Move whitespace-only behavior to preview style

The new criteria to reformat empty and whitespace-only files should go
into the preview style

Signed-off-by: Antonio Ossa Guerra <aaossa@uc.cl>

CHANGES.md

@@ -31,8 +31,6 @@
 - Parsing support has been added for walruses inside generator expression that are
   passed as function args (for example,
   `any(match := my_re.match(text) for text in texts)`) (#3327).
-- Reformat empty and whitespace-only files as either an empty file (if no newline is
-  present) or as a single newline character (if a newline is present) (#3348)
 
 ### Performance
 
@@ -71,6 +69,8 @@
 
 - Fix a crash when formatting some dicts with parenthesis-wrapped long string keys
   (#3262)
+- Reformat empty and whitespace-only files as either an empty file (if no newline is
+  present) or as a single newline character (if a newline is present) (#3348)
 
 ### Configuration
 

src/black/__init__.py

@@ -914,6 +914,9 @@ 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 mode.preview and not src_contents.strip():
+        raise NothingChanged
+
     if mode.is_ipynb:
         dst_contents = format_ipynb_string(src_contents, fast=fast, mode=mode)
     else:
@@ -1008,7 +1011,7 @@ 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:
+    if mode.preview and not src_contents:
         raise NothingChanged
 
     trailing_newline = src_contents[-1] == "\n"
@@ -1103,7 +1106,7 @@ def _format_str_once(src_contents: str, *, mode: Mode) -> str:
     dst_contents = []
     for block in dst_blocks:
         dst_contents.extend(block.all_lines())
-    if not dst_contents:
+    if mode.preview and not dst_contents:
         normalized_content, _, newline = decode_bytes(src_contents.encode("utf-8"))
         if "\n" in normalized_content:
             return newline

tests/test_black.py

@@ -156,14 +156,16 @@ def test_empty_ff(self) -> None:
 
     @patch("black.dump_to_file", dump_to_stderr)
     def test_one_empty_line(self) -> None:
+        mode = black.Mode(preview=True)
         for nl in ["\n", "\r\n"]:
             source = expected = nl
-            actual = fs(source)
+            actual = fs(source, mode=mode)
             self.assertFormatEqual(expected, actual)
             black.assert_equivalent(source, actual)
-            black.assert_stable(source, actual, DEFAULT_MODE)
+            black.assert_stable(source, actual, mode)
 
     def test_one_empty_line_ff(self) -> None:
+        mode = black.Mode(preview=True)
         for nl in ["\n", "\r\n"]:
             expected = nl
             tmp_file = Path(black.dump_to_file(nl))
@@ -174,7 +176,9 @@ def test_one_empty_line_ff(self) -> None:
                 with open(tmp_file, "wb") as f:
                     f.write(nl.encode("utf-8"))
             try:
-                self.assertFalse(ff(tmp_file, write_back=black.WriteBack.YES))
+                self.assertFalse(
+                    ff(tmp_file, mode=mode, write_back=black.WriteBack.YES)
+                )
                 with open(tmp_file, "rb") as f:
                     actual = f.read().decode("utf8")
             finally:
@@ -957,22 +961,13 @@ def err(msg: str, **kwargs: Any) -> None:
         )
 
     def test_format_file_contents(self) -> None:
-        empty = ""
         mode = DEFAULT_MODE
+        empty = ""
         with self.assertRaises(black.NothingChanged):
             black.format_file_contents(empty, mode=mode, fast=False)
         just_nl = "\n"
         with self.assertRaises(black.NothingChanged):
             black.format_file_contents(just_nl, mode=mode, fast=False)
-        just_crlf = "\r\n"
-        with self.assertRaises(black.NothingChanged):
-            black.format_file_contents(just_crlf, mode=mode, fast=False)
-        just_whitespace_nl = "\n\t\n \n\t \n \t\n\n"
-        actual = black.format_file_contents(just_whitespace_nl, mode=mode, fast=False)
-        self.assertEqual("\n", actual)
-        just_whitespace_crlf = "\r\n\t\r\n \r\n\t \r\n \t\r\n\r\n"
-        actual = black.format_file_contents(just_whitespace_crlf, 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)
@@ -985,6 +980,17 @@ def test_format_file_contents(self) -> None:
             black.format_file_contents(invalid, mode=mode, fast=False)
         self.assertEqual(str(e.exception), "Cannot parse: 1:7: return if you can")
 
+        mode = black.Mode(preview=True)
+        just_crlf = "\r\n"
+        with self.assertRaises(black.NothingChanged):
+            black.format_file_contents(just_crlf, mode=mode, fast=False)
+        just_whitespace_nl = "\n\t\n \n\t \n \t\n\n"
+        actual = black.format_file_contents(just_whitespace_nl, mode=mode, fast=False)
+        self.assertEqual("\n", actual)
+        just_whitespace_crlf = "\r\n\t\r\n \r\n\t \r\n \t\r\n\r\n"
+        actual = black.format_file_contents(just_whitespace_crlf, mode=mode, fast=False)
+        self.assertEqual("\r\n", actual)
+
     def test_endmarker(self) -> None:
         n = black.lib2to3_parse("\n")
         self.assertEqual(n.type, black.syms.file_input)
@@ -1300,6 +1306,7 @@ def get_output(*args: Any, **kwargs: Any) -> io.TextIOWrapper:
 
             return get_output
 
+        mode = black.Mode(preview=True)
         for content, expected in cases:
             output = io.StringIO()
             io_TextIOWrapper = io.TextIOWrapper
@@ -1310,12 +1317,27 @@ def get_output(*args: Any, **kwargs: Any) -> io.TextIOWrapper:
                         fast=True,
                         content=content,
                         write_back=black.WriteBack.YES,
-                        mode=DEFAULT_MODE,
+                        mode=mode,
                     )
                 except io.UnsupportedOperation:
                     pass  # StringIO does not support detach
                 assert output.getvalue() == expected
 
+        # An empty string is the only test case for `preview=False`
+        output = io.StringIO()
+        io_TextIOWrapper = io.TextIOWrapper
+        with patch("io.TextIOWrapper", _new_wrapper(output, io_TextIOWrapper)):
+            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_invalid_cli_regex(self) -> None:
         for option in ["--include", "--exclude", "--extend-exclude", "--force-exclude"]:
             self.invokeBlack(["-", option, "**()(!!*)"], exit_code=2)