diff --git a/.travis.yml b/.travis.yml
index 4bef66638..1dd1e896e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,7 +8,7 @@ matrix:
     - python: "3.6"
       env: TEST_CMD="./tests/mypy_selftest.py"
     - python: "3.5"
-      env: TEST_CMD="./tests/mypy_test.py"
+      env: TEST_CMD="./tests/mypy_test.py --no-implicit-optional"
     - python: "2.7"
       env: TEST_CMD="./tests/pytype_test.py --num-parallel=4"
 
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index fd3678f5c..5bf687557 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -135,7 +135,8 @@ rule is that they should be as concise as possible.  Specifically:
   names, or methods and fields within a single class;
 * use a single blank line between top-level class definitions, or none
   if the classes are very small;
-* do not use docstrings.
+* do not use docstrings;
+* for arguments with a type and a default, use spaces around the `=`.
 
 Imports in stubs are considered private (not part of the exported API)
 unless:
@@ -144,6 +145,14 @@ unless:
 * they use the form ``from library import *`` which means all names
   from that library are exported.
 
+For arguments with type and a default value of `None`, PEP 484
+prescribes that the type automatically becomes `Optional`.  However we
+prefer explicit over implicit in this case, and require the explicit
+`Optional[]` around the type.  The mypy tests enforce this (through
+the use of --no-implicit-optional) and the error looks like
+`Incompatible types in assignment (expression has type None, variable
+has type "Blah") `.
+
 Stub files support forward references natively.  In other words, the
 order of class declarations and type aliases does not matter in
 a stub file.  You can also use the name of the class within its own
diff --git a/stdlib/3.4/asyncio/locks.pyi b/stdlib/3.4/asyncio/locks.pyi
index 685402511..16cb02b30 100644
--- a/stdlib/3.4/asyncio/locks.pyi
+++ b/stdlib/3.4/asyncio/locks.pyi
@@ -37,7 +37,7 @@ class Event:
     def wait(self) -> Generator[Any, None, bool]: ...
 
