Fix unchanged string mutants (#437)

* Add test for github issue 435

* Filter out identical string mutants with different values

As at the status quo, operator_string took the prefix of the node's
value, trimming that off the rest to leave an approximation.

This worked well enough until some wombat (hi) fed in a regex
string consisting only of escape sequences and whitespace.  For
whatever reason, the node represented that value as r'(rest of text...)
which the status quo dutifully trimmed to (rest of text...).

As the two text strings didn't match, status quo dutifully let the
mutant through despite it being a no-op.

This commit adds an additional check to catch the _unmodified_ node
value being equal to the mutated node value, skipping those
candidate mutants.

Author: CyberiaResurrection

mutmut/node_mutation.py

@@ -31,6 +31,7 @@ def operator_string(
 ) -> Iterable[cst.BaseString]:
     if isinstance(node, cst.SimpleString):
         value = node.value
+        old_value = value
         prefix = value[
             : min([x for x in [value.find('"'), value.find("'")] if x != -1])
         ]
@@ -52,6 +53,8 @@ def operator_string(
             new_value = f"{prefix}{value[0]}{mut_func(value[1:-1])}{value[-1]}"
             if new_value == value:
                 continue
+            if new_value == old_value:
+                continue
             yield node.with_changes(value=new_value)
 
 

tests/test_mutation.py

@@ -418,6 +418,30 @@ def test_bug_github_issue_77():
     assert mutants_for_source('') == []
 
 
+def test_bug_github_issue_435():
+    source = """
+    def parse(self, text: str) -> tuple[Tree[Token], str]:
+        text = re.sub(r'[\w\-]  [\w\-]', dashrepl, text)
+
+        return self.parser.parse(text), text
+    """.strip()
+
+    mutants = mutants_for_source(source)
+
+    expected = [
+        'def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = None\n\n        return self.parser.parse(text), text',
+        'def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = re.sub(None, dashrepl, text)\n\n        return self.parser.parse(text), text',
+        "def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = re.sub(r'[\\w\\-]  [\\w\\-]', None, text)\n\n        return self.parser.parse(text), text",
+        "def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = re.sub(r'[\\w\\-]  [\\w\\-]', dashrepl, None)\n\n        return self.parser.parse(text), text",
+        'def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = re.sub(dashrepl, text)\n\n        return self.parser.parse(text), text',
+        "def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = re.sub(r'[\\w\\-]  [\\w\\-]', text)\n\n        return self.parser.parse(text), text",
+        "def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = re.sub(r'[\\w\\-]  [\\w\\-]', dashrepl, )\n\n        return self.parser.parse(text), text",
+        "def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = re.sub(r'XX[\\w\\-]  [\\w\\-]XX', dashrepl, text)\n\n        return self.parser.parse(text), text",
+        "def parse(self, text: str) -> tuple[Tree[Token], str]:\n        text = re.sub(r'[\\w\\-]  [\\w\\-]', dashrepl, text)\n\n        return self.parser.parse(None), text"
+    ]
+    assert sorted(mutants) == sorted(expected)
+
+
 def test_multiline_dunder_whitelist():
     source = """
 __all__ = [