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I’m using Python 3.9 and vtk 9.1.0 on Windows and find that vtkDataAssembly.SelectNodes() is not working correctly. The below example populates an assembly from XML The assembly has a root node named "Model_26", with a single child named "Assembly_1" which in turn has 19 children. The example then make tqo SelectNodes calls - the first is epected to return the ID of the "Assembly_1" node and the second is expected to return the IDs of the 19 children of "Assembly_1" The first selection behaves as expected but the second returns nothing Further testing shows that any selection path with a trailing '/' returns nothing instead of returning the childern of the last node in the path

import vtk

data_assembly_xml="""<?xml version="1.0"?>
<Model_26 type="vtkDataAssembly" version="1.0" id="0">
 <Assembly_1 id="1">
  <Part_462 id="2">
   <Solid_1 id="3">
    <dataset id="0" />
   </Solid_1>
  </Part_462>
  <Part_463 id="4">
   <Mesh_1 id="5">
    <dataset id="1" />
    <dataset id="2" />
   </Mesh_1>
   <Solid_2 id="6" />
  </Part_463>
  <Part_464 id="7">
   <Solid_3 id="8">
    <dataset id="3" />
   </Solid_3>
  </Part_464>
  <Part_465 id="9">
   <Solid_4 id="10">
    <dataset id="4" />
   </Solid_4>
  </Part_465>
  <Part_466 id="11">
   <Solid_5 id="12">
    <dataset id="5" />
   </Solid_5>
  </Part_466>
  <Part_467 id="13">
   <Solid_6 id="14">
    <dataset id="6" />
   </Solid_6>
  </Part_467>
  <Part_468 id="15">
   <Solid_7 id="16">
    <dataset id="7" />
   </Solid_7>
  </Part_468>
  <Part_469 id="17">
   <Mesh_2 id="18">
    <dataset id="8" />
    <dataset id="9" />
   </Mesh_2>
   <Solid_8 id="19" />
  </Part_469>
  <Part_470 id="20">
   <Solid_9 id="21">
    <dataset id="10" />
   </Solid_9>
  </Part_470>
  <Part_471 id="22">
   <Solid_10 id="23">
    <dataset id="11" />
   </Solid_10>
  </Part_471>
  <Part_479 id="24">
   <Solid_18 id="25">
    <dataset id="12" />
   </Solid_18>
  </Part_479>
  <Part_484 id="26">
   <Solid_23 id="27">
    <dataset id="13" />
   </Solid_23>
  </Part_484>
  <Part_485 id="28">
   <Solid_24 id="29">
    <dataset id="14" />
   </Solid_24>
  </Part_485>
  <Part_488 id="30">
   <Solid_27 id="31">
    <dataset id="15" />
   </Solid_27>
  </Part_488>
  <Part_490 id="32">
   <Solid_29 id="33">
    <dataset id="16" />
   </Solid_29>
  </Part_490>
  <Part_517 id="34">
   <Solid_56 id="35">
    <dataset id="17" />
   </Solid_56>
  </Part_517>
  <Part_518 id="36">
   <Solid_57 id="37">
    <dataset id="18" />
   </Solid_57>
  </Part_518>
  <Part_519 id="38">
   <Solid_58 id="39">
    <dataset id="19" />
   </Solid_58>
  </Part_519>
  <Part_520 id="40">
   <Solid_59 id="41">
    <dataset id="20" />
   </Solid_59>
  </Part_520>
 </Assembly_1>
</Model_26>"""

das = vtk.vtkDataAssembly()
das.InitializeFromXML(data_assembly_xml)

print(das.SelectNodes(['/Model_26/Assembly_1']))

print(das.SelectNodes(['/Model_26/Assembly_1/']))

This produces the following output,

(1,)
()