Empty and all-whitespace files become 1 empty line

Fixes psf#2382

src/black/__init__.py

@@ -844,7 +844,7 @@ def format_stdin_to_stdout(
         )
         if write_back == WriteBack.YES:
             # Make sure there's a newline after the content
-            if dst and dst[-1] != "\n":
+            if not dst or dst[-1] != "\n":
                 dst += "\n"
             f.write(dst)
         elif write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
@@ -889,9 +889,6 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `mode` is passed to :func:`format_str`.
     """
-    if not src_contents.strip():
-        raise NothingChanged
-
     if mode.is_ipynb:
         dst_contents = format_ipynb_string(src_contents, fast=fast, mode=mode)
     else:
@@ -979,6 +976,8 @@ def format_ipynb_string(src_contents: str, *, fast: bool, mode: Mode) -> FileCon
     Operate cell-by-cell, only on code cells, only for Python notebooks.
     If the ``.ipynb`` originally had a trailing newline, it'll be preserved.
     """
+    if not src_contents:
+        return ""
     trailing_newline = src_contents[-1] == "\n"
     modified = False
     nb = json.loads(src_contents)
@@ -1061,6 +1060,9 @@ def f(
             current_line, mode=mode, features=split_line_features
         ):
             dst_contents.append(str(line))
+    if not dst_contents:
+        _, _, newline = decode_bytes(src_contents.encode("utf-8"))
+        return newline
     return "".join(dst_contents)
 
 

tests/data/whitespace.py

@@ -0,0 +1,6 @@
+
+
+
+
+
+# output

tests/test_black.py

@@ -137,19 +137,39 @@ def invokeBlack(
 
     @patch("black.dump_to_file", dump_to_stderr)
     def test_empty(self) -> None:
-        source = expected = ""
+        source = ""
+        expected = "\n"
         actual = fs(source)
         self.assertFormatEqual(expected, actual)
         black.assert_equivalent(source, actual)
         black.assert_stable(source, actual, DEFAULT_MODE)
 
     def test_empty_ff(self) -> None:
-        expected = ""
+        expected = "\n"
         tmp_file = Path(black.dump_to_file())
+        try:
+            self.assertTrue(ff(tmp_file, write_back=black.WriteBack.YES))
+            with open(tmp_file, "rb") as f:
+                actual = f.read().decode("utf8")
+        finally:
+            os.unlink(tmp_file)
+        self.assertFormatEqual(expected, actual)
+
+    @patch("black.dump_to_file", dump_to_stderr)
+    def test_one_empty_line(self) -> None:
+        source = expected = os.linesep
+        actual = fs(source)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(source, actual)
+        black.assert_stable(source, actual, DEFAULT_MODE)
+
+    def test_one_empty_line_ff(self) -> None:
+        expected = os.linesep
+        tmp_file = Path(black.dump_to_file("\n"))
         try:
             self.assertFalse(ff(tmp_file, write_back=black.WriteBack.YES))
-            with open(tmp_file, encoding="utf8") as f:
-                actual = f.read()
+            with open(tmp_file, "rb") as f:
+                actual = f.read().decode("utf8")
         finally:
             os.unlink(tmp_file)
         self.assertFormatEqual(expected, actual)
@@ -990,11 +1010,17 @@ def err(msg: str, **kwargs: Any) -> None:
     def test_format_file_contents(self) -> None:
         empty = ""
         mode = DEFAULT_MODE
-        with self.assertRaises(black.NothingChanged):
-            black.format_file_contents(empty, mode=mode, fast=False)
+        actual = black.format_file_contents(empty, mode=mode, fast=False)
+        self.assertEqual("\n", actual)
         just_nl = "\n"
         with self.assertRaises(black.NothingChanged):
             black.format_file_contents(just_nl, mode=mode, fast=False)
+        just_whitespace_unix = "\n\t\n    \n\t    \n    \t\n\n"
+        actual = black.format_file_contents(just_whitespace_unix, mode=mode, fast=False)
+        self.assertEqual("\n", actual)
+        just_whitespace_win = "\r\n\t\r\n    \r\n\t    \r\n    \t\r\n\r\n"
+        actual = black.format_file_contents(just_whitespace_win, mode=mode, fast=False)
+        self.assertEqual("\r\n", actual)
         same = "j = [1, 2, 3]\n"
         with self.assertRaises(black.NothingChanged):
             black.format_file_contents(same, mode=mode, fast=False)
@@ -1687,21 +1713,22 @@ 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:
+    @parameterized.expand([("", "\n"), (os.linesep, os.linesep)])
+    def test_reformat_one_with_stdin_empty(self, content, expected) -> None:
         output = io.StringIO()
         mock_io = Mock()
         mock_io.TextIOWrapper.return_value = output
         with patch.object(black, "io", mock_io):
             try:
                 black.format_stdin_to_stdout(
                     fast=True,
-                    content="",
+                    content=content,
                     write_back=black.WriteBack.YES,
                     mode=DEFAULT_MODE,
                 )
             except io.UnsupportedOperation:
                 pass  # StringIO does not support detach
-            assert output.getvalue() == ""
+            assert output.getvalue() == expected
 
     def test_gitignore_exclude(self) -> None:
         path = THIS_DIR / "data" / "include_exclude_tests"

tests/test_format.py

@@ -52,6 +52,7 @@
     "string_prefixes",
     "tricky_unicode_symbols",
     "tupleassign",
+    "whitespace",
 ]
 
 SIMPLE_CASES_PY2 = [