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 :class:`~astropy.utils.state.ScienceState` classes
in the top-level package. Setting the prior versions at the start of a Python
session will allow consistent units.
Example
=======
.. EXAMPLE START: Using Prior Versions of Constants
To initialize units to a prior version of constants:
>>> import astropy
>>> astropy.physical_constants.set('codata2010') # doctest: +SKIP
>>> astropy.astronomical_constants.set('iau2012') # doctest: +SKIP
>>> import astropy.units as u
>>> import astropy.constants as const
>>> (const.M_sun / u.M_sun).to(u.dimensionless_unscaled) - 1 # doctest: +SKIP
>>> print(const.M_sun) # doctest: +SKIP
Name = Solar mass
Value = 1.9891e+30
Uncertainty = 5e+25
Unit = kg
Reference = Allen's Astrophysical Quantities 4th Ed.
If :mod:`astropy.units` has already been imported, a :class:`RuntimeError` is
raised.
.. EXAMPLE END