code: def test_subfield():
      myfield = basic_field_random()
      valid = nr.rand(myfield.V) > 0.1
      sf = myfield.subfield(valid)
      assert sf.V == np.sum(valid)
  
imports: import numpy
  import numpy.random
  from numpy.testing import assert_array_equal
  from ..field import field_from_coo_matrix_and_data, field_from_graph_and_data
  from ..graph import wgraph_from_3d_grid
label_desc: Use image node_id 27 for calling a global function with example usage: test_subfield() without return types