code: def _get_formula_class(self, formula):
          
          # recursive import otherwise
          from sprinter.formula.base import FormulaBase
          if formula in LEGACY_MAPPINGS:
              formula = LEGACY_MAPPINGS[formula]
          formula_class, formula_url = formula, None
          if ':' in formula:
              formula_class, formula_url = formula.split(":", 1)
          if formula_class not in self._formula_dict:
              try:
                  self._formula_dict[formula_class] = lib.get_subclass_from_module(formula_class, FormulaBase)
              except (SprinterException, ImportError):
                  logger.info("Downloading %s..." % formula_class)
                  try:
                      self._pip.install_egg(formula_url or formula_class)
                      try:
                          self._formula_dict[formula_class] = lib.get_subclass_from_module(formula_class, FormulaBase)
                      except ImportError:
                          logger.debug("FeatureDict import Error", exc_info=sys.exc_info())
                          raise SprinterException("Error: Unable to retrieve formula %s!" % formula_class)
                  except PipException:
                      logger.error("ERROR: Unable to download %s!" % formula_class)
          return self._formula_dict[formula_class]
docs: get a formula class object if it exists, else
          create one, add it to the dict, and pass return it.