class_docstr: Test the dependency tree.
  
  This is what we are working with::
  
      None (No Data)
       +DataID(name='comp19')
       + +DataID(name='ds5', resolution=250, modifiers=('res_change',))
       + + +DataID(name='ds5', resolution=250, modifiers=())
       + + +__EMPTY_LEAF_SENTINEL__ (No Data)
       + +DataID(name='comp13')
       + + +DataID(name='ds5', resolution=250, modifiers=('res_change',))
       + + + +DataID(name='ds5', resolution=250, modifiers=())
       + + + +__EMPTY_LEAF_SENTINEL__ (No Data)
       + +DataID(name='ds2', resolution=250, calibration=<calibration.reflectance>, modifiers=())
docstr: Test that dependency tree copy preserves the uniqueness of the empty node.
docstr_tok: ["Test","that","dependency","tree","copy","preserves","the","uniqueness","of","the","empty","node","."]
code: def test_copy_preserves_unique_empty_node(self):
      """Test that dependency tree copy preserves the uniqueness of the empty node."""
      new_dependency_tree = self.dependency_tree.copy()
      assert (
          self.dependency_tree.empty_node
          is new_dependency_tree.empty_node
      )
  
      assert (
          self.dependency_tree._root.children[0].children[0].children[1]
          is self.dependency_tree.empty_node
      )
      assert (
          new_dependency_tree._root.children[0].children[0].children[1]
          is self.dependency_tree.empty_node
      )
  
imports: import os
  import unittest
  from satpy.dependency_tree import DependencyTree
  from satpy.tests.utils import make_cid, make_dataid
label_desc: Use image node_id 4 for calling the TestDependencyTree obj's underlying member method code with example usage: obj.test_copy_preserves_unique_empty_node() without return types