first commit

backend/app/dataset/rules/python/_custom_demo.py

@@ -0,0 +1,47 @@
+"""Custom rule demonstrating pluggable extensibility (A1b)."""
+
+import ast
+from typing import Optional
+
+from app.dataset.rules.base import Mutation, MutationRule
+
+
+class UnusedVariableRule(MutationRule):
+    """A custom rule: replace a variable read with an undefined name.
+
+    This is intentionally simple and demonstrates that adding a new file under
+    app/dataset/rules/<language>/ is enough to register a rule.
+    """
+
+    name = "unused_variable_demo"
+    language = "python"
+    defect_type = "custom_demo"
+
+    def detect_and_mutate(self, source: str, filename: str = "") -> Optional[Mutation]:
+        try:
+            tree = ast.parse(source)
+        except SyntaxError:
+            return None
+
+        for node in ast.walk(tree):
+            if isinstance(node, ast.FunctionDef) and node.body:
+                first = node.body[0]
+                if isinstance(first, ast.Assign) and isinstance(first.targets[0], ast.Name):
+                    var_name = first.targets[0].id
+                    line_no = first.lineno
+                    lines = source.splitlines(keepends=True)
+                    line = lines[line_no - 1]
+                    mutated_line = line.replace(var_name, "__undefined_" + var_name, 1)
+                    if mutated_line == line:
+                        continue
+                    mutated = "".join(lines[:line_no - 1] + [mutated_line] + lines[line_no:])
+                    return Mutation(
+                        defect_type=self.defect_type,
+                        language=self.language,
+                        line_start=line_no,
+                        line_end=line_no,
+                        mutated_source=mutated,
+                        reference_fix=f"Use the original variable name '{var_name}'.",
+                        description=f"Custom rule: replaced '{var_name}' with an undefined placeholder.",
+                    )
+        return None