diff --git a/docs/source/config_file.rst b/docs/source/config_file.rst
index 504d62b05347..821905345add 100644
--- a/docs/source/config_file.rst
+++ b/docs/source/config_file.rst
@@ -172,6 +172,8 @@ overridden by the pattern sections matching the module name.
 - ``strict_boolean`` (Boolean, default False) makes using non-boolean
   expressions in conditions an error.
 
+- ``no_implicit_optional`` (Boolean, default false) changes the treatment of
+  arguments with a default value of None by not implicitly making their type Optional
 
 Example
 *******
diff --git a/mypy/fastparse.py b/mypy/fastparse.py
index 19619cf58c6b..c5249a8c9588 100644
--- a/mypy/fastparse.py
+++ b/mypy/fastparse.py
@@ -28,6 +28,7 @@
 from mypy import experiments
 from mypy import messages
 from mypy.errors import Errors
+from mypy.options import Options
 
 try:
     from typed_ast import ast3
@@ -58,14 +59,12 @@
 
 
 def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
-          pyversion: Tuple[int, int] = defaults.PYTHON3_VERSION,
-          custom_typing_module: str = None) -> MypyFile:
+          options: Options = Options()) -> MypyFile:
+
     """Parse a source file, without doing any semantic analysis.
 
     Return the parse tree. If errors is not provided, raise ParseError
     on failure. Otherwise, use the errors object to report parse errors.
-
-    The pyversion (major, minor) argument determines the Python syntax variant.
     """
     raise_on_error = False
     if errors is None:
@@ -74,14 +73,16 @@ def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
     errors.set_file('<input>' if fnam is None else fnam, None)
     is_stub_file = bool(fnam) and fnam.endswith('.pyi')
     try:
-        assert pyversion[0] >= 3 or is_stub_file
-        feature_version = pyversion[1] if not is_stub_file else defaults.PYTHON3_VERSION[1]
+        if is_stub_file:
+            feature_version = defaults.PYTHON3_VERSION[1]
+        else:
+            assert options.python_version[0] >= 3
+            feature_version = options.python_version[1]
         ast = ast3.parse(source, fnam, 'exec', feature_version=feature_version)
 
-        tree = ASTConverter(pyversion=pyversion,
+        tree = ASTConverter(options=options,
                             is_stub=is_stub_file,
                             errors=errors,
-                            custom_typing_module=custom_typing_module,
                             ).visit(ast)
         tree.path = fnam
         tree.is_stub = is_stub_file
@@ -136,17 +137,15 @@ def is_no_type_check_decorator(expr: ast3.expr) -> bool:
 
 class ASTConverter(ast3.NodeTransformer):  # type: ignore  # typeshed PR #931
     def __init__(self,
-                 pyversion: Tuple[int, int],
+                 options: Options,
                  is_stub: bool,
-                 errors: Errors,
-                 custom_typing_module: str = None) -> None:
+                 errors: Errors) -> None:
         self.class_nesting = 0
         self.imports = []  # type: List[ImportBase]
 
-        self.pyversion = pyversion
+        self.options = options
         self.is_stub = is_stub
         self.errors = errors
-        self.custom_typing_module = custom_typing_module
 
     def fail(self, msg: str, line: int, column: int) -> None:
         self.errors.report(line, column, msg)
@@ -260,9 +259,9 @@ def translate_module_id(self, id: str) -> str:
 
         For example, translate '__builtin__' in Python 2 to 'builtins'.
         """
-        if id == self.custom_typing_module:
+        if id == self.options.custom_typing_module:
             return 'typing'
-        elif id == '__builtin__' and self.pyversion[0] == 2:
+        elif id == '__builtin__' and self.options.python_version[0] == 2:
             # HACK: __builtin__ in Python 2 is aliases to builtins. However, the implementation
             #   is named __builtin__.py (there is another layer of translation elsewhere).
             return 'builtins'
@@ -388,7 +387,7 @@ def do_func_def(self, n: Union[ast3.FunctionDef, ast3.AsyncFunctionDef],
             return func_def
 
     def set_type_optional(self, type: Type, initializer: Expression) -> None:
-        if not experiments.STRICT_OPTIONAL:
+        if self.options.no_implicit_optional or not experiments.STRICT_OPTIONAL:
             return
         # Indicate that type should be wrapped in an Optional if arg is initialized to None.
         optional = isinstance(initializer, NameExpr) and initializer.name == 'None'
@@ -855,16 +854,13 @@ def visit_Num(self, n: ast3.Num) -> Union[IntExpr, FloatExpr, ComplexExpr]:
     # Str(string s)
     @with_line
     def visit_Str(self, n: ast3.Str) -> Union[UnicodeExpr, StrExpr]:
-        if self.pyversion[0] >= 3 or self.is_stub:
-            # Hack: assume all string literals in Python 2 stubs are normal
-            # strs (i.e. not unicode).  All stubs are parsed with the Python 3
-            # parser, which causes unprefixed string literals to be interpreted
-            # as unicode instead of bytes.  This hack is generally okay,
-            # because mypy considers str literals to be compatible with
-            # unicode.
-            return StrExpr(n.s)
-        else:
-            return UnicodeExpr(n.s)
+        # Hack: assume all string literals in Python 2 stubs are normal
+        # strs (i.e. not unicode).  All stubs are parsed with the Python 3
+        # parser, which causes unprefixed string literals to be interpreted
+        # as unicode instead of bytes.  This hack is generally okay,
+        # because mypy considers str literals to be compatible with
+        # unicode.
+        return StrExpr(n.s)
 
