than that, I'd observe that variables with unit information were
a significant source of bugs and additional effort in our development
effort. To be fair, they probably helped identify a lot of bugs too.
But we had to write a fair bit of code whose sole purpose was to add
or remove units from variables.
For example, we have a solver in the application which uses
scipy.optimize.fsolve. To make it work correctly, we had to take our
variables with user-supplied units, remove the units, but remember
them in context, and then inside the function we were trying to solve
for we would have to re-apply the units before calling the code which
performed the computations (which then usually did its own unit
conversions before actually computing anything meaningful).
We also hit some of the limitations of a simple physical quantities
system: this particular application involved fluid dynamics, and in
particular power-law fluids, where one of the key physical quantities,
consistency, has units of Pa*s**n where n is the shear index, another
key physical quantity. In particular, n was variable in the
situations we were dealing with, and so we had to special-case the
consistency variables and ensure that they were always in a particular
set of units everywhere in our code.
I haven't had time yet to sit back and look hard at how the unit
libraries could have been improved to make our experience smoother.
As I mentioned earlier, if I have time today I'll respond to this
thread in detail and try and make some concrete constructive
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