 class Condition(_ContextManagerMixin):
-    def __init__(self, lock: Lock = None, *, loop: Optional[AbstractEventLoop] = ...) -> None: ...
+    def __init__(self, lock: Optional[Lock] = None, *, loop: Optional[AbstractEventLoop] = ...) -> None: ...
     def locked(self) -> bool: ...
     @coroutine
     def acquire(self) -> Generator[Any, None, bool]: ...
diff --git a/stdlib/3/builtins.pyi b/stdlib/3/builtins.pyi
index 27d9c5eed..19586b75d 100644
--- a/stdlib/3/builtins.pyi
+++ b/stdlib/3/builtins.pyi
@@ -250,8 +250,8 @@ class str(Sequence[str]):
     def center(self, width: int, fillchar: str = ' ') -> str: ...
     def count(self, x: str, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def encode(self, encoding: str = 'utf-8', errors: str = 'strict') -> bytes: ...
-    def endswith(self, suffix: Union[str, Tuple[str, ...]], start: int = None,
-                 end: int = None) -> bool: ...
+    def endswith(self, suffix: Union[str, Tuple[str, ...]], start: Optional[int] = None,
+                 end: Optional[int] = None) -> bool: ...
     def expandtabs(self, tabsize: int = 8) -> str: ...
     def find(self, sub: str, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def format(self, *args: Any, **kwargs: Any) -> str: ...
@@ -271,20 +271,20 @@ class str(Sequence[str]):
     def join(self, iterable: Iterable[str]) -> str: ...
     def ljust(self, width: int, fillchar: str = ' ') -> str: ...
     def lower(self) -> str: ...
-    def lstrip(self, chars: str = None) -> str: ...
+    def lstrip(self, chars: Optional[str] = None) -> str: ...
     def partition(self, sep: str) -> Tuple[str, str, str]: ...
     def replace(self, old: str, new: str, count: int = -1) -> str: ...
     def rfind(self, sub: str, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def rindex(self, sub: str, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def rjust(self, width: int, fillchar: str = ' ') -> str: ...
     def rpartition(self, sep: str) -> Tuple[str, str, str]: ...
-    def rsplit(self, sep: str = None, maxsplit: int = -1) -> List[str]: ...
-    def rstrip(self, chars: str = None) -> str: ...
-    def split(self, sep: str = None, maxsplit: int = -1) -> List[str]: ...
+    def rsplit(self, sep: Optional[str] = None, maxsplit: int = -1) -> List[str]: ...
+    def rstrip(self, chars: Optional[str] = None) -> str: ...
+    def split(self, sep: Optional[str] = None, maxsplit: int = -1) -> List[str]: ...
     def splitlines(self, keepends: bool = ...) -> List[str]: ...
-    def startswith(self, prefix: Union[str, Tuple[str, ...]], start: int = None,
-                   end: int = None) -> bool: ...
-    def strip(self, chars: str = None) -> str: ...
+    def startswith(self, prefix: Union[str, Tuple[str, ...]], start: Optional[int] = None,
+                   end: Optional[int] = None) -> bool: ...
+    def strip(self, chars: Optional[str] = None) -> str: ...
     def swapcase(self) -> str: ...
     def title(self) -> str: ...
     def translate(self, table: Dict[int, Any]) -> str: ...
@@ -333,22 +333,22 @@ class bytes(ByteString):
     def capitalize(self) -> bytes: ...
     def center(self, width: int, fillchar: bytes = ...) -> bytes: ...
     if sys.version_info >= (3, 3):
-        def count(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def count(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def count(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def count(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     def decode(self, encoding: str = 'utf-8', errors: str = 'strict') -> str: ...
     def endswith(self, suffix: Union[bytes, Tuple[bytes, ...]]) -> bool: ...
     def expandtabs(self, tabsize: int = 8) -> bytes: ...
     if sys.version_info >= (3, 3):
-        def find(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def find(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def find(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def find(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     if sys.version_info >= (3, 5):
         def hex(self) -> str: ...
     if sys.version_info >= (3, 3):
-        def index(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def index(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def index(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def index(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     def isalnum(self) -> bool: ...
     def isalpha(self) -> bool: ...
     def isdigit(self) -> bool: ...
@@ -359,25 +359,25 @@ class bytes(ByteString):
     def join(self, iterable: Iterable[bytes]) -> bytes: ...
     def ljust(self, width: int, fillchar: bytes = ...) -> bytes: ...
     def lower(self) -> bytes: ...
-    def lstrip(self, chars: bytes = None) -> bytes: ...
+    def lstrip(self, chars: Optional[bytes] = None) -> bytes: ...
     def partition(self, sep: bytes) -> Tuple[bytes, bytes, bytes]: ...
     def replace(self, old: bytes, new: bytes, count: int = -1) -> bytes: ...
     if sys.version_info >= (3, 3):
-        def rfind(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def rfind(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def rfind(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def rfind(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     if sys.version_info >= (3, 3):
