code: def test_mgrid_size_none_handling(self, start, stop, step, expected):
      # regression test None value handling for
      # start and step values used by mgrid;
      # internally, this aims to cover previously
      # unexplored code paths in nd_grid()
      grid = mgrid[start:stop:step, start:stop:step]
      # need a smaller grid to explore one of the
      # untested code paths
      grid_small = mgrid[start:stop:step]
      assert_equal(grid.size, expected[0])
      assert_equal(grid_small.size, expected[1])
  
imports: import pytest
  import numpy
  from numpy.testing import assert_, assert_equal, assert_array_equal, assert_almost_equal, assert_array_almost_equal, assert_raises, assert_raises_regex
  from numpy.lib._index_tricks_impl import mgrid, ogrid, ndenumerate, fill_diagonal, diag_indices, diag_indices_from, index_exp, ndindex, c_, r_, s_, ix_
label_desc: Use image node_id 5 for calling the TestGrid obj's underlying member method code with example usage: obj.test_mgrid_size_none_handling(start, stop, step, expected) without return types