# [Python-ideas] the Quantity pattern

Robert Kern robert.kern at gmail.com
Wed Mar 3 04:35:00 CET 2010

```On 2010-03-02 20:14 , Darren Dale wrote:
> On Tue, Mar 2, 2010 at 2:17 PM, Robert Kern<robert.kern at gmail.com>  wrote:
>> On 2010-03-02 12:35 PM, Bill Janssen wrote:
>>>
>>> I was looking at Martin Fowler's Quantity pattern earlier.
>>>
>>> http://martinfowler.com/ap2/quantity.html
>>>
>>> I remember writing this up as an idea for Fortran back in the early
>>> 80's, only to find a CACM paper from 1978 exploring the idea:
>>> "Incorporation of Units into Programming Languages", Karr&    Loveman, May
>>> 1978.
>>>
>>> But it would still be a cool idea for Python.  Perhaps it's already
>>> there and I haven't noticed?
>>
>> Tons of implementations (in no particular order):
>>
>> http://pypi.python.org/pypi/quantities/
>> http://pypi.python.org/pypi/Unum/
>> http://pypi.python.org/pypi/magnitude/
>> http://pypi.python.org/pypi/units/
>> http://pypi.python.org/pypi/ScientificPython/
>> http://pypi.python.org/pypi/SciMath/
>>
>> And quite a few more that are part of other packages or otherwise not on
>> PyPI. It's ridiculously easy to write something that what people think are
>> the common cases and so everyone does. It's a lot harder to write something
>> that robustly handles what are actually common cases (absolute temperature
>> scales, logarithmic scales, etc.).
>
> I prefer to think of this as two separate issues. One issue is a
> Quantity pattern for dealing with values that have magnitude and
> dimensionality, and the other is coordinate systems (requiring a point
> of reference, like temperature scales).

Theoretically and implementation-wise, absolutely. However, users want to
convert Fahrenheit to Celsius with the same tool they use to convert meters to
feet. To them, it's the same problem.

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma