# UnitSystem¶

class gala.units.UnitSystem(units, *args)[source]

Bases: object

Represents a system of units. At minimum, this consists of a set of length, time, mass, and angle units, but may also contain preferred representations for composite units. For example, the base unit system could be {kpc, Myr, Msun, radian}, but you can also specify a preferred speed, such as km/s.

This class functions like a dictionary with keys set by physical types. If a unit for a particular physical type is not specified on creation, a composite unit will be created with the base units. See Examples below for some demonstrations.

Parameters
*units

The units that define the unit system. At minimum, this must contain length, time, mass, and angle units.

Examples

If only base units are specified, any physical type specified as a key to this object will be composed out of the base units:

>>> usys = UnitSystem(u.m, u.s, u.kg, u.radian)
>>> usys['energy']
Unit("kg m2 / s2")


However, custom representations for composite units can also be specified when initializing:

>>> usys = UnitSystem(u.m, u.s, u.kg, u.radian, u.erg)
>>> usys['energy']
Unit("erg")


This is useful for Galactic dynamics where lengths and times are usually given in terms of kpc and Myr, but speeds are given in km/s:

>>> usys = UnitSystem(u.kpc, u.Myr, u.Msun, u.radian, u.km/u.s)
>>> usys['speed']
Unit("km / s")


Methods Summary

 A thin wrapper around astropy.units.Quantity.decompose() that knows how to handle Quantities with physical types with non-default representations. get_constant(name) Retrieve a constant with specified name in this unit system. Return a dictionary representation of the unit system with keys set by the physical types and values set by the unit objects.

Methods Documentation

decompose(q)[source]

A thin wrapper around astropy.units.Quantity.decompose() that knows how to handle Quantities with physical types with non-default representations.

Parameters
qQuantity

An instance of an astropy Quantity object.

Returns
qQuantity

A new quantity, decomposed to represented in this unit system.

get_constant(name)[source]

Retrieve a constant with specified name in this unit system.

Parameters
namestr

The name of the constant, e.g., G.

Returns
constfloat

The value of the constant represented in this unit system.

Examples

>>> usys = UnitSystem(u.kpc, u.Myr, u.radian, u.Msun)
>>> usys.get_constant('c')
306.6013937855506

to_dict()[source]

Return a dictionary representation of the unit system with keys set by the physical types and values set by the unit objects.