Low-Level Unit Conversion#

Conversion of quantities from one unit to another is handled using the Quantity.to() method. This page describes some low-level features for handling unit conversion that are rarely required in user code.

Direct Conversion#

In this case, given a source and destination unit, the values in the new units are returned.

>>> from astropy import units as u
>>> u.pc.to(u.m, 3.26)

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.])

Obtaining a Conversion Function#

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_kmph = cms.get_converter(u.km / u.hour)
>>> cms_to_kmph(125.)
>>> cms_to_kmph([1000, 2000])
array([36., 72.])

Incompatible Conversions#

If you attempt to convert to a incompatible unit, a UnitConversionError will result:

>>> cms = u.cm / u.s
>>> cms.to(u.km)  
Traceback (most recent call last):
UnitConversionError: 'cm / s' (speed) and 'km' (length) are not convertible

You can check whether a particular conversion is possible using the is_equivalent() method:

>>> u.m.is_equivalent(u.pc)
>>> u.m.is_equivalent("second")
>>> (u.m ** 3).is_equivalent(u.l)