code: def get_constant(self, name):
          
          try:
              c = getattr(const, name)
          except AttributeError:
              raise ValueError("Constant name '{}' doesn't exist in astropy.constants".format(name))
  
          return c.decompose(self._core_units).value
docs: Retrieve a constant with specified name in this unit system.
  
          Parameters
          ----------
          name : str
              The name of the constant, e.g., G.
  
          Returns
          -------
          const : float
              The value of the constant represented in this unit system.
  
          Examples
          --------
  
              >>> usys = UnitSystem(u.kpc, u.Myr, u.radian, u.Msun)
              >>> usys.get_constant('c')
              306.6013937879527