[Python] Return None instead of undefined from resolve_template

because undefined is for internal puprposes only

Author: dmitryashutov

python/fpml/core/extract.py

@@ -55,7 +55,7 @@ def resolve_template(
         FHIRPathMappingLanguage Specification:
         https://github.com/beda-software/FHIRPathMappingLanguage/tree/main?tab=readme-ov-file#specification
     """  # noqa: E501
-    return resolve_template_recur(
+    result = resolve_template_recur(
         [],
         guarded_resource if strict else resource,
         template,
@@ -64,6 +64,8 @@ def resolve_template(
         fp_options=fp_options,
     )
 
+    return None if result == undefined else result
+
 
 def resolve_template_recur(
     start_path: Path,

python/tests/core/test_extract.py

@@ -58,11 +58,11 @@ def test_transformation_works_on_accessing_props_of_implicit_context_in_strict_m
 
 
 def test_transformation_for_empty_object_return_empty_object() -> None:
-    assert resolve_template({}, {}) == undefined
+    assert resolve_template({}, {}) is None
 
 
 def test_transformation_for_empty_array_return_empty_array() -> None:
-    assert resolve_template({}, [])  == undefined
+    assert resolve_template({}, [])  is None
 
 
 def test_transformation_for_array_of_arrays_returns_flattened_array() -> None:
@@ -82,7 +82,7 @@ def test_transformation_for_object_with_null_keys_returns_null_keys() -> None:
 
 
 def test_transformation_for_object_with_undefined_keys_clears_undefined_keys() -> None:
-    assert resolve_template({}, {"key": undefined})  == undefined
+    assert resolve_template({}, {"key": undefined}) is None
 
 
 def test_transformation_for_object_with_non_null_keys_returns_non_null_keys() -> None:
@@ -123,7 +123,7 @@ def test_transformation_for_non_empty_array_expression_return_first_element() ->
 
 def test_transformation_for_empty_array_expression_clears_undefined_keys() -> None:
     resource: Resource = {"list": []}
-    assert resolve_template(resource, {"result": "{{ list.where($this = 0) }}"}) == undefined
+    assert resolve_template(resource, {"result": "{{ list.where($this = 0) }}"}) is None
 
 
 def test_transformation_for_empty_array_nullable_expression_returns_null() -> None:
@@ -148,7 +148,7 @@ def test_transformation_for_empty_array_template_expression_returns_undefined()
             resource,
             "/Patient/{{ list.where($this = 0) }}/_history/{{ list.last() }}",
         )
-        == undefined
+        is None
     )
 
 
@@ -237,7 +237,7 @@ def test_assign_block_with_undefined_intermediate_values() -> None:
                 "valueA": "{{ %varB }}",
             },
         )
-        == undefined
+        is None
     )
 
 
@@ -511,7 +511,7 @@ def test_if_block_clears_undefined_keys_for_falsy_condition_without_else_branch(
             },
         },
     )
-    assert result == undefined
+    assert result is None
 
 
 def test_if_block_returns_null_for_falsy_condition_with_nullable_else_branch() -> None: