A python package for scientific study

One of my on-going projects is the development of a body of python code for the representation of scientific quantities. While software for scientific computation generally concerns itself with attaining the best precision available, I have concerned myself more with having the numeric types involved keep track of the uncertainties in data and the units in which things are measured.

Units are generally ignored in scientific computation; the system of units in use implies the units associated with each quantity so, the reasoning goes, one only needs to know its value. There are two flaws with this, from a software maintenance (i.e. we know there exist bugs) point of view: conversion factors between units of the same kind should not be omitted (e.g. when one converts from electron-Volts to Joules, or gallons per minute to cubic feet per second); and quantities with different units should not be added together. Having quantities know their own units makes it possible for a program to raise an error in the latter case; while eliminating the former issue. It also provides a handy check, when one does a quick computation in an interactive session, that one has computed what one expected – if the units aren't right, something went wrong !

Since python supports emulation of numeric types via magic methods of a class, it's entirely practical to ensure that quantities behave as required, even to the extent of getting nice display. When displayed (i.e. when repr or str is called on it), a number only displays one more decimal place than is justified by its precision. Thus, when it displays Hubble's constant, study.chemy.physics.Cosmos.Hubble, as 2.3 * atto / second it's saying that the data at my disposal seem to indicate that there's at least a fifty percent likelihood that the value lies between 1.5 aHz and 2.5 aHz (values in this interval would all round to 2 aHz; in which a is the abbreviation for quantifier atto = 1e-18 and Hz is the short form for Hertz, an alternate name for 1/second) and the next digit of my best estimate of the value is a 3, but there's less than a fifty percent likelihood of the value falling between 2.25 aHz and 2.35 aHz.

I'm also a big fan of lazy evaluation; most of the types used in the package support attributes which are only computed when they are first referenced. Thus specifying one (or a few) of an object's attributes can suffice to determine various others. This carries the added advantage of simplifying some cases where specifying enough of a set of attributes suffices to determine the rest of the set, without it mattering (much) which attributes in the set were specified. For example, specifying momentum, energy, frequency or wavelength of a photon is sufficient to determine all of the others.

The current version of this software is what evolved out of my messing around with wanting to represent error bars and units. I sporadically dream of doing a major re-write which resolves assorted issues I'm lumbered with by how I got to where I am; however, it may be some time before I even complete thinking through the design of that (I can fairly be accused of a bad case of the second-system effect), let alone have anything better to show for it than what I have now. In the mean time, what I've got works and I sporadically improve it; if anyone else wants to play with it, I have a bzip2-ed tar-ball available; I (as copyright holder) grant permission for anyone to download it and use it for their own amusement. If you want to do anything else with it, I'd be most interested, but please get in touch with me. The principal reason why I haven't yet released it under the GPL is the thought that I really should make it redundant with my planned next edition first, so don't be afraid to ask.

For documentation, print the __doc__ attributes of relevant objects (or just read doc-strings in the source). Unpack the tar-ball (everything it contains is in a directory called study) as a sub-directory of some directory in your $PYTHONPATH (or simply in sys.path) and, in a python session, import study or assorted sub-objects of it. The package gives an over-view of the sub-packages it contains, each sub-package explains what it contains. I make extensive use of hierarchical name-spaces, each step of which indicates what it makes available.

Note that:


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