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Low-Level Unit Conversion
*************************
Conversion of quantities from one unit to another is handled using the
`Quantity.to() <astropy.units.quantity.Quantity.to>` method. This page
describes some low-level features for handling unit conversion that
are rarely required in user code.
Direct Conversion
=================
.. EXAMPLE START: Direct Conversions Between Units
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)
1.0059308915661856e+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.])
.. EXAMPLE END
Obtaining a Conversion Function
-------------------------------
.. EXAMPLE START: 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.)
4.5
>>> cms_to_kmph([1000, 2000])
array([36., 72.])
.. EXAMPLE END
Incompatible Conversions
========================
.. EXAMPLE START: Conversions Between Incompatible Units
If you attempt to convert to a incompatible unit, a
:class:`~astropy.units.UnitConversionError` will result:
>>> cms = u.cm / u.s
>>> cms.to(u.km) # doctest: +IGNORE_EXCEPTION_DETAIL
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
:meth:`~astropy.units.core.UnitBase.is_equivalent` method::
>>> u.m.is_equivalent(u.pc)
True
>>> u.m.is_equivalent("second")
False
>>> (u.m ** 3).is_equivalent(u.l)
True
.. EXAMPLE END
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