code: def count_nonzero(self):
      return sum(x != 0 for x in self.values())
  
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 5 for calling the _dok_base obj's underlying member method code with example usage: obj.count_nonzero() and returns: sum