code: def __init__(self, stemmer=null_stemmer):
      if not callable(stemmer):
          raise ValueError("Stemmer has to be a callable object")
  
      self._stemmer = stemmer
  
file_path: sumy/sumy/summarizers/_summarizer.py
imports: from __future__ import absolute_import
  from __future__ import division, print_function, unicode_literals
  from collections import namedtuple
  from operator import attrgetter
  from ..utils import ItemsCount
  from .._compat import to_unicode
  from ..nlp.stemmers import null_stemmer
label_desc: Use image node_id 1 to create a new AbstractSummarizer object from inherited base classes: object with example: obj = AbstractSummarizer(stemmer)