Provide a more useful error when parsing fails during AST safety checks (#2218)

CHANGES.md

@@ -5,6 +5,7 @@
 ### _Black_
 
 - Refactor `src/black/__init__.py` into many files (#2206)
+- Provide a more useful error when parsing fails during AST safety checks (#2218)
 
 ### Documentation
 

src/black/parsing.py

@@ -108,26 +108,34 @@ def lib2to3_unparse(node: Node) -> str:
 
 def parse_ast(src: str) -> Union[ast.AST, ast3.AST, ast27.AST]:
     filename = "<unknown>"
+    error = None
     if sys.version_info >= (3, 8):
         # TODO: support Python 4+ ;)
         for minor_version in range(sys.version_info[1], 4, -1):
             try:
                 return ast.parse(src, filename, feature_version=(3, minor_version))
-            except SyntaxError:
+            except SyntaxError as e:
+                if error is None:
+                    error = str(e)
                 continue
     else:
         for feature_version in (7, 6):
             try:
                 return ast3.parse(src, filename, feature_version=feature_version)
-            except SyntaxError:
+            except SyntaxError as e:
+                if error is None:
+                    error = str(e)
                 continue
     if ast27.__name__ == "ast":
         raise SyntaxError(
             "The requested source code has invalid Python 3 syntax.\n"
             "If you are trying to format Python 2 files please reinstall Black"
             " with the 'python2' extra: `python3 -m pip install black[python2]`."
         )
-    return ast27.parse(src)
+    try:
+        return ast27.parse(src)
+    except SyntaxError as e:
+        raise SyntaxError(error or str(e))
 
 
 def stringify_ast(

tests/test_black.py

@@ -496,6 +496,29 @@ def test_python2_should_fail_without_optional_install(self) -> None:
         )
         self.assertIn(msg, actual)
 
+    @pytest.mark.python2
+    def test_safety_check_syntax_error(self) -> None:
+        source = "e = 'test'\nx = f'{'"
+        tmp_file = Path(black.dump_to_file(source))
+        try:
+            runner = BlackRunner()
+            result = runner.invoke(black.main, [str(tmp_file)])
+            self.assertEqual(result.exit_code, 123)
+        finally:
+            os.unlink(tmp_file)
+        actual = (
+            runner.stderr_bytes.decode()
+            .replace("\n", "")
+            .replace("\\n", "")
+            .replace("\\r", "")
+            .replace("\r", "")
+        )
+        msg = (
+            "cannot use --safe with this file; failed to parse source file."
+            "  AST error message: f-string: expecting '}' (<unknown>, line 2)"
+        )
+        self.assertIn(msg, actual)
+
     @pytest.mark.python2
     @patch("black.dump_to_file", dump_to_stderr)
     def test_python2_print_function(self) -> None: