Fix string mutation (#388)

String mutation would otherwise fail when escape sequences are used,
changing `"\n"` to `"\N"`, which is a syntax error.

Author: fabi321

mutmut/node_mutation.py

@@ -1,4 +1,5 @@
 """This module contains the mutations for indidvidual nodes, e.g. replacing a != b with a == b."""
+import re
 from typing import Any, Union
 from collections.abc import Callable, Iterable, Sequence
 import libcst as cst
@@ -11,6 +12,8 @@
     ]
 ]
 
+CHARACTER_AFTER_BACKSLASH = re.compile(r"\\([A-Z])")
+
 def operator_number(
     node: cst.BaseNumber
 ) -> Iterable[cst.BaseNumber]:
@@ -40,7 +43,7 @@ def operator_string(
         supported_str_mutations: list[Callable[[str], str]] = [
             lambda x: "XX" + x + "XX",
             lambda x: x.lower(),
-            lambda x: x.upper(),
+            lambda x: CHARACTER_AFTER_BACKSLASH.sub(lambda match: f"\\{match.group(1).lower()}", x.upper()),
             lambda x: x.capitalize(),
         ]
 

tests/test_mutation.py

@@ -126,6 +126,7 @@ def mutated_module(source: str) -> str:
         ('a: Optional[int] = None', 'a: Optional[int] = ""'),
         ('a: int = 1', ['a: int = 2', 'a: int = None']),
         ('a: str = "FoO"', ['a: str = "XXFoOXX"', 'a: str = "foo"', 'a: str = "FOO"', 'a: str = "Foo"', 'a: str = None']),
+        ('a: str = "Fo\\t"', ['a: str = "XXFo\\tXX"', 'a: str = "fo\\t"', 'a: str = "FO\\t"', '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']),