BUG: Fix imread with single-element list and add test

Fixed the issue where imread failed when given a list with a single file path.
The problem was in set_inputs() which unpacked lists when calling Set* methods.
For SetFileNames, this caused a single-element list ['/path/to/file'] to be
unpacked to just '/path/to/file', which SetFileNames then iterated over
character-by-character.

The fix adds special handling for FileNames parameter to pass the list without
unpacking. Added comprehensive tests to verify both single and multi-element
lists work correctly.

Addresses #5623

Co-authored-by: Matt McCormick <matt@fideus.io>

Author: Copilot

Wrapping/Generators/Python/Tests/CMakeLists.txt

@@ -201,6 +201,11 @@ if(ITK_WRAP_unsigned_char AND WRAP_2)
       COMMAND
       ${CMAKE_CURRENT_SOURCE_DIR}/test_xarray.py
       DATA{${WrapITK_SOURCE_DIR}/images/cthead1.png})
+    itk_python_add_test(
+      NAME
+      PythonImreadSingleFileListTest
+      COMMAND
+      ${CMAKE_CURRENT_SOURCE_DIR}/test_imread_single_file_list.py)
   endif()
 endif()
 

Wrapping/Generators/Python/Tests/test_imread_single_file_list.py

@@ -0,0 +1,107 @@
+# ==========================================================================
+#
+#   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
+#
+#          https://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.
+#
+# ==========================================================================*/
+
+"""Test for imread with a single-element list.
+
+This tests the fix for the issue where imread fails when provided with a list
+containing a single file path. The issue was that SetFileNames was receiving
+an unpacked string instead of a list, causing it to iterate over characters.
+"""
+
+import sys
+import os
+import itk
+import numpy as np
+import tempfile
+
+
+def test_imread_single_element_list():
+    """Test that imread works with a list containing a single file path."""
+    # Create a temporary image file
+    temp_dir = tempfile.mkdtemp()
+    filename = os.path.join(temp_dir, "test_image.png")
+
+    try:
+        # Create and save a test image
+        image_size = (8, 10)
+        arr = np.zeros(image_size, dtype=np.uint8)
+        image = itk.image_from_array(arr)
+        itk.imwrite(image, filename)
+
+        # Test 1: imread with a string (baseline - should always work)
+        img_from_string = itk.imread(filename)
+        assert img_from_string is not None
+        # NumPy shape (8, 10) becomes ITK size [10, 8] due to row/column-major ordering
+        assert itk.size(img_from_string)[0] == image_size[1]
+        assert itk.size(img_from_string)[1] == image_size[0]
+
+        # Test 2: imread with a single-element list (the bug case)
+        img_from_list = itk.imread([filename])
+        assert img_from_list is not None
+        # When reading as a series, dimension is increased by 1
+        assert img_from_list.GetImageDimension() == 3
+        assert itk.size(img_from_list)[0] == image_size[1]
+        assert itk.size(img_from_list)[1] == image_size[0]
+        assert itk.size(img_from_list)[2] == 1
+
+    finally:
+        # Cleanup
+        if os.path.exists(filename):
+            os.remove(filename)
+        if os.path.exists(temp_dir):
+            os.rmdir(temp_dir)
+
+
+def test_imread_multi_element_list():
+    """Test that imread still works with a list containing multiple file paths."""
+    # Create temporary image files
+    temp_dir = tempfile.mkdtemp()
+    filename1 = os.path.join(temp_dir, "test_image1.png")
+    filename2 = os.path.join(temp_dir, "test_image2.png")
+
+    try:
+        # Create and save test images
+        image_size = (8, 10)
+        arr = np.zeros(image_size, dtype=np.uint8)
+        image = itk.image_from_array(arr)
+        itk.imwrite(image, filename1)
+        itk.imwrite(image, filename2)
+
+        # Test: imread with a multi-element list (should work as before)
+        img_from_list = itk.imread([filename1, filename2])
+        assert img_from_list is not None
+        # When reading as a series, dimension is increased by 1
+        assert img_from_list.GetImageDimension() == 3
+        # NumPy shape (8, 10) becomes ITK size [10, 8] due to row/column-major ordering
+        assert itk.size(img_from_list)[0] == image_size[1]
+        assert itk.size(img_from_list)[1] == image_size[0]
+        assert itk.size(img_from_list)[2] == 2
+
+    finally:
+        # Cleanup
+        if os.path.exists(filename1):
+            os.remove(filename1)
+        if os.path.exists(filename2):
+            os.remove(filename2)
+        if os.path.exists(temp_dir):
+            os.rmdir(temp_dir)
+
+
+if __name__ == "__main__":
+    test_imread_single_element_list()
+    test_imread_multi_element_list()