# Combining and defining units¶

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

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


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

>>> 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)]


Users are free to define new units, either fundamental or compound using the def_unit function. For example:

>>> 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 simple:

>>> 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>


One 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()
[]