Fix/obfuscation application (#62)

* Add deepcopy

* Add a test

* Fix tests

src/hutch_bunny/core/obfuscation.py

@@ -1,3 +1,6 @@
+from copy import deepcopy
+
+
 def low_number_suppression(value: int | float, threshold: int = 10) -> int | float:
     """Suppress values that fall below a given threshold.
 
@@ -39,6 +42,8 @@ def rounding(value: int | float, nearest: int = 10) -> int:
 def apply_filters(value: int | float, filters: list) -> int | float:
     """Iterate over a list of filters and apply them to the supplied value.
 
+    Makes a deep copy of the filters list to avoid mutating the original list.
+
     Args:
         value (int | float): The value to be filtered.
         filters (list): The filters applied to the value.
@@ -48,7 +53,8 @@ def apply_filters(value: int | float, filters: list) -> int | float:
     """
     actions = {"Low Number Suppression": low_number_suppression, "Rounding": rounding}
     result = value
-    for f in filters:
+    filters_copy = deepcopy(filters)
+    for f in filters_copy:
         if action := actions.get(f.pop("id", None)):
             result = action(result, **f)
             if result == 0:

tests/test_obfuscation.py

@@ -3,6 +3,7 @@
     low_number_suppression,
     rounding,
 )
+from copy import deepcopy
 
 
 def test_low_number_suppression():
@@ -64,3 +65,25 @@ def test_apply_filters_combined_leak():
 def test_apply_filters_combined_empty_filter():
     # Test that an empty filter list returns the original value
     assert apply_filters(9, []) == 9
+
+
+def test_apply_filters_preserves_original_filters():
+    # Test that the original filters list is not modified
+    original_filters = [
+        {"id": "Low Number Suppression", "threshold": 100},
+        {"id": "Rounding", "nearest": 100},
+    ]
+
+    # Make a deep copy to compare
+    filters_before = deepcopy(original_filters)
+
+    # Apply filters in a loop to simulate daemon behavior
+    for _ in range(3): 
+        result = apply_filters(123, filters=original_filters)
+        # Verify each iteration still produces the expected result
+        assert result == 100
+        # Verify filters still contain their 'id' keys after each iteration
+        assert all('id' in f for f in original_filters)
+
+    # Verify the original filters list remains completely unchanged
+    assert original_filters == filters_before