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 # doctest: +FLOAT_CMP >>> 52.0 * fluxunit / u.s # doctest: +FLOAT_CMP 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 # doctest: +FLOAT_CMP 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) # doctest: +FLOAT_CMP One can see the definition of a unit and its :ref:`decomposition ` 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]) >>> (u.m / u.s).find_equivalent_units() Primary name | Unit definition | Aliases [ kmph | 0.277778 m / s | , ]