Treat black's NothingChanged as a successful no-op

src/fixit/format.py

@@ -48,9 +48,16 @@ def format(self, module: Module, path: Path) -> FileContent:
         import ufmt.util
 
         mode = ufmt.util.make_black_config(path)
-        content = black.format_file_contents(
-            module.bytes.decode("utf-8"), fast=False, mode=mode
-        )
+        try:
+            content = black.format_file_contents(
+                module.bytes.decode("utf-8"), fast=False, mode=mode
+            )
+        except black.NothingChanged:
+            # black signals "this is already formatted" by raising rather than
+            # by returning the content unchanged. That is a successful no-op,
+            # and letting it propagate discards the autofix that produced this
+            # module.
+            return module.bytes
         return content.encode("utf-8")
 
 

src/fixit/tests/__init__.py

@@ -9,6 +9,7 @@
 from fixit.testing import add_lint_rule_tests_to_module
 from .config import ConfigTest
 from .engine import EngineTest
+from .format import FormatTest
 from .ftypes import TypesTest
 from .rule import RuleTest, RunnerTest
 from .smoke import SmokeTest

src/fixit/tests/format.py

@@ -0,0 +1,34 @@
+# 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 unittest import TestCase
+
+import libcst
+
+from ..format import format_module
+from ..ftypes import Config
+
+FAKE_PATH = Path("fake.py")
+
+
+class FormatTest(TestCase):
+    def test_black_already_formatted(self) -> None:
+        # black reports "already formatted" by raising rather than by returning
+        # the content unchanged. Letting that escape loses the autofix that
+        # produced the module, so the formatter has to treat it as a no-op.
+        config = Config(path=FAKE_PATH, formatter="black")
+        for content, expected in (
+            (b"x = 1\n", b"x = 1\n"),
+            (b"x=1\n", b"x = 1\n"),
+        ):
+            with self.subTest(content=content):
+                module = libcst.parse_module(content)
+                self.assertEqual(expected, format_module(module, FAKE_PATH, config))
+
+    def test_no_formatter_returns_module_bytes(self) -> None:
+        config = Config(path=FAKE_PATH)
+        module = libcst.parse_module(b"x=1\n")
+        self.assertEqual(b"x=1\n", format_module(module, FAKE_PATH, config))