docstr: Cache "filename" as a pickle file
code: def persist(self, filename=None):
      """
      Cache "filename" as a pickle file
      """
      if filename is None:
          filename = self.filename
      with open(filename, "w") as f:
          pickle.dump(self.cache, f)
  
file_path: Theano/theano/gof/callcache.py
imports: from __future__ import absolute_import, print_function, division
  import logging
  import six.moves.cPickle
label_desc: Use image node_id 2 for calling the CallCache obj's underlying member method code with example usage: obj.persist(filename) without return types