Add mutation for ternary operators

src/mutmut/node_mutation.py

@@ -242,6 +242,23 @@ def operator_assignment(
     yield node.with_changes(value=mutated_value)
 
 
+def _wrap_low_precedence_expression(expression: cst.BaseExpression) -> cst.BaseExpression:
+    """Wrap low-precedence expressions before appending boolean operators."""
+    if expression.lpar or expression.rpar:
+        return expression
+
+    if isinstance(expression, cst.BooleanOperation | cst.IfExp | cst.Lambda | cst.NamedExpr):
+        return expression.with_changes(lpar=[cst.LeftParen()], rpar=[cst.RightParen()])
+
+    return expression
+
+
+def operator_if_exp(node: cst.IfExp) -> Iterable[cst.IfExp]:
+    test = _wrap_low_precedence_expression(node.test)
+    yield node.with_changes(test=cst.BooleanOperation(left=test, operator=cst.And(), right=cst.Name("False")))
+    yield node.with_changes(test=cst.BooleanOperation(left=test, operator=cst.Or(), right=cst.Name("True")))
+
+
 def operator_match(node: cst.Match) -> Iterable[cst.CSTNode]:
     """Drop the case statements in a match."""
     if len(node.cases) > 1:
@@ -263,6 +280,7 @@ def operator_match(node: cst.Match) -> Iterable[cst.CSTNode]:
     (cst.Call, operator_symmetric_string_methods_swap),
     (cst.Call, operator_unsymmetrical_string_methods_swap),
     (cst.Lambda, operator_lambda),
+    (cst.IfExp, operator_if_exp),
     (cst.CSTNode, operator_keywords),  # type: ignore[type-abstract]
     (cst.CSTNode, operator_swap_op),  # type: ignore[type-abstract]
     (cst.Match, operator_match),

tests/test_mutation.py

@@ -244,6 +244,7 @@ def mutated_module(source: str) -> str:
         ("foo is not foo", "foo is foo"),
         ("a or b", "a and b"),
         ("a and b", "a or b"),
+        ("a if b else c", ["a if b and False else c", "a if b or True else c"]),
         ("not a", "a"),
         ("a < b", ["a <= b"]),
         ("a <= b", ["a < b"]),
@@ -325,6 +326,13 @@ def foo() -> int:
     assert not mutants
 
 
+def test_ternary_mutation_preserves_boolean_precedence():
+    mutants = mutants_for_source("a if b or c else d")
+
+    assert "a if (b or c) and False else d" in mutants
+    assert "a if (b or c) or True else d" in mutants
+
+
 def test_do_not_mutate_specific_functions():
     source = """
 class A:
@@ -401,13 +409,15 @@ def test_function_with_annotation():
     mutated_code = mutated_module(source)
     print(mutated_code)
 
-    expected_defs = [
-        "def x_capitalize__mutmut_1(s : str):\n    return s[0].title() - s[1:] if s else s",
-        "def x_capitalize__mutmut_2(s : str):\n    return s[1].title() + s[1:] if s else s",
-        "def x_capitalize__mutmut_3(s : str):\n    return s[0].title() + s[2:] if s else s",
+    expected_snippets = [
+        "return s[0].title() + s[1:] if s and False else s",
+        "return s[0].title() + s[1:] if s or True else s",
+        "return s[0].title() - s[1:] if s else s",
+        "return s[1].title() + s[1:] if s else s",
+        "return s[0].title() + s[2:] if s else s",
     ]
 
-    for expected in expected_defs:
+    for expected in expected_snippets:
         print(expected)
         assert expected in mutated_code