-        def rindex(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def rindex(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def rindex(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def rindex(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     def rjust(self, width: int, fillchar: bytes = ...) -> bytes: ...
     def rpartition(self, sep: bytes) -> Tuple[bytes, bytes, bytes]: ...
-    def rsplit(self, sep: bytes = None, maxsplit: int = -1) -> List[bytes]: ...
-    def rstrip(self, chars: bytes = None) -> bytes: ...
-    def split(self, sep: bytes = None, maxsplit: int = -1) -> List[bytes]: ...
+    def rsplit(self, sep: Optional[bytes] = None, maxsplit: int = -1) -> List[bytes]: ...
+    def rstrip(self, chars: Optional[bytes] = None) -> bytes: ...
+    def split(self, sep: Optional[bytes] = None, maxsplit: int = -1) -> List[bytes]: ...
     def splitlines(self, keepends: bool = ...) -> List[bytes]: ...
     def startswith(self, prefix: Union[bytes, Tuple[bytes, ...]]) -> bool: ...
-    def strip(self, chars: bytes = None) -> bytes: ...
+    def strip(self, chars: Optional[bytes] = None) -> bytes: ...
     def swapcase(self) -> bytes: ...
     def title(self) -> bytes: ...
     def translate(self, table: Optional[bytes], delete: bytes = ...) -> bytes: ...
@@ -424,22 +424,22 @@ class bytearray(MutableSequence[int], ByteString):
     def capitalize(self) -> bytearray: ...
     def center(self, width: int, fillchar: bytes = ...) -> bytearray: ...
     if sys.version_info >= (3, 3):
-        def count(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def count(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def count(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def count(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     def decode(self, encoding: str = 'utf-8', errors: str = 'strict') -> str: ...
     def endswith(self, suffix: bytes) -> bool: ...
     def expandtabs(self, tabsize: int = 8) -> bytearray: ...
     if sys.version_info >= (3, 3):
-        def find(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def find(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def find(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def find(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     if sys.version_info >= (3, 5):
         def hex(self) -> str: ...
     if sys.version_info >= (3, 3):
-        def index(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def index(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def index(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def index(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     def insert(self, index: int, object: int) -> None: ...
     def isalnum(self) -> bool: ...
     def isalpha(self) -> bool: ...
@@ -451,25 +451,25 @@ class bytearray(MutableSequence[int], ByteString):
     def join(self, iterable: Iterable[bytes]) -> bytearray: ...
     def ljust(self, width: int, fillchar: bytes = ...) -> bytearray: ...
     def lower(self) -> bytearray: ...
-    def lstrip(self, chars: bytes = None) -> bytearray: ...
+    def lstrip(self, chars: Optional[bytes] = None) -> bytearray: ...
     def partition(self, sep: bytes) -> Tuple[bytearray, bytearray, bytearray]: ...
     def replace(self, old: bytes, new: bytes, count: int = -1) -> bytearray: ...
     if sys.version_info >= (3, 3):
-        def rfind(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def rfind(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def rfind(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def rfind(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     if sys.version_info >= (3, 3):
-        def rindex(self, sub: Union[bytes, int], start: int = None, end: int = None) -> int: ...
+        def rindex(self, sub: Union[bytes, int], start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     else:
-        def rindex(self, sub: bytes, start: int = None, end: int = None) -> int: ...
+        def rindex(self, sub: bytes, start: Optional[int] = None, end: Optional[int] = None) -> int: ...
     def rjust(self, width: int, fillchar: bytes = ...) -> bytearray: ...
     def rpartition(self, sep: bytes) -> Tuple[bytearray, bytearray, bytearray]: ...
-    def rsplit(self, sep: bytes = None, maxsplit: int = -1) -> List[bytearray]: ...
-    def rstrip(self, chars: bytes = None) -> bytearray: ...
-    def split(self, sep: bytes = None, maxsplit: int = -1) -> List[bytearray]: ...
+    def rsplit(self, sep: Optional[bytes] = None, maxsplit: int = -1) -> List[bytearray]: ...
+    def rstrip(self, chars: Optional[bytes] = None) -> bytearray: ...
+    def split(self, sep: Optional[bytes] = None, maxsplit: int = -1) -> List[bytearray]: ...
     def splitlines(self, keepends: bool = ...) -> List[bytearray]: ...
     def startswith(self, prefix: bytes) -> bool: ...
-    def strip(self, chars: bytes = None) -> bytearray: ...
+    def strip(self, chars: Optional[bytes] = None) -> bytearray: ...
     def swapcase(self) -> bytearray: ...
     def title(self) -> bytearray: ...
     def translate(self, table: Optional[bytes], delete: bytes = ...) -> bytearray: ...
@@ -556,7 +556,7 @@ class slice:
     @overload
     def __init__(self, stop: Optional[int]) -> None: ...
     @overload
-    def __init__(self, start: Optional[int], stop: Optional[int], step: int = None) -> None: ...
+    def __init__(self, start: Optional[int], stop: Optional[int], step: Optional[int] = None) -> None: ...
     def indices(self, len: int) -> Tuple[int, int, int]: ...
 
