Return new list of changed strings

soundchanger/change.py

@@ -2,7 +2,10 @@ import re
 
 
 def apply(changes, strings, categories={}, ignore_errors=True, apply=True, zero_characters=['∅']):
-    """Apply a sound change to a given string"""
+    """
+    Applies a sound change or a list of sound changes to a string or
+    a list of given strings.
+    """
 
     if not apply:
         return strings
@@ -13,13 +16,15 @@ def apply(changes, strings, categories={}, ignore_errors=True, apply=True, zero_
     return_str = isinstance(strings, str)
     if isinstance(strings, str):
         strings = [strings]
+    else:
+        strings = strings.copy()
 
     for change in changes:
-        v = validate_change(change, ignore_errors=ignore_errors)
-        if v == False:
-            continue
-        change = convert_change_to_regex(change, categories=categories, zero_characters=zero_characters)
         
+        if validate_change(change, ignore_errors=ignore_errors) == False:
+            continue
+
+        change = convert_change_to_regex(change, categories=categories, zero_characters=zero_characters)
         original, change_to, before, after = split_change(change)
 
         pattern = f"({before})({original})({after})"

tests/test_change.py

@@ -92,3 +92,8 @@ def test_multiple_changes_and_strings():
 def test_applying_invalid_change():
     assert apply('', 'pana') == 'pana'
     assert apply(['p>f', '>', 'f>h'], 'pana') == 'hana'
+
+def test_original_list_remains_unchanged():
+    strings = ['abc', 'bcd', 'cde']
+    new_strings = apply('c>f', strings)
+    assert strings != new_strings