Using Prior Versions of Constants

By default, astropy.units are initialized upon first import to use the current versions of astropy.constants. For units to initialize properly to a prior version of constants, the constants versions must be set before the first import of astropy.units or astropy.constants.

This is accomplished using ScienceState classes in the top-level package. Setting the prior versions at the start of a Python session will allow consistent units.

Example

To initialize units to a prior version of constants:

>>> import astropy
>>> astropy.physical_constants.set('codata2010')  
<ScienceState physical_constants: 'codata2010'>
>>> astropy.astronomical_constants.set('iau2012')  
<ScienceState astronomical_constants: 'iau2012'>
>>> import astropy.units as u
>>> import astropy.constants as const
>>> (const.M_sun / u.M_sun).to(u.dimensionless_unscaled) - 1  
<Quantity 0.>
>>> const.M_sun  
  Name   = Solar mass
  Value  = 1.9891e+30
  Uncertainty  = 5e+25
  Unit  = kg
  Reference = Allen's Astrophysical Quantities 4th Ed.

If astropy.units has already been imported, a RuntimeError is raised.