class_docstr: Wrap a generator to a DataFlow.
  The dataflow will not have length.
class_docstr_tok: ["Wrap","a","generator","to","a","DataFlow",".","The","dataflow","will","not","have","length","."]
code: def __len__(self):
      return len(self._gen)
  
imports: import copy
  import numpy
  import six
  from .base import DataFlow, RNGDataFlow
label_desc: Use image node_id 3 for calling the DataFromGenerator obj's underlying member method code with example usage: obj.__len__() and returns: len