Turn --fast-parser on by default

mypy/fastparse.py

@@ -57,8 +57,10 @@ def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
 
     The pyversion (major, minor) argument determines the Python syntax variant.
     """
+    raise_on_error = False
     if errors is None:
         errors = Errors()
+        raise_on_error = True
     errors.set_file('<input>' if fnam is None else fnam)
     is_stub_file = bool(fnam) and fnam.endswith('.pyi')
     try:
@@ -72,14 +74,14 @@ def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
                             ).visit(ast)
         tree.path = fnam
         tree.is_stub = is_stub_file
-        return tree
     except SyntaxError as e:
-        if errors:
-            errors.report(e.lineno, e.offset, e.msg)
-        else:
-            raise
+        errors.report(e.lineno, e.offset, e.msg)
+        tree = MypyFile([], [], False, set())
+
+    if raise_on_error and errors.is_errors():
+        errors.raise_error()
 
-    return MypyFile([], [], False, set())
+    return tree
 
 
 def parse_type_comment(type_comment: str, line: int, errors: Errors) -> Optional[Type]:

mypy/fastparse2.py

@@ -74,8 +74,10 @@ def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
 
     The pyversion (major, minor) argument determines the Python syntax variant.
     """
+    raise_on_error = False
     if errors is None:
         errors = Errors()
+        raise_on_error = True
     errors.set_file('<input>' if fnam is None else fnam)
     is_stub_file = bool(fnam) and fnam.endswith('.pyi')
     try:
@@ -89,14 +91,14 @@ def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
         assert isinstance(tree, MypyFile)
         tree.path = fnam
         tree.is_stub = is_stub_file
-        return tree
     except SyntaxError as e:
-        if errors:
-            errors.report(e.lineno, e.offset, e.msg)
-        else:
-            raise
+        errors.report(e.lineno, e.offset, e.msg)
+        tree = MypyFile([], [], False, set())
+
+    if raise_on_error and errors.is_errors():
+        errors.raise_error()
 
-    return MypyFile([], [], False, set())
+    return tree
 
 
 def with_line(f: Callable[['ASTConverter', T], U]) -> Callable[['ASTConverter', T], U]:

mypy/main.py

@@ -225,8 +225,8 @@ def add_invertible_flag(flag: str,
     add_invertible_flag('--show-error-context', default=True,
                         dest='hide_error_context',
                         help='Precede errors with "note:" messages explaining context')
-    add_invertible_flag('--fast-parser', default=False,
-                        help="enable fast parser (recommended)")
+    add_invertible_flag('--no-fast-parser', default=True, dest='fast_parser',
+                        help="disable the fast parser (not recommended)")
     parser.add_argument('-i', '--incremental', action='store_true',
                         help="enable experimental module cache")
     parser.add_argument('--cache-dir', action='store', metavar='DIR',

mypy/options.py

@@ -114,7 +114,7 @@ def __init__(self) -> None:
         self.use_builtins_fixtures = False
 
         # -- experimental options --
-        self.fast_parser = False
+        self.fast_parser = True
         self.incremental = False
         self.cache_dir = defaults.CACHE_DIR
         self.debug_cache = False