class_docstr: Default implementation of dataset indexing.
  
  DatasetMixin provides the :meth:`__getitem__` operator. The default
  implementation uses :meth:`get_example` to extract each example, and
  combines the results into a list. This mixin makes it easy to implement a
  new dataset that does not support efficient slicing.
  
  Dataset implementation using DatasetMixin still has to provide the
  :meth:`__len__` operator explicitly.
docstr: Returns the number of data points.
docstr_tok: ["Returns","the","number","of","data","points","."]
code: def __len__(self):
      """Returns the number of data points."""
      raise NotImplementedError
  
imports: import numpy
  import six
label_desc: Use image node_id 2 for calling the DatasetMixin obj's underlying member method code with example usage: obj.__len__() without return types