Add .upper() .lower() and .capitalize() as mutations (#384)

* feat(str_mut): add .upper() .lower() and .capitalize() as mutations + tests

* fix(str_mut): add check on useless mutants

* style(str_mut): clear code and fix tests

Author: CHB-0r1s

mutmut/node_mutation.py

@@ -37,7 +37,18 @@ def operator_string(
             # that mutation is meaningless for
             return
 
-        yield node.with_changes(value=f"{prefix}{value[0]}XX{value[1:-1]}XX{value[-1]}")
+        supported_str_mutations: list[Callable[[str], str]] = [
+            lambda x: "XX" + x + "XX",
+            lambda x: x.lower(),
+            lambda x: x.upper(),
+            lambda x: x.capitalize(),
+        ]
+
+        for mut_func in supported_str_mutations:
+            new_value = f"{prefix}{value[0]}{mut_func(value[1:-1])}{value[-1]}"
+            if new_value == value:
+                continue
+            yield node.with_changes(value=new_value)
 
 
 def operator_lambda(

tests/e2e/snapshots/config.json

@@ -1,6 +1,8 @@
 {
   "mutants/config_pkg/__init__.py.meta": {
-    "config_pkg.x_hello__mutmut_1": 1
+    "config_pkg.x_hello__mutmut_1": 1,
+    "config_pkg.x_hello__mutmut_2": 1,
+    "config_pkg.x_hello__mutmut_3": 1
   },
   "mutants/config_pkg/math.py.meta": {
     "config_pkg.math.x_add__mutmut_1": 0,

tests/e2e/snapshots/my_lib.json

@@ -1,11 +1,21 @@
 {
   "mutants/src/my_lib/__init__.py.meta": {
     "my_lib.x_hello__mutmut_1": 1,
+    "my_lib.x_hello__mutmut_2": 1,
+    "my_lib.x_hello__mutmut_3": 1,
     "my_lib.x_badly_tested__mutmut_1": 0,
+    "my_lib.x_badly_tested__mutmut_2": 0,
+    "my_lib.x_badly_tested__mutmut_3": 0,
     "my_lib.x_untested__mutmut_1": 33,
+    "my_lib.x_untested__mutmut_2": 33,
+    "my_lib.x_untested__mutmut_3": 33,
     "my_lib.x_make_greeter__mutmut_1": 1,
     "my_lib.x_make_greeter__mutmut_2": 1,
-    "my_lib.x_make_greeter__mutmut_3": 0,
+    "my_lib.x_make_greeter__mutmut_3": 1,
+    "my_lib.x_make_greeter__mutmut_4": 1,
+    "my_lib.x_make_greeter__mutmut_5": 0,
+    "my_lib.x_make_greeter__mutmut_6": 0,
+    "my_lib.x_make_greeter__mutmut_7": 0,
     "my_lib.x_fibonacci__mutmut_1": 1,
     "my_lib.x_fibonacci__mutmut_2": 0,
     "my_lib.x_fibonacci__mutmut_3": 0,

tests/test_mutation.py

@@ -58,7 +58,7 @@ def mutated_module(source: str) -> str:
         ('x: list[A | None]', []),
         ('a: Optional[int] = None', 'a: Optional[int] = ""'),
         ('a: int = 1', ['a: int = 2', 'a: int = None']),
-        ('a: str = "foo"', ['a: str = "XXfooXX"', 'a: str = None']),
+        ('a: str = "FoO"', ['a: str = "XXFoOXX"', 'a: str = "foo"', 'a: str = "FOO"', 'a: str = "Foo"', 'a: str = None']),
         ('lambda: 0', ['lambda: 1', 'lambda: None']),
         ("1 in (1, 2)", ['2 in (1, 2)', '1 not in (1, 2)', '1 in (2, 2)', '1 in (1, 3)']),
         ('1+1', ['2+1', '1 - 1', '1+2']),
@@ -81,9 +81,9 @@ def mutated_module(source: str) -> str:
         ('1e-3', '1.001'),
         ('True', 'False'),
         ('False', 'True'),
-        ('"foo"', '"XXfooXX"'),
-        ("'foo'", "'XXfooXX'"),
-        ("u'foo'", "u'XXfooXX'"),
+        ('"FoO"', ['"XXFoOXX"', '"foo"', '"FOO"', '"Foo"']),
+        ("'FoO'", ["'XXFoOXX'", "'foo'", "'FOO'", "'Foo'"]),
+        ("u'FoO'", ["u'XXFoOXX'", "u'foo'", "u'FOO'", "u'Foo'"]),
         ("10", "11"),
         ("10.", "11.0"),
         ("0o10", "9"),