From 017aafea992ca1c6d7af45d3013af7ddb7fda12a Mon Sep 17 00:00:00 2001
From: simaki <shota.imaki.0801@gmail.com>
Date: Thu, 24 Jun 2021 04:11:23 +0900
Subject: [PATCH] Accept empty stdin (close #2337) (#2346)

Commit history before merge:

* Accept empty stdin (close #2337)
* Update tests/test_black.py
* Add changelog
* Assert Black reformats an empty string to an empty string (#2337) (#2346)
* fix
---
 CHANGES.md            |  1 +
 src/black/__init__.py |  3 ++-
 tests/test_black.py   | 15 +++++++++++++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/CHANGES.md b/CHANGES.md
index 12e8fcc..ea5196e 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -6,6 +6,7 @@
 
 - Add primer support and test for code piped into black via STDIN (#2315)
 - Fix internal error when `FORCE_OPTIONAL_PARENTHESES` feature is enabled (#2332)
+- Accept empty stdin (#2346)
 
 ## 21.6b0
 
diff --git a/src/black/__init__.py b/src/black/__init__.py
index a985926..8e2123d 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -803,7 +803,8 @@ def format_stdin_to_stdout(
         )
         if write_back == WriteBack.YES:
             # Make sure there's a newline after the content
-            dst += "" if dst[-1] == "\n" else "\n"
+            if dst and dst[-1] != "\n":
+                dst += "\n"
             f.write(dst)
         elif write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
             now = datetime.utcnow()
diff --git a/tests/test_black.py b/tests/test_black.py
index 5262c0e..42ac119 100644
--- a/tests/test_black.py
+++ b/tests/test_black.py
@@ -6,6 +6,7 @@ from concurrent.futures import ThreadPoolExecutor
 from contextlib import contextmanager
 from dataclasses import replace
 import inspect
+import io
 from io import BytesIO
 import os
 from pathlib import Path
@@ -1682,6 +1683,20 @@ class BlackTestCase(BlackBaseTestCase):
             # __BLACK_STDIN_FILENAME__ should have been stripped
             report.done.assert_called_with(expected, black.Changed.YES)
 
+    def test_reformat_one_with_stdin_empty(self) -> None:
+        output = io.StringIO()
+        with patch("io.TextIOWrapper", lambda *args, **kwargs: output):
+            try:
+                black.format_stdin_to_stdout(
+                    fast=True,
+                    content="",
+                    write_back=black.WriteBack.YES,
+                    mode=DEFAULT_MODE,
+                )
+            except io.UnsupportedOperation:
+                pass  # StringIO does not support detach
+            assert output.getvalue() == ""
+
     def test_gitignore_exclude(self) -> None:
         path = THIS_DIR / "data" / "include_exclude_tests"
         include = re.compile(r"\.pyi?$")
-- 
2.39.5