code: def _get_value(quantitystr, fitparams, fixedparams):
      
  
      # for Mandel-Agol fitting, sometimes we want to fix some parameters,
      # and fit others. this function allows that flexibility.
      fitparamskeys, fixedparamskeys = fitparams.keys(), fixedparams.keys()
      if quantitystr in fitparamskeys:
          quantity = fitparams[quantitystr]
      elif quantitystr in fixedparamskeys:
          quantity = fixedparams[quantitystr]
      return quantity
docs: This decides if a value is to be fit for or is fixed in a model fit.
  
      When you want to get the value of some parameter, but you're not sure if
      it's being fit or if it is fixed. then, e.g. for `period`::
  
          period_value = _get_value('period', fitparams, fixedparams)