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- Units and Quantities (`astropy.units`) »
- Using Prior Versions of Constants

# 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.>
    >>> print(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.