Combining and Defining Units

Basic example

Units and quantities can be combined together using the regular Python numeric operators:

>>> from astropy import units as u
>>> fluxunit = u.erg / (u.cm ** 2 * u.s)
>>> fluxunit
Unit("erg / (s cm2)")
>>> 52.0 * fluxunit  
<Quantity  52. erg / (s cm2)>
>>> 52.0 * fluxunit / u.s  
<Quantity  52. erg / (cm2 s2)>

Fractional powers

Units support fractional powers, which retain their precision through complex operations. To do this, it is recommended to use fractions.Fraction objects:

>>> from fractions import Fraction
>>> Franklin = u.g ** Fraction(1, 2) * u.cm ** Fraction(3, 2) * u.s ** -1

Note

Floating-point powers that are effectively the same as fractions with a denominator less than 10 are implicitly converted to Fraction objects under the hood. Therefore, the following are equivalent:

>>> x = u.m ** Fraction(1, 3)
>>> x.powers
[Fraction(1, 3)]
>>> x = u.m ** (1. / 3.)
>>> x.powers
[Fraction(1, 3)]

Defining units

Users are free to define new units, either fundamental or compound, using the def_unit() function:

>>> bakers_fortnight = u.def_unit('bakers_fortnight', 13 * u.day)

The addition of a string gives the new unit a name that will show up when the unit is printed:

>>> 10. * bakers_fortnight  
<Quantity  10. bakers_fortnight>

Creating a new fundamental unit is also possible:

>>> titter = u.def_unit('titter')
>>> chuckle = u.def_unit('chuckle', 5 * titter)
>>> laugh = u.def_unit('laugh', 4 * chuckle)
>>> guffaw = u.def_unit('guffaw', 3 * laugh)
>>> rofl = u.def_unit('rofl', 4 * guffaw)
>>> death_by_laughing = u.def_unit('death_by_laughing', 10 * rofl)
>>> (1. * rofl).to(titter)  
<Quantity  240. titter>

Users can see the definition of a unit and its decomposition via:

>>> rofl.represents
Unit("4 guffaw")
>>> rofl.decompose()
Unit("240 titter")

By default, custom units are not searched by methods such as find_equivalent_units(). However, they can be enabled by calling add_enabled_units():

>>> kmph = u.def_unit('kmph', u.km / u.h)
>>> (u.m / u.s).find_equivalent_units()
There are no equivalent units
>>> u.add_enabled_units([kmph])
<astropy.units.core._UnitContext object at ...>
>>> (u.m / u.s).find_equivalent_units()
  Primary name | Unit definition | Aliases
[
  kmph         | 0.277778 m / s  |         ,
]