Support passing --preview through to Black (#273)

CHANGELOG.rst

@@ -16,6 +16,8 @@ Changelog
 
   Thanks to Julianus Pfeuffer for the report in `Issue #218 <https://github.com/adamchainz/blacken-docs/issues/218>`__.
 
+* Support passing the ``--preview`` option through to Black, to select the future style.
+
 * Remove ``language_version`` from ``.pre-commit-hooks.yaml``.
   This change allows ``default_language_version`` in ``.pre-commit-config.yaml` to take precedence.
 

README.rst

@@ -94,9 +94,25 @@ __ https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core
 
 blacken-docs currently passes the following options through to Black:
 
-* ``-l`` / ``--line-length``
-* ``-t`` / ``--target-version``
-* ``-S`` / ``--skip-string-normalization``
+* |-l / --line-length|__
+
+  .. |-l / --line-length| replace:: ``-l`` / ``--line-length``
+  __ https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#l-line-length
+
+* |--preview|__
+
+  .. |--preview| replace:: ``--preview``
+  __ https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#preview
+
+* |-S / --skip-string-normalization|__
+
+  .. |-S / --skip-string-normalization| replace:: ``-S`` / ``--skip-string-normalization``
+  __ https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#s-skip-string-normalization
+
+* |-t / --target-version|__
+
+  .. |-t / --target-version| replace:: ``-t`` / ``--target-version``
+  __ https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#t-target-version
 
 It also has the below extra options:
 

src/blacken_docs/__init__.py

@@ -261,6 +261,12 @@ def main(argv: Sequence[str] | None = None) -> int:
         type=int,
         default=DEFAULT_LINE_LENGTH,
     )
+    parser.add_argument("--preview", action="store_true")
+    parser.add_argument(
+        "-S",
+        "--skip-string-normalization",
+        action="store_true",
+    )
     parser.add_argument(
         "-t",
         "--target-version",
@@ -270,11 +276,6 @@ def main(argv: Sequence[str] | None = None) -> int:
         help=f"choices: {[v.name.lower() for v in TargetVersion]}",
         dest="target_versions",
     )
-    parser.add_argument(
-        "-S",
-        "--skip-string-normalization",
-        action="store_true",
-    )
     parser.add_argument("-E", "--skip-errors", action="store_true")
     parser.add_argument(
         "--rst-literal-blocks",
@@ -284,9 +285,10 @@ def main(argv: Sequence[str] | None = None) -> int:
     args = parser.parse_args(argv)
 
     black_mode = black.FileMode(
-        target_versions=set(args.target_versions),
         line_length=args.line_length,
+        preview=args.preview,
         string_normalization=not args.skip_string_normalization,
+        target_versions=set(args.target_versions),
     )
 
     retv = 0

tests/test_blacken_docs.py

@@ -470,9 +470,10 @@ def test_integration_line_length(tmp_path):
     )
 
     result = blacken_docs.main((str(f), "--line-length=80"))
-
     assert result == 0
-    assert blacken_docs.main((str(f), "--line-length=50"))
+
+    result2 = blacken_docs.main((str(f), "--line-length=50"))
+    assert result2 == 1
     assert f.read_text() == (
         "```python\n"
         "foo(\n"
@@ -483,6 +484,30 @@ def test_integration_line_length(tmp_path):
     )
 
 
+def test_integration_preview(tmp_path):
+    f = tmp_path / "f.md"
+    f.write_text(
+        dedent(
+            """\
+            ```python
+            x = 'a' 'b'
+            ```
+            """
+        )
+    )
+
+    result = blacken_docs.main((str(f), "--preview"))
+
+    assert result == 1
+    assert f.read_text() == dedent(
+        """\
+        ```python
+        x = "ab"
+        ```
+        """
+    )
+
+
 def test_integration_py36(tmp_path):
     f = tmp_path / "f.md"
     f.write_text(