units.py

Units.py is a Python library that makes it easy to work with units in physics without having to worry about unit conversions. For example, if you have an energy in electronvolts and you have the value of Planck's constant in Joules times seconds, normally you would have to convert one of those values to match the units of the other one. But with units.py, you don't have to do that. You can easily multiply two values in different units and get the correct result.

Setup

Units.py can be installed using pip:

pip install unitspy

You can then import it into your Python script:

import unitspy as up

Usage

Introduction

Units.py creates a Quantity class. A Quantity object represents a physical quantitiy, for example a length, a time or a mass. To create a Quantity object, simply multiply a number by a unit provided by units.py. For example, to create a quantity representing 3 meters, do this:

threeMeters = 3 * up.m

You can multiply and divide Quantity objects just like numbers, and you can add, subtract and compare two Quantity objects as long as they have compatible units (for example seconds and days are compatible units as they're both units of time, seconds and meters are not as they measure different things). Example:

myLength = 10 * up.m
myTime = 5 * up.s
print(myLength / myTime)    #Result: 2m/s

Units

Most units are available, and their names are often their SI abbreviations. Here is a list of the most common units:

Units.py name	Unit
g	Gram
kg	Kilogram
m	Meter
Å or Angstrom (the two are equivalent)	Ångström
au	Astronomical unit
ly	Light year
pc	Parsec
s	Second
minute	Minute
hr	Hour
d	Day
y	Year
A	Ampere
K	Kelvin
Hz	Hertz
N	Newton
Pa	Pascal
J	Joule
eV	Electronvolt
W	Watt
C	Coulomb
V	Volt
F	Farad
Ohm	Ohm
T	Tesla

You can also add SI prefixes before some of these units to get multiples of them. For example, write cm for centimeter or ms for millisecond. For units starting with "micro", you can either precede the unit with µ (for example µm), or write out the the name of the entire unit (for example micrometer).

You can also multiply, divide and take the power of units. For example, you can write m / s for meters per second or m ** 2 for square meters.

Angles and dimensionless quantities

Units.py also provides a dimensionless unit in order to represent dimensionless quantities. A Quantity object with the dimensionless unit is meant to work exactly like an object of Python's built-in float type.

For angles, there is also a radians unit, which is simply an alias of dimensionless, and a degrees unit, where 1 * up.degrees is equal to pi / 180 * up.radians. That way, you can easily specify angles in degrees, and for example pass that as an argument to trigonometric functions like numpy.sin, for example

import unitspy as up
import numpy as np
print(np.sin(float(90 * up.degree)))    #Result: 1.0

There is also up.arcmin and up.arcsec, representing arcminutes and arcseconds respectively.

Temperatures

Units.py stores temperatures in Kelvins, and can't store temperatures in Celsius. However, it defines Quantity object named zeroCelsius equal to the value of 0 Celsius in Kelvin, which can be used to convert between Kelvin and Celsius. If you have a temperature in Celsius, add zeroCelsius to get the temperature in Kelvin, and if you have a temperature in Kelvin, subtract zeroCelsius to get the temperature in Celsius. Example:

temperatureInCelsius = 15 * up.K    #Define this quantity in Kelvin even though it really represents a unit in Celsius
temperatureInKelvin = temperatureInCelsius + up.zeroCelsius
print(temperatureInKelvin)    #Result: 288.15K

Physical quantities

Units.py also provides pre-defined Quantity objects corresponding to physical constants. These are some common ones:

Units.py name (should be preceded by up.)	Quantity
kB	Boltzmann constant
e	Elementary charge
G	Gravitational constant
h	Planck constant
hbar	Reduced Planck constant
c	Speed of light
epsilon0	Vacuum permittivity
mu0	Vacuum permeability
me	Electron mass
mp	Proton mass
mn	Neutron mass
solarMass	Solar mass
pi	Pi

Conversions between units

If you want to output the value of a Quantity object in a specific unit, you will need the toUnit method. This method takes one argument, which is the unit in which you want to output the quantity. For example, here is how you output the value of one light year in meters:

myDistance = 1 * up.ly
print(myDistance.toUnit(up.m))    #Result: 9460500000000000.0m

Quantity objects also have a toSiUnits method which converts it to SI units and a toPlanckUnits method which converts it to Planck units. These methods take no arguments. Example:

myDistance = 1 * up.ly
print(myDistance.toSiUnits())    #Result: 9460500000000000.0m
print(myDistance.toPlanckUnits())    #Result: 5.853346072890479e+50lₚ