code: def lookupEnvVar(name, envName, defaultValue):
      
      try:
          value = os.environ[envName]
      except KeyError:
          log.info('Using default %s of %s as %s is not set.', name, defaultValue, envName)
          return defaultValue
      else:
          log.info('Overriding %s of %s with %s from %s.', name, defaultValue, value, envName)
          return value
docs: Use this for looking up environment variables that control Toil and are important enough to
      log the result of that lookup.
  
      :param str name: the human readable name of the variable
      :param str envName: the name of the environment variable to lookup
      :param str defaultValue: the fall-back value
      :return: the value of the environment variable or the default value the variable is not set
      :rtype: str