     # Only available with typed_ast >= 0.6.2
     if hasattr(ast3, 'JoinedStr'):
@@ -894,11 +890,7 @@ def visit_Bytes(self, n: ast3.Bytes) -> Union[BytesExpr, StrExpr]:
         # The following line is a bit hacky, but is the best way to maintain
         # compatibility with how mypy currently parses the contents of bytes literals.
         contents = str(n.s)[2:-1]
-
-        if self.pyversion[0] >= 3:
-            return BytesExpr(contents)
-        else:
-            return StrExpr(contents)
+        return BytesExpr(contents)
 
     # NameConstant(singleton value)
     def visit_NameConstant(self, n: ast3.NameConstant) -> NameExpr:
diff --git a/mypy/fastparse2.py b/mypy/fastparse2.py
index aca04187e57c..ef6c6d00c4fa 100644
--- a/mypy/fastparse2.py
+++ b/mypy/fastparse2.py
@@ -38,11 +38,11 @@
 from mypy.types import (
     Type, CallableType, AnyType, UnboundType, EllipsisType
 )
-from mypy import defaults
 from mypy import experiments
 from mypy import messages
 from mypy.errors import Errors
 from mypy.fastparse import TypeConverter, parse_type_comment
+from mypy.options import Options
 
 try:
     from typed_ast import ast27
@@ -74,14 +74,11 @@
 
 
 def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
-          pyversion: Tuple[int, int] = defaults.PYTHON3_VERSION,
-          custom_typing_module: str = None) -> MypyFile:
+          options: Options = Options()) -> MypyFile:
     """Parse a source file, without doing any semantic analysis.
 
     Return the parse tree. If errors is not provided, raise ParseError
     on failure. Otherwise, use the errors object to report parse errors.
-
-    The pyversion (major, minor) argument determines the Python syntax variant.
     """
     raise_on_error = False
     if errors is None:
@@ -90,12 +87,11 @@ def parse(source: Union[str, bytes], fnam: str = None, errors: Errors = None,
     errors.set_file('<input>' if fnam is None else fnam, None)
     is_stub_file = bool(fnam) and fnam.endswith('.pyi')
     try:
-        assert pyversion[0] < 3 and not is_stub_file
+        assert options.python_version[0] < 3 and not is_stub_file
         ast = ast27.parse(source, fnam, 'exec')
-        tree = ASTConverter(pyversion=pyversion,
+        tree = ASTConverter(options=options,
                             is_stub=is_stub_file,
                             errors=errors,
-                            custom_typing_module=custom_typing_module,
                             ).visit(ast)
         assert isinstance(tree, MypyFile)
         tree.path = fnam
@@ -137,17 +133,15 @@ def is_no_type_check_decorator(expr: ast27.expr) -> bool:
 
 class ASTConverter(ast27.NodeTransformer):
     def __init__(self,
-                 pyversion: Tuple[int, int],
+                 options: Options,
                  is_stub: bool,
-                 errors: Errors,
-                 custom_typing_module: str = None) -> None:
+                 errors: Errors) -> None:
         self.class_nesting = 0
         self.imports = []  # type: List[ImportBase]
 
-        self.pyversion = pyversion
+        self.options = options
         self.is_stub = is_stub
         self.errors = errors
-        self.custom_typing_module = custom_typing_module
 
     def fail(self, msg: str, line: int, column: int) -> None:
         self.errors.report(line, column, msg)
@@ -262,9 +256,9 @@ def translate_module_id(self, id: str) -> str:
 
