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

@@ -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
 

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()

tests/test_black.py

@@ -6,6 +6,7 @@
 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 @@ def test_reformat_one_with_stdin_and_existing_path(self) -> None:
             # __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?$")