Refactor violation diffing into separate function

src/fixit/engine.py

@@ -28,6 +28,26 @@
 LOG = logging.getLogger(__name__)
 
 
+def diff_violation(path: Path, module: Module, violation: LintViolation) -> str:
+    """
+    Generate string diff representation of a violation.
+    """
+
+    orig = module.code
+    mod = module.deep_replace(  # type:ignore # LibCST#906
+        violation.node, violation.replacement
+    )
+    assert isinstance(mod, Module)
+    change = mod.code
+
+    return unified_diff(
+        orig,
+        change,
+        path.name,
+        n=1,
+    )
+
+
 class LintRunner:
     def __init__(self, path: Path, source: FileContent) -> None:
         self.path = path
@@ -87,19 +107,7 @@ def visit_hook(name: str) -> Iterator[None]:
                 count += 1
 
                 if violation.replacement:
-                    orig = self.module.code
-                    mod = self.module.deep_replace(  # type:ignore # LibCST#906
-                        violation.node, violation.replacement
-                    )
-                    assert isinstance(mod, Module)
-                    change = mod.code
-
-                    diff = unified_diff(
-                        orig,
-                        change,
-                        self.path.name,
-                        n=1,
-                    )
+                    diff = diff_violation(self.path, self.module, violation)
                     violation = replace(violation, diff=diff)
 
                 yield violation

src/fixit/testing.py

@@ -10,9 +10,7 @@
 from pathlib import Path
 from typing import Any, Callable, Collection, Dict, List, Mapping, Sequence, Type, Union
 
-from moreorless import unified_diff
-
-from .engine import LintRunner
+from .engine import diff_violation, LintRunner
 from .ftypes import Config
 from .rule import Invalid, LintRule, Valid
 
@@ -112,9 +110,7 @@ def _test_method(
 
             if len(reports) == 1:
                 # make sure we generated a reasonable diff
-                expected_diff = unified_diff(
-                    source_code, expected_code, filename=path.name, n=1
-                )
+                expected_diff = diff_violation(path, runner.module, reports[0])
                 self.assertEqual(expected_diff, report.diff)
 
 

src/fixit/tests/__init__.py

@@ -8,6 +8,7 @@
 
 from fixit.testing import add_lint_rule_tests_to_module
 from .config import ConfigTest
+from .engine import EngineTest
 from .ftypes import TypesTest
 from .rule import RuleTest, RunnerTest
 from .smoke import SmokeTest

src/fixit/tests/engine.py

@@ -0,0 +1,61 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+
+from pathlib import Path
+from textwrap import dedent
+from unittest import TestCase
+
+from libcst import (
+    Call,
+    ensure_type,
+    Expr,
+    parse_module,
+    SimpleStatementLine,
+    SimpleString,
+)
+from libcst.metadata import CodePosition, CodeRange
+
+from ..engine import diff_violation
+from ..ftypes import LintViolation
+
+
+class EngineTest(TestCase):
+    def test_diff_violation(self):
+        src = dedent(
+            """\
+                import sys
+                print("hello world")
+            """
+        )
+        path = Path("foo.py")
+        module = parse_module(src)
+        node = ensure_type(
+            ensure_type(
+                ensure_type(module.body[-1], SimpleStatementLine).body[0], Expr
+            ).value,
+            Call,
+        ).args[0]
+        repl = node.with_changes(value=SimpleString('"goodnight moon"'))
+
+        violation = LintViolation(
+            "Fake",
+            CodeRange(CodePosition(1, 1), CodePosition(2, 2)),
+            message="some error",
+            node=node,
+            replacement=repl,
+        )
+
+        expected = dedent(
+            """\
+                --- a/foo.py
+                +++ b/foo.py
+                @@ -1,2 +1,2 @@
+                 import sys
+                -print("hello world")
+                +print("goodnight moon")
+            """
+        )
+        result = diff_violation(path, module, violation)
+        self.assertEqual(expected, result)