A unit can be decomposed into its irreducible parts using the decompose method:
>>> from astropy import units as u
>>> u.Ry
Unit("Ry")
>>> u.Ry.decompose()
Unit("2.18e-18 m2 kg / (s2)")
You can limit the selection of units that you want to decompose to using the bases keyword argument:
>>> u.Ry.decompose(bases=[u.m, u.N])
Unit("2.179872e-18 m N")
This is also useful to decompose to a particular system. For example, to decompose the Rydberg unit in terms of CGS units:
>>> u.Ry.decompose(bases=u.cgs.bases)
Unit("2.179872e-11 cm2 g / (s2)")
Conversely, a unit may be recomposed back into more complex units using the compose method. Since there may be multiple equally good results, a list is always returned:
>>> x = u.Ry.decompose()
>>> x.compose()
[Unit("1.000000e+00 Ry"),
Unit("5.210019e-22 kcal"),
Unit("5.210019e-19 cal"),
Unit("2.066120e-21 BTU"),
Unit("1.360569e+01 eV"),
Unit("2.179872e-18 J"),
Unit("2.179872e-11 erg")]
Some other interesting examples:
>>> (u.s ** -1).compose()
[Unit("Hz"),
Unit("1 / (s)"),
Unit("3.155693e+07 / (yr)"),
Unit("8.640000e+04 / (d)"),
Unit("6.000000e+01 / (min)"),
Unit("3.600000e+03 / (h)"),
Unit("6.048000e+05 / (wk)"),
Unit("3.155693e+07 / (a)"),
Unit("8.616409e+04 / (sday)"),
Unit("1.209600e+06 / (fortnight)")]
Composition can be combined with Equivalencies:
>>> (u.s ** -1).compose(equivalencies=u.spectral())
[Unit("Hz"),
Unit("J"),
Unit("m"),
Unit("1 / (s)"),
Unit("2.390057e-01 cal"),
Unit("1.057001e-16 lyr"),
Unit("1.093613e+00 yd"),
Unit("3.240779e-17 pc"),
Unit("4.587425e+17 Ry"),
Unit("1.000000e+06 micron"),
Unit("3.937008e+01 inch"),
Unit("6.684587e-12 AU"),
Unit("1.000000e+02 cm"),
Unit("1.437798e-09 solRad"),
Unit("6.241509e+18 eV"),
Unit("3.280840e+00 ft"),
Unit("2.390057e-04 kcal"),
Unit("1.000000e+10 Angstrom"),
Unit("1.000000e+07 erg"),
Unit("9.478171e-04 BTU"),
Unit("6.213712e-04 mi"),
Unit("6.000000e+01 / (min)"),
Unit("3.600000e+03 / (h)"),
Unit("6.048000e+05 / (wk)"),
Unit("8.616409e+04 / (sday)"),
Unit("1.209600e+06 / (fortnight)"),
Unit("8.640000e+04 / (d)"),
Unit("3.155693e+07 / (yr)"),
Unit("3.155693e+07 / (a)")]
Obviously a name doesn’t exist for every arbitrary derived unit imaginable. In that case, the system will do its best to reduce the unit to the fewest possible symbols:
>>> (u.cd * u.sr * u.V * u.s).compose()
[Unit("lm Wb")]
Built on top of this functionality is a convenience method to convert between unit systems.
>>> u.Pa.to_system(u.cgs)
[Unit("1.000000e+01 Ba")]
This is equivalent to decomposing into the new system and then composing into the most complex units possible, though to_system adds some extra logic to return the results sorted in the most useful order:
>>> u.Pa.decompose(bases=u.cgs.bases)
Unit("1.000000e+01 g / (cm s2)")
>>> _.compose(units=u.cgs)
[Unit("1.000000e+01 Ba")]