code: def parse_env(config_schema, env):
      
      try:
          return {
              key: item_schema.parse(key, env.get(key))
              for key, item_schema in config_schema.items()
          }
      except KeyError as error:
          raise MissingConfigError(
              "Required config not set: {}".format(error.args[0])
          )
docs: Parse the values from a given environment against a given config schema
  
      Args:
          config_schema: A dict which maps the variable name to a Schema object
              that describes the requested value.
          env: A dict which represents the value of each variable in the
              environment.