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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 52.0 erg / (cm2 s)>
>>> 52.0 * fluxunit / u.s
<Quantity 52.0 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
`~fractions.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 `~astropy.units.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 10.0 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 240.0 titter>
One can see the definition of a unit and its :ref:`decomposition <decomposing>`
via::
>>> rofl.represents
Unit("4 guffaw")
>>> rofl.decompose()
Unit("240 titter")
By default, custom units are not searched by methods such as
`~astropy.units.core.UnitBase.find_equivalent_units`. However, they
can be enabled by calling `~astropy.units.add_enabled_units`::
>>> kmph = u.def_unit('kmph', u.km / u.h)
>>> (u.m / u.s).find_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 | ,
]
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