 class tuple(Sequence[_T_co], Generic[_T_co]):
@@ -604,7 +604,7 @@ class list(MutableSequence[_T], Generic[_T]):
     def insert(self, index: int, object: _T) -> None: ...
     def remove(self, object: _T) -> None: ...
     def reverse(self) -> None: ...
-    def sort(self, *, key: Callable[[_T], Any] = None, reverse: bool = ...) -> None: ...
+    def sort(self, *, key: Optional[Callable[[_T], Any]] = None, reverse: bool = ...) -> None: ...
 
     def __len__(self) -> int: ...
     def __iter__(self) -> Iterator[_T]: ...
@@ -646,7 +646,7 @@ class dict(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
     def clear(self) -> None: ...
     def copy(self) -> Dict[_KT, _VT]: ...
     def popitem(self) -> Tuple[_KT, _VT]: ...
-    def setdefault(self, k: _KT, default: _VT = None) -> _VT: ...
+    def setdefault(self, k: _KT, default: Optional[_VT] = None) -> _VT: ...
     @overload
     def update(self, __m: Mapping[_KT, _VT], **kwargs: _VT) -> None: ...
     @overload
@@ -740,7 +740,7 @@ class range(Sequence[int]):
     @overload
     def __init__(self, start: int, stop: int, step: int = 1) -> None: ...
     def count(self, value: int) -> int: ...
-    def index(self, value: int, start: int = 0, stop: int = None) -> int: ...
+    def index(self, value: int, start: int = 0, stop: Optional[int] = None) -> int: ...
     def __len__(self) -> int: ...
     def __contains__(self, o: object) -> bool: ...
     def __iter__(self) -> Iterator[int]: ...
@@ -752,13 +752,14 @@ class range(Sequence[int]):
     def __reversed__(self) -> Iterator[int]: ...
 
 class property:
-    def __init__(self, fget: Callable[[Any], Any] = None,
-                 fset: Callable[[Any, Any], None] = None,
-                 fdel: Callable[[Any], None] = None, doc: str = None) -> None: ...
+    def __init__(self, fget: Optional[Callable[[Any], Any]] = None,
+                 fset: Optional[Callable[[Any, Any], None]] = None,
+                 fdel: Optional[Callable[[Any], None]] = None,
+                 doc: Optional[str] = None) -> None: ...
     def getter(self, fget: Callable[[Any], Any]) -> property: ...
     def setter(self, fset: Callable[[Any, Any], None]) -> property: ...
     def deleter(self, fdel: Callable[[Any], None]) -> property: ...
-    def __get__(self, obj: Any, type: type=None) -> Any: ...
+    def __get__(self, obj: Any, type: Optional[type] = None) -> Any: ...
     def __set__(self, obj: Any, value: Any) -> None: ...
     def __delete__(self, obj: Any) -> None: ...
     def fget(self) -> Any: ...
@@ -782,10 +783,10 @@ def delattr(o: Any, name: str) -> None: ...
 def dir(o: object = ...) -> List[str]: ...
 _N = TypeVar('_N', int, float)
 def divmod(a: _N, b: _N) -> Tuple[_N, _N]: ...
-def eval(source: str, globals: Dict[str, Any] = None,
-         locals: Mapping[str, Any] = None) -> Any: ...  # TODO code object as source
-def exec(object: str, globals: Dict[str, Any] = None,
-         locals: Mapping[str, Any] = None) -> Any: ...  # TODO code object as source
+def eval(source: str, globals: Optional[Dict[str, Any]] = None,
+         locals: Optional[Mapping[str, Any]] = None) -> Any: ...  # TODO code object as source
+def exec(object: str, globals: Optional[Dict[str, Any]] = None,
+         locals: Optional[Mapping[str, Any]] = None) -> Any: ...  # TODO code object as source
 def exit(code: Any = ...) -> NoReturn: ...
 @overload
 def filter(function: Optional[Callable[[_T], Any]],
@@ -800,7 +801,7 @@ def hash(o: object) -> int: ...
 def help(*args: Any, **kwds: Any) -> None: ...
 def hex(i: int) -> str: ...  # TODO __index__
 def id(o: object) -> int: ...
-def input(prompt: Any = None) -> str: ...
+def input(prompt: Optional[Any] = None) -> str: ...
 @overload
 def iter(iterable: Iterable[_T]) -> Iterator[_T]: ...
 @overload
@@ -836,15 +837,15 @@ if sys.version_info >= (3, 6):
     class _PathLike(Generic[AnyStr]):
         def __fspath__(self) -> AnyStr: ...
 
