Fix blackd error handling: split SourceASTParseError from ASTSafetyError

CHANGES.md

@@ -40,10 +40,17 @@
 
 <!-- Changes to Black's terminal output and error messages -->
 
+- Add `SourceASTParseError` to distinguish source parse failures from internal safety
+  errors, improving error reporting when Black's lenient parser accepts input that
+  `ast.parse()` rejects (#5080)
+
 ### _Blackd_
 
 <!-- Changes to blackd -->
 
+- Return HTTP 400 (Bad Request) for source parse failures instead of HTTP 500, keeping
+  HTTP 500 only for genuine internal safety errors (#5080)
+
 ### Integrations
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->

src/black/__init__.py

@@ -66,6 +66,7 @@
 from black.parsing import (  # noqa F401
     ASTSafetyError,
     InvalidInput,
+    SourceASTParseError,
     lib2to3_parse,
     parse_ast,
     stringify_ast,
@@ -1086,6 +1087,8 @@ def check_stability_and_equivalence(
     """
     try:
         assert_equivalent(src_contents, dst_contents)
+    except SourceASTParseError:
+        raise
     except ASTSafetyError:
         if _target_versions_exceed_runtime(mode.target_versions):
             raise ASTSafetyError(
@@ -1632,7 +1635,7 @@ def assert_equivalent(src: str, dst: str) -> None:
     try:
         src_ast = parse_ast(src)
     except Exception as exc:
-        raise ASTSafetyError(
+        raise SourceASTParseError(
             "cannot use --safe with this file; failed to parse source file AST: "
             f"{exc}\n"
             "This could be caused by running Black with an older Python version "

src/black/parsing.py

@@ -106,6 +106,15 @@ class ASTSafetyError(Exception):
     """Raised when Black's generated code is not equivalent to the old AST."""
 
 
+class SourceASTParseError(Exception):
+    """Raised when the source file cannot be parsed by ast.parse().
+
+    This is not a bug in Black — Black's lib2to3-based parser is more lenient
+    than Python's ast.parse(), so it may accept code that ast.parse() rejects.
+    In blackd, this should be reported as a 400 Bad Request.
+    """
+
+
 def _parse_single_version(
     src: str, version: tuple[int, int], *, type_comments: bool
 ) -> ast.AST:

src/blackd/__init__.py

@@ -206,6 +206,8 @@ async def handle(
         return web.Response(status=204, headers=headers)
     except black.InvalidInput as e:
         return web.Response(status=400, headers=headers, text=str(e))
+    except black.SourceASTParseError as e:
+        return web.Response(status=400, headers=headers, text=str(e))
     except web.HTTPException:
         raise
     except Exception as e:

tests/test_black.py

@@ -38,7 +38,7 @@
 from black.debug import DebugVisitor
 from black.mode import Mode, Preview
 from black.output import color_diff, diff
-from black.parsing import ASTSafetyError
+from black.parsing import ASTSafetyError, SourceASTParseError
 from black.report import Report
 from black.strings import lines_with_leading_tabs_expanded
 
@@ -3212,7 +3212,7 @@ def test_assert_equivalent_fstring(self) -> None:
         )
 
     def test_equivalency_ast_parse_failure_includes_error(self) -> None:
-        with pytest.raises(ASTSafetyError) as err:
+        with pytest.raises(SourceASTParseError) as err:
             black.assert_equivalent("a«»a  = 1", "a«»a  = 1")
 
         err.match("--safe")