         For example, translate '__builtin__' in Python 2 to 'builtins'.
         """
-        if id == self.custom_typing_module:
+        if id == self.options.custom_typing_module:
             return 'typing'
-        elif id == '__builtin__' and self.pyversion[0] == 2:
+        elif id == '__builtin__':
             # HACK: __builtin__ in Python 2 is aliases to builtins. However, the implementation
             #   is named __builtin__.py (there is another layer of translation elsewhere).
             return 'builtins'
@@ -370,7 +364,7 @@ def visit_FunctionDef(self, n: ast27.FunctionDef) -> Statement:
             return func_def
 
     def set_type_optional(self, type: Type, initializer: Expression) -> None:
-        if not experiments.STRICT_OPTIONAL:
+        if self.options.no_implicit_optional or not experiments.STRICT_OPTIONAL:
             return
         # Indicate that type should be wrapped in an Optional if arg is initialized to None.
         optional = isinstance(initializer, NameExpr) and initializer.name == 'None'
@@ -872,16 +866,9 @@ def visit_Str(self, s: ast27.Str) -> Expression:
             # The following line is a bit hacky, but is the best way to maintain
             # compatibility with how mypy currently parses the contents of bytes literals.
             contents = str(n)[2:-1]
-
-            if self.pyversion[0] >= 3:
-                return BytesExpr(contents)
-            else:
-                return StrExpr(contents)
+            return StrExpr(contents)
         else:
-            if self.pyversion[0] >= 3 or self.is_stub:
-                return StrExpr(s.s)
-            else:
-                return UnicodeExpr(s.s)
+            return UnicodeExpr(s.s)
 
     # Ellipsis
     def visit_Ellipsis(self, n: ast27.Ellipsis) -> EllipsisExpr:
diff --git a/mypy/main.py b/mypy/main.py
index b90d82a30979..79690b08d100 100644
--- a/mypy/main.py
+++ b/mypy/main.py
@@ -236,6 +236,8 @@ def add_invertible_flag(flag: str,
     add_invertible_flag('--show-error-context', default=False,
                         dest='show_error_context',
                         help='Precede errors with "note:" messages explaining context')
+    add_invertible_flag('--no-implicit-optional', default=False, strict_flag=True,
+                        help="don't assume arguments with default values of None are Optional")
     parser.add_argument('-i', '--incremental', action='store_true',
                         help="enable module cache")
     parser.add_argument('--quick-and-dirty', action='store_true',
diff --git a/mypy/options.py b/mypy/options.py
index 8c8764200800..5e841bee0c6e 100644
--- a/mypy/options.py
+++ b/mypy/options.py
@@ -29,6 +29,7 @@ class Options:
         "warn_return_any",
         "ignore_errors",
         "strict_boolean",
+        "no_implicit_optional",
     }
 
     OPTIONS_AFFECTING_CACHE = PER_MODULE_OPTIONS | {"strict_optional", "quick_and_dirty"}
@@ -92,6 +93,9 @@ def __init__(self) -> None:
         # Alternate way to show/hide strict-None-checking related errors
         self.show_none_errors = True
 
+        # Don't assume arguments with default values of None are Optional
+        self.no_implicit_optional = False
+
         # Use script name instead of __main__
         self.scripts_are_modules = False
 
diff --git a/mypy/parse.py b/mypy/parse.py
index ddcd226b9ff2..72e5ab468fed 100644
--- a/mypy/parse.py
+++ b/mypy/parse.py
@@ -22,12 +22,10 @@ def parse(source: Union[str, bytes],
         return mypy.fastparse.parse(source,
                                     fnam=fnam,
                                     errors=errors,
-                                    pyversion=options.python_version,
-                                    custom_typing_module=options.custom_typing_module)
+                                    options=options)
     else:
         import mypy.fastparse2
         return mypy.fastparse2.parse(source,
                                      fnam=fnam,
                                      errors=errors,
-                                     pyversion=options.python_version,
-                                     custom_typing_module=options.custom_typing_module)
+                                     options=options)
diff --git a/test-data/unit/check-optional.test b/test-data/unit/check-optional.test
index e4fb2a16025c..b0d8769384c0 100644
--- a/test-data/unit/check-optional.test
+++ b/test-data/unit/check-optional.test
@@ -125,11 +125,10 @@ def f(x: int = None) -> None:
 f(None)
 [out]
 
-[case testInferOptionalFromDefaultNoneWithFastParser]
-
-def f(x: int = None) -> None:
-  x + 1  # E: Unsupported left operand type for + (some union)
-f(None)
+[case testNoInferOptionalFromDefaultNone]
+# flags: --no-implicit-optional
+def f(x: int = None) -> None:  # E: Incompatible types in assignment (expression has type None, variable has type "int")
+  pass
 [out]
 
 [case testInferOptionalFromDefaultNoneComment]
@@ -139,12 +138,11 @@ def f(x=None):
 f(None)
 [out]
 
-[case testInferOptionalFromDefaultNoneCommentWithFastParser]
-
-def f(x=None):
+[case testNoInferOptionalFromDefaultNoneComment]
+# flags: --no-implicit-optional
+def f(x=None):  # E: Incompatible types in assignment (expression has type None, variable has type "int")
   # type: (int) -> None
-  x + 1  # E: Unsupported left operand type for + (some union)
-f(None)
+  pass
 [out]
 
 [case testInferOptionalType]