Format ourselves in preview mode

pyproject.toml

@@ -17,7 +17,10 @@ extend-exclude = '''
   | profiling
 )/
 '''
-
+# We use preview style for formatting Black itself. If you
+# want stable formatting across releases, you should keep
+# this off.
+preview = true
 
 # Build system information below.
 # NOTE: You don't need this in your own Black configuration.

src/black/__init__.py

@@ -1343,10 +1343,10 @@ def assert_equivalent(src: str, dst: str) -> None:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(
-            f"cannot use --safe with this file; failed to parse source file AST: "
+            "cannot use --safe with this file; failed to parse source file AST: "
             f"{exc}\n"
-            f"This could be caused by running Black with an older Python version "
-            f"that does not support new syntax used in your source file."
+            "This could be caused by running Black with an older Python version "
+            "that does not support new syntax used in your source file."
         ) from exc
 
     try:
@@ -1365,7 +1365,7 @@ def assert_equivalent(src: str, dst: str) -> None:
         log = dump_to_file(diff(src_ast_str, dst_ast_str, "src", "dst"))
         raise AssertionError(
             "INTERNAL ERROR: Black produced code that is not equivalent to the"
-            f" source.  Please report a bug on "
+            " source.  Please report a bug on "
             f"https://github.com/psf/black/issues.  This diff might be helpful: {log}"
         ) from None
 

tests/test_ipynb.py

@@ -415,7 +415,7 @@ def test_ipynb_diff_with_change() -> None:
             f"--config={EMPTY_CONFIG}",
         ],
     )
-    expected = "@@ -1,3 +1,3 @@\n %%time\n \n-print('foo')\n" '+print("foo")\n'
+    expected = "@@ -1,3 +1,3 @@\n %%time\n \n-print('foo')\n+print(\"foo\")\n"
     assert expected in result.output
 
 
@@ -494,7 +494,7 @@ def test_ipynb_flag(tmp_path: pathlib.Path) -> None:
             f"--config={EMPTY_CONFIG}",
         ],
     )
-    expected = "@@ -1,3 +1,3 @@\n %%time\n \n-print('foo')\n" '+print("foo")\n'
+    expected = "@@ -1,3 +1,3 @@\n %%time\n \n-print('foo')\n+print(\"foo\")\n"
     assert expected in result.output