There are two ways of handling conversions between units.
In this case, given a source and destination unit, the value(s) in the new units is(are) returned.
>>> from astropy import units as u
>>> u.pc.to(u.m, 3.26)
1.0059317615e+17
This converts 3.26 parsecs to meters.
Arrays are permitted as arguments.
>>> u.h.to(u.s, [1, 2, 5, 10.1])
array([ 3600., 7200., 18000., 36360.])
Finally, one may obtain a function that can be used to convert to the new unit. Normally this may seem like overkill when all one needs to do is multiply by a scale factor, but there are cases when the transformation between units may not be as simple as a single scale factor, for example when a custom equivalency table is in use.
Conversion to different units involves obtaining a conversion function and then applying it to the value, or values to be converted.
>>> cms = u.cm / u.s
>>> cms_to_mph = cms.get_converter(u.mile / u.hour)
>>> cms_to_mph(100.)
2.2366936292054402
>>> cms_to_mph([1000, 2000])
array([ 22.36936292, 44.73872584])
If you attempt to convert to a incompatible unit, an exception will result:
>>> cms.to(u.mile)
...
UnitsException: 'cm / (s)' (speed) and 'mi' (length) are not convertible
You can check whether a particular conversion is possible using the is_equivalent method:
>>> u.m.is_equivalent(u.foot)
True
>>> u.m.is_equivalent("second")
False
>>> (u.m ** 2).is_equivalent(u.acre)
True