-    def open(file: Union[str, bytes, int, _PathLike], mode: str = 'r', buffering: int = -1, encoding: str = None,
-             errors: str = None, newline: str = None, closefd: bool = ...) -> IO[Any]: ...
+    def open(file: Union[str, bytes, int, _PathLike], mode: str = 'r', buffering: int = -1, encoding: Optional[str] = None,
+             errors: Optional[str] = None, newline: Optional[str] = None, closefd: bool = ...) -> IO[Any]: ...
 else:
-    def open(file: Union[str, bytes, int], mode: str = 'r', buffering: int = -1, encoding: str = None,
-             errors: str = None, newline: str = None, closefd: bool = ...) -> IO[Any]: ...
+    def open(file: Union[str, bytes, int], mode: str = 'r', buffering: int = -1, encoding: Optional[str] = None,
+             errors: Optional[str] = None, newline: Optional[str] = None, closefd: bool = ...) -> IO[Any]: ...
 
 def ord(c: Union[str, bytes, bytearray]) -> int: ...
 # TODO: in Python 3.2, print() does not support flush
-def print(*values: Any, sep: str = ' ', end: str = '\n', file: IO[str] = None, flush: bool = False) -> None: ...
+def print(*values: Any, sep: str = ' ', end: str = '\n', file: Optional[IO[str]] = None, flush: bool = False) -> None: ...
 @overload
 def pow(x: int, y: int) -> Any: ...  # The return type can be int or float, depending on y
 @overload
@@ -853,7 +854,7 @@ def pow(x: int, y: int, z: int) -> Any: ...
 def pow(x: float, y: float) -> float: ...
 @overload
 def pow(x: float, y: float, z: float) -> float: ...
-def quit(code: int = None) -> None: ...
+def quit(code: Optional[int] = None) -> None: ...
 @overload
 def reversed(object: Reversible[_T]) -> Iterator[_T]: ...
 @overload
@@ -868,7 +869,8 @@ def round(number: SupportsRound[_T]) -> _T: ...
 @overload
 def round(number: SupportsRound[_T], ndigits: int) -> _T: ...
 def setattr(object: Any, name: str, value: Any) -> None: ...
-def sorted(iterable: Iterable[_T], *, key: Callable[[_T], Any] = None,
+def sorted(iterable: Iterable[_T], *,
+           key: Optional[Callable[[_T], Any]] = None,
            reverse: bool = False) -> List[_T]: ...
 def sum(iterable: Iterable[_T], start: _T = ...) -> _T: ...
 def vars(object: Any = ...) -> Dict[str, Any]: ...
diff --git a/stdlib/3/configparser.pyi b/stdlib/3/configparser.pyi
index a990d8f31..ff90651ff 100644
--- a/stdlib/3/configparser.pyi
+++ b/stdlib/3/configparser.pyi
@@ -80,9 +80,9 @@ class RawConfigParser(_parser):
     def has_option(self, section: str, option: str) -> bool: ...
 
     def read(self, filenames: Union[str, Sequence[str]],
-             encoding: str = None) -> List[str]: ...
+             encoding: Optional[str] = None) -> List[str]: ...
 
-    def read_file(self, f: Iterable[str], source: str = None) -> None: ...
+    def read_file(self, f: Iterable[str], source: Optional[str] = None) -> None: ...
 
     def read_string(self, string: str, source: str = ...) -> None: ...
 
