code: def test_subgraph_1(n=10):
      x = nr.randn(n, 2)
      G = WeightedGraph(x.shape[0])
      valid = np.zeros(n)
      assert G.subgraph(valid) is None
  
imports: import numpy
  import numpy.random
  from numpy.testing import assert_almost_equal, assert_array_almost_equal, assert_array_equal
  from ..graph import WeightedGraph, complete_graph, concatenate_graphs, eps_nn, knn, mst, wgraph_from_3d_grid, wgraph_from_adjacency, wgraph_from_coo_matrix
label_desc: Use image node_id 41 for calling a global function with example usage: test_subgraph_1(n) without return types