code: def _getnnz(self, axis=None):
      if axis is not None:
          raise NotImplementedError(
              "_getnnz over an axis is not implemented "
              "for DOK format."
          )
      return len(self._dict)
  
imports: import itertools
  import numpy
  from ._matrix import spmatrix
  from ._base import _spbase, sparray, issparse
  from ._index import IndexMixin
  from ._sputils import isdense, getdtype, isshape, isintlike, isscalarlike, upcast, upcast_scalar, check_shape
label_desc: Use image node_id 4 for calling the _dok_base obj's underlying member method code with example usage: obj._getnnz(axis) and returns: len