@@ -117,16 +117,16 @@ class RawConfigParser(_parser):
 
 class ConfigParser(RawConfigParser):
     def __init__(self,
-                 defaults: _section = None,
+                 defaults: Optional[_section] = None,
                  dict_type: Mapping[str, str] = ...,
                  allow_no_value: bool = ...,
                  delimiters: Sequence[str] = ...,
                  comment_prefixes: Sequence[str] = ...,
-                 inline_comment_prefixes: Sequence[str] = None,
+                 inline_comment_prefixes: Optional[Sequence[str]] = None,
                  strict: bool = ...,
                  empty_lines_in_values: bool = ...,
                  default_section: str = ...,
-                 interpolation: Interpolation = None,
+                 interpolation: Optional[Interpolation] = None,
                  converters: _converters = ...) -> None: ...
 
 class SafeConfigParser(ConfigParser): ...
diff --git a/stdlib/3/getpass.pyi b/stdlib/3/getpass.pyi
index 73c0cb918..f55023c95 100644
--- a/stdlib/3/getpass.pyi
+++ b/stdlib/3/getpass.pyi
@@ -1,9 +1,9 @@
 # Stubs for getpass
 
-from typing import TextIO
+from typing import Optional, TextIO
 
 
-def getpass(prompt: str = ..., stream: TextIO = None) -> str: ...
+def getpass(prompt: str = ..., stream: Optional[TextIO] = None) -> str: ...
 
 
 def getuser() -> str: ...
diff --git a/stdlib/3/importlib/__init__.pyi b/stdlib/3/importlib/__init__.pyi
index f44c2a799..08b3a8188 100644
--- a/stdlib/3/importlib/__init__.pyi
+++ b/stdlib/3/importlib/__init__.pyi
@@ -4,14 +4,15 @@ import sys
 import types
 from typing import Any, Mapping, Optional, Sequence
 
-def __import__(name: str, globals: Mapping[str, Any] = None,
-               locals: Mapping[str, Any] = None, fromlist: Sequence[str] = ...,
+def __import__(name: str, globals: Optional[Mapping[str, Any]] = None,
+               locals: Optional[Mapping[str, Any]] = None,
+               fromlist: Sequence[str] = ...,
                level: int = ...) -> types.ModuleType: ...
 
-def import_module(name: str, package: str = None) -> types.ModuleType: ...
+def import_module(name: str, package: Optional[str] = None) -> types.ModuleType: ...
 
 if sys.version_info >= (3, 3):
-    def find_loader(name: str, path: str = None) -> Optional[Loader]: ...
+    def find_loader(name: str, path: Optional[str] = None) -> Optional[Loader]: ...
 
     def invalidate_caches() -> None: ...
 
diff --git a/stdlib/3/importlib/abc.pyi b/stdlib/3/importlib/abc.pyi
index f7c746ac7..f7cd81d21 100644
--- a/stdlib/3/importlib/abc.pyi
+++ b/stdlib/3/importlib/abc.pyi
@@ -19,7 +19,7 @@ class Finder(metaclass=ABCMeta):
     # easier to simply ignore that this method exists.
     # @abstractmethod
     # def find_module(self, fullname: str,
-    #                 path: Sequence[_Path] = None) -> Optional[Loader]: ...
+    #                 path: Optional[Sequence[_Path]] = None) -> Optional[Loader]: ...
 
 class ResourceLoader(Loader):
     @abstractmethod
@@ -64,7 +64,7 @@ if sys.version_info >= (3, 3):
             # Not defined on the actual class, but expected to exist.
             def find_spec(
                 self, fullname: str, path: Optional[Sequence[_Path]],
-                target: types.ModuleType = None
+                target: Optional[types.ModuleType] = None
             ) -> Optional[ModuleSpec]:
                 ...
 
