From 8daa64a2e10907539094df51f4c51306bb426f07 Mon Sep 17 00:00:00 2001
From: KotlinIsland <65446343+KotlinIsland@users.noreply.github.com>
Date: Thu, 7 Sep 2023 17:11:50 +1000
Subject: [PATCH] blackd: fix mishandling of single character input (#3558)

---
 CHANGES.md             | 2 ++
 src/blackd/__init__.py | 3 ++-
 tests/test_blackd.py   | 6 ++++++
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/CHANGES.md b/CHANGES.md
index 2168c1b..af9fc49 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -45,6 +45,8 @@
 
 <!-- Changes to blackd -->
 
+- Fix an issue in `blackd` with single character input (#3558)
+
 ### Integrations
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->
diff --git a/src/blackd/__init__.py b/src/blackd/__init__.py
index 4f2d87d..6b0f3d3 100644
--- a/src/blackd/__init__.py
+++ b/src/blackd/__init__.py
@@ -152,7 +152,8 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
         )
 
         # Preserve CRLF line endings
-        if req_str[req_str.find("\n") - 1] == "\r":
+        nl = req_str.find("\n")
+        if nl > 0 and req_str[nl - 1] == "\r":
             formatted_str = formatted_str.replace("\n", "\r\n")
             # If, after swapping line endings, nothing changed, then say so
             if formatted_str == req_str:
diff --git a/tests/test_blackd.py b/tests/test_blackd.py
index 325bd7d..dd2126e 100644
--- a/tests/test_blackd.py
+++ b/tests/test_blackd.py
@@ -240,3 +240,9 @@ class BlackDTestCase(AioHTTPTestCase):  # type: ignore[misc]
             response = await self.client.post("/", data=data)
             self.assertEqual(await response.text(), expected)
             self.assertEqual(response.status, 200)
+
+    @unittest_run_loop
+    async def test_single_character(self) -> None:
+        response = await self.client.post("/", data="1")
+        self.assertEqual(await response.text(), "1\n")
+        self.assertEqual(response.status, 200)
-- 
2.39.5