code: def compose_suffix(num_docs=0, num_topics=0, suffix=None):
      
      if not isinstance(suffix, basestring):
          suffix = '_{}X{}'.format(num_docs, num_topics)
      return suffix
docs: Create a short, informative, but not-so-unique identifying string for a trained model
      If a str suffix is provided then just pass it through.
      >>> compose_suffix(num_docs=100, num_topics=20)
      '_100X20'
      >>> compose_suffix(suffix='_sfx')
      '_sfx'
      >>> compose_suffix(suffix='')
      ''
      >>> compose_suffix(suffix=None)
      '_0X0'