Module magnitude

source code

A physical quantity is a number with a unit, like 10 km/h. Units can be any of the SI units, plus a bunch of non-SI, bits, dollars, and any combination of them. They can include the standard SI prefixes. Magnitude can operate with physical quantities, parse their units, and print them. You don't have to worry about unit consistency or conversions; everything is handled transparently. By default output is done in basic SI units, but you can specify any output unit, as long as it can be reduced to the basic units of the physical quantity.

The basic units understood by the magnitude module are:

indicator meaning --------- ------- $ dollar ('dollar' is also acceptable) A amperes bit binary digit cd candela K degrees Kelvin kg kilograms m meters mol amount of substance s seconds

From these basic units you can derive many other units. The magnitude package predefines these derived units:

Bq becquerel C coulomb c speed of light (m/s) day degC degree Celsius dpi dots per inch F farad ft feet ("'" is also acceptable) g gram gravity acceleration due to gravity (m/s**2) Gy gray H henry h hour Hz Hertz inch ('"' is also acceptable) ips inches per second J joule kat byte l liter lightyear light year lm lumen lpi lines per inch lux min minute N newton ohm Pa pascal S siemens Sv sievert T tesla V volt W watt Wb weber year

Two magnitudes have no units, 'rad' (radian - unit of plane angle) and 'sr' (steradian - unit of solid angle).

Any of the above units can be augmented with the following set of scale prefixes:

letter scale name ------ ----- ---- y 1e-24 yocto z 1e-21 zepto a 1e-18 atto f 1e-15 femto p 1e-12 pico n 1e-9 nano u 1e-6 micro m 1e-3 mili c 1e-2 centi d 1e-1 deci k 1e3 kilo Ki 2^10 Kibi M 1e6 mega Mi 2^20 Mebi G 1e9 giga Gi 2^30 Gibi T 1e12 tera Ti 2^40 Tebi P 1e15 peta Pi 2^50 Pebi E 1e18 exa Ei 2^60 Exbi Z 1e21 zetta Y 1e24 yotta

You can define new magnitudes by instantiating the Magnitude class. Suppose you want to define pounds as a magnitude and associate with it the unit 'lb'. A pound is 0.45359237 kilograms, so we have

>>> lb = Magnitude(0.45359237, kg=1)

To make it recognized automatically you also have to present it to the system:

>>> new_mag('lb', lb)

You can then use it as you would any other predefined physical quantity:

>>> me = mg(180, 'lb')
>>> print me.ounit('kg').toval()
81.6466266

The following online references provide more detail about physical units and the SI system.

http://physics.nist.gov/cuu/Units/units.html http://en.wikipedia.org/wiki/SI http://www.gnu.org/software/units/units.html for units.dat http://www.cip.physik.uni-muenchen.de/~tf/misc/etools.lisp

This code was very much inspired by

http://www.cs.utexas.edu/users/novak/units.html

and its associated paper,

http://www.cs.utexas.edu/users/novak/units95.html