permits black to run in AWS Lambda:

black.py

@@ -541,7 +541,14 @@ def reformat_many(
     if sys.platform == "win32":
         # Work around https://bugs.python.org/issue26903
         worker_count = min(worker_count, 61)
-    executor = ProcessPoolExecutor(max_workers=worker_count)
+    try:
+        executor = ProcessPoolExecutor(max_workers=worker_count)
+    except OSError:
+        # we arrive here if the underlying system does not support multi-processing
+        # like in AWS Lambda, in which case we gracefully fallback to the default
+        # mono-process Executor by using None
+        executor = None
+
     try:
         loop.run_until_complete(
             schedule_formatting(
@@ -556,7 +563,8 @@ def reformat_many(
         )
     finally:
         shutdown(loop)
-        executor.shutdown()
+        if executor is not None:
+            executor.shutdown()
 
 
 async def schedule_formatting(
@@ -566,7 +574,7 @@ async def schedule_formatting(
     mode: FileMode,
     report: "Report",
     loop: asyncio.AbstractEventLoop,
-    executor: Executor,
+    executor: Optional[Executor],
 ) -> None:
     """Run formatting of `sources` in parallel using the provided `executor`.
 

tests/test_black.py

@@ -1159,6 +1159,27 @@ def test_cache_multiple_files(self) -> None:
             self.assertIn(one, cache)
             self.assertIn(two, cache)
 
+    @patch("black.ProcessPoolExecutor", autospec=True)
+    def test_works_in_mono_process_only_environment(self, mock_executor) -> None:
+        mock_executor.side_effect = OSError()
+        mode = black.FileMode()
+        with cache_dir() as workspace:
+            one = (workspace / "one.py").resolve()
+            with one.open("w") as fobj:
+                fobj.write("print('hello')")
+            two = (workspace / "two.py").resolve()
+            with two.open("w") as fobj:
+                fobj.write("print('hello')")
+            black.write_cache({}, [one], mode)
+            self.invokeBlack([str(workspace)])
+            with one.open("r") as fobj:
+                self.assertEqual(fobj.read(), "print('hello')")
+            with two.open("r") as fobj:
+                self.assertEqual(fobj.read(), 'print("hello")\n')
+            cache = black.read_cache(mode)
+            self.assertIn(one, cache)
+            self.assertIn(two, cache)
+
     def test_no_cache_when_writeback_diff(self) -> None:
         mode = black.FileMode()
         with cache_dir() as workspace: