Fix module resolution for relative subdirectory paths

plain_file.py

@@ -651,11 +651,7 @@ def plain_file_parser(  # noqa: C901
             f"Plain syntax error: Invalid plain file extension: {plain_source_file_path.suffix}. Expected: {PLAIN_SOURCE_FILE_EXTENSION}."
         )
 
-    module_name = (
-        plain_source_file_path.stem
-        if plain_source_file_path.is_absolute()
-        else plain_source_file_path.with_suffix("").as_posix()
-    )
+    module_name = plain_source_file_path.stem
 
     code_variables = {}
 

tests/test_plainfileparser.py

@@ -27,6 +27,19 @@ def test_regular_plain_source(get_test_data_path):
     }
 
 
+def test_relative_subdirectory_path_resolves(get_test_data_path):
+    # Regression: the CLI passes the full path as the filename and its directory as template_dirs[0].
+    # For a relative path with subdirectories, module_name must be the bare stem; otherwise the
+    # subdirectory prefix double-joins with template_dirs[0] and the module is reported as missing.
+    data_dir = get_test_data_path("data/plainfileparser")
+    rel_file = os.path.relpath(os.path.join(data_dir, "regular_plain_source.plain"))
+    assert os.sep in rel_file  # ensure the relative path actually has a subdirectory prefix
+
+    module_name, _, _ = plain_file.plain_file_parser(rel_file, [os.path.dirname(rel_file)])
+
+    assert module_name == "regular_plain_source"
+
+
 def test_unknown_section():
     plain_source = """
 ***definitions***