ENH: Expose itk::VariableLengthVector operators in Python wrapping

Allows `itk.VariableLengthVector`s to be added directly in Python, i.e.
`v3 = v1 + v2` now succeeds where `v1`, `v2`, `v3` are all the same
specialization of `itk.VariableLengthVector`.

Modules/Core/Common/wrapping/test/CMakeLists.txt

@@ -9,4 +9,5 @@ if(ITK_WRAP_PYTHON)
   itk_python_add_test(NAME itkImageDuplicatorPythonTest COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/itkImageDuplicatorTest.py)
   itk_python_add_test(NAME itkImagePythonTest COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/itkImageTest.py)
   itk_python_add_test(NAME itkMatrixPythonTest COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/itkMatrixTest.py)
+  itk_python_add_test(NAME itkVariableLengthVectorPythonTest COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/itkVariableLengthVectorTest.py)
 endif()

Modules/Core/Common/wrapping/test/itkVariableLengthVectorTest.py

@@ -0,0 +1,57 @@
+# ==========================================================================
+#
+#   Copyright NumFOCUS
+#
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+#
+#          http://www.apache.org/licenses/LICENSE-2.0.txt
+#
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+#
+# ==========================================================================*/
+
+import itk
+itk.auto_progress(2)
+
+n_channels = 31
+
+# Verify UC addition operation
+vector_type = itk.VariableLengthVector[itk.UC]
+vector1 = vector_type(n_channels)
+vector2 = vector_type(n_channels)
+assert len(vector1) == n_channels and len(vector2) == n_channels
+
+vector1.Fill(16)
+for idx in range(n_channels):
+    vector2[idx] = idx
+
+sum = vector1 + vector2
+print(f'UC sum: {sum}')
+
+for idx in range(n_channels):
+    assert sum[idx] == 16 + idx, "Got unexpected result from vector sum"
+
+# Verify float addition operation
+vector_float_type = itk.VariableLengthVector[itk.F]
+vector3 = vector_float_type(n_channels)
+vector4 = vector_float_type(n_channels)
+assert len(vector3) == n_channels and len(vector4) == n_channels
+
+vector3.Fill(0.5)
+for idx in range(n_channels):
+    vector4.SetElement(idx, 0.1 * idx)
+
+float_sum = vector3 + vector4
+print(f'float sum: {float_sum}')
+
+tolerance = 1e-6
+for idx in range(n_channels):
+    diff = abs(float_sum[idx] - (0.5 + 0.1 * idx))
+    print(f'float sum[{idx}]: {float_sum[idx]:0.9f} diff: {diff:0.2e}')
+    assert diff < tolerance, "Got unexpected result from vector float sum"

Wrapping/Generators/Python/CMakeLists.txt

@@ -726,6 +726,12 @@ macro(itk_wrap_simple_type_python wrap_class swig_name)
     ADD_PYTHON_CONFIG_TEMPLATE("list" "std::list" "listitkLightObject" "itk::LightObject")
   endif()
 
+  if("${cpp_name}" STREQUAL "itk::VariableLengthVector")
+    if(NOT ("${template_params}" MATCHES "std::complex")) # TODO cover complex types
+      set(ITK_WRAP_PYTHON_SWIG_EXT "${ITK_WRAP_PYTHON_SWIG_EXT}DECL_PYTHON_VARIABLELENGTHVECTOR_CLASS(${swig_name}, ${template_params})\n")
+    endif()
+  endif()
+
   if("${swig_name}" STREQUAL "itkObject")
     set(ITK_WRAP_PYTHON_SWIG_EXT "${ITK_WRAP_PYTHON_SWIG_EXT}DECL_PYTHON_OBJECT_CLASS(${swig_name})\n")
   endif()

Wrapping/Generators/Python/PyBase/pyBase.i

@@ -271,6 +271,36 @@ str = str
 %enddef
 
 
+%define DECL_PYTHON_VARIABLELENGTHVECTOR_CLASS(swig_name, type)
+
+    %extend swig_name {
+        swig_name __add__(swig_name v2) {
+            return *self + v2;
+        }
+        swig_name __sub__(swig_name v2) {
+            return *self - v2;
+        }
+        type __getitem__(unsigned long d) {
+            if (d >= self->GetNumberOfElements()) { throw std::out_of_range("swig_name index out of range."); }
+            return self->operator[]( d );
+        }
+        void __setitem__(unsigned long d, type v) {
+            if (d >= self->GetNumberOfElements()) { throw std::out_of_range("swig_name index out of range."); }
+            self->operator[]( d ) = v;
+        }
+        unsigned int __len__() {
+            return self->GetNumberOfElements();
+        }
+        std::string __repr__() {
+            std::ostringstream msg;
+            msg << "swig_name (" << *self << ")";
+            return msg.str();
+        }
+    }
+
+%enddef
+
+
 %define DECL_PYTHON_OBJECT_CLASS(swig_name)
 
     %pythonprepend itkObject::AddObserver %{