feat(#3195): add preview option support for blackd

CHANGES.md

@@ -29,6 +29,8 @@
 
 <!-- Changes to blackd -->
 
+- `blackd` now supports preview style via `X-PREVIEW` header (#3195)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

docs/usage_and_configuration/black_as_a_server.md

@@ -56,6 +56,9 @@ The headers controlling how source code is formatted are:
 - `X-Skip-Magic-Trailing-Comma`: corresponds to the `--skip-magic-trailing-comma`
   command line flag. If present and its value is not the empty string, trailing commas
   will not be used as a reason to split lines.
+- `X-Preview`: corresponds to the `--preview`command line flag. If present and its value
+  is not the empty string, experimental and potentially disruptive style changes will be
+  used.
 - `X-Fast-Or-Safe`: if set to `fast`, `blackd` will act as _Black_ does when passed the
   `--fast` command line flag.
 - `X-Python-Variant`: if set to `pyi`, `blackd` will act as _Black_ does when passed the

src/blackd/__init__.py

@@ -32,6 +32,7 @@
 PYTHON_VARIANT_HEADER = "X-Python-Variant"
 SKIP_STRING_NORMALIZATION_HEADER = "X-Skip-String-Normalization"
 SKIP_MAGIC_TRAILING_COMMA = "X-Skip-Magic-Trailing-Comma"
+PREVIEW = "X-PREVIEW"
 FAST_OR_SAFE_HEADER = "X-Fast-Or-Safe"
 DIFF_HEADER = "X-Diff"
 
@@ -41,6 +42,7 @@
     PYTHON_VARIANT_HEADER,
     SKIP_STRING_NORMALIZATION_HEADER,
     SKIP_MAGIC_TRAILING_COMMA,
+    PREVIEW,
     FAST_OR_SAFE_HEADER,
     DIFF_HEADER,
 ]
@@ -109,6 +111,7 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
         skip_magic_trailing_comma = bool(
             request.headers.get(SKIP_MAGIC_TRAILING_COMMA, False)
         )
+        preview = bool(request.headers.get(PREVIEW, False))
         fast = False
         if request.headers.get(FAST_OR_SAFE_HEADER, "safe") == "fast":
             fast = True
@@ -118,6 +121,7 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
             line_length=line_length,
             string_normalization=not skip_string_normalization,
             magic_trailing_comma=not skip_magic_trailing_comma,
+            preview=preview,
         )
         req_bytes = await request.content.read()
         charset = request.charset if request.charset is not None else "utf8"

tests/test_blackd.py

@@ -167,6 +167,13 @@ async def test_blackd_invalid_line_length(self) -> None:
         )
         self.assertEqual(response.status, 400)
 
+    @unittest_run_loop
+    async def test_blackd_preview(self) -> None:
+        response = await self.client.post(
+            "/", data=b'print("hello")\n', headers={blackd.PREVIEW: "true"}
+        )
+        self.assertEqual(response.status, 204)
+
     @unittest_run_loop
     async def test_blackd_response_black_version_header(self) -> None:
         response = await self.client.post("/")