@@ -78,7 +78,7 @@ if sys.version_info >= (3, 3):
             # Not defined on the actual class, but expected to exist.
             def find_spec(
                 self, fullname: str,
-                target: types.ModuleType = None
+                target: Optional[types.ModuleType] = None
             ) -> Optional[ModuleSpec]: ...
 
     class FileLoader(ResourceLoader, ExecutionLoader):
diff --git a/stdlib/3/importlib/machinery.pyi b/stdlib/3/importlib/machinery.pyi
index 32be0627e..f9e1907bd 100644
--- a/stdlib/3/importlib/machinery.pyi
+++ b/stdlib/3/importlib/machinery.pyi
@@ -22,7 +22,7 @@ if sys.version_info >= (3, 3):
             @classmethod
             def find_spec(cls, fullname: str,
                           path: Optional[Sequence[importlib.abc._Path]],
-                          target: types.ModuleType = None) -> Optional[ModuleSpec]:
+                          target: Optional[types.ModuleType] = None) -> Optional[ModuleSpec]:
                 ...
         # InspectLoader
         @classmethod
@@ -79,7 +79,7 @@ if sys.version_info >= (3, 3):
             @classmethod
             def find_spec(cls, fullname: str,
                           path: Optional[Sequence[importlib.abc._Path]],
-                          target: types.ModuleType = None) -> Optional[ModuleSpec]:
+                          target: Optional[types.ModuleType] = None) -> Optional[ModuleSpec]:
                 ...
         # InspectLoader
         @classmethod
@@ -136,7 +136,7 @@ if sys.version_info >= (3, 3):
             @classmethod
             def find_spec(cls, fullname: str,
                           path: Optional[Sequence[importlib.abc._Path]],
-                          target: types.ModuleType = None) -> Optional[ModuleSpec]:
+                          target: Optional[types.ModuleType] = None) -> Optional[ModuleSpec]:
                 ...
 else:
     class WindowsRegisteryFinder:
diff --git a/stdlib/3/importlib/util.pyi b/stdlib/3/importlib/util.pyi
index 6f0232b00..0a56b9ca7 100644
--- a/stdlib/3/importlib/util.pyi
+++ b/stdlib/3/importlib/util.pyi
@@ -20,22 +20,22 @@ if sys.version_info >= (3, 3):
 if sys.version_info >= (3, 4):
     MAGIC_NUMBER = ...  # type: bytes
 
-    def cache_from_source(path: str, debug_override: bool = None, *,
-                          optimization: Any = None) -> str: ...
+    def cache_from_source(path: str, debug_override: Optional[bool] = None, *,
+                          optimization: Optional[Any] = None) -> str: ...
     def source_from_cache(path: str) -> str: ...
     def decode_source(source_bytes: bytes) -> str: ...
     def find_spec(
-        name: str, package: str = None
+        name: str, package: Optional[str] = None
     ) -> importlib.machinery.ModuleSpec: ...
     def spec_from_loader(
         name: str, loader: Optional[importlib.abc.Loader], *,
-        origin: str = None, loader_state: Any = None,
-        is_package: bool = None
+        origin: Optional[str] = None, loader_state: Optional[Any] = None,
+        is_package: Optional[bool] = None
     ) -> importlib.machinery.ModuleSpec: ...
     def spec_from_file_location(
         name: str, location: str, *,
-        loader: importlib.abc.Loader = None,
-        submodule_search_locations: List[str]=None
+        loader: Optional[importlib.abc.Loader] = None,
+        submodule_search_locations: Optional[List[str]] = None
     ) -> importlib.machinery.ModuleSpec: ...
 
 if sys.version_info >= (3, 5):
diff --git a/stdlib/3/io.pyi b/stdlib/3/io.pyi
index a22955bb1..4b1deb742 100644
--- a/stdlib/3/io.pyi
+++ b/stdlib/3/io.pyi
@@ -106,8 +106,8 @@ class BytesIO(BinaryIO):
     def __iter__(self) -> Iterator[bytes]: ...
     def __next__(self) -> bytes: ...
     def __enter__(self) -> 'BytesIO': ...
-    def __exit__(self, t: type = None, value: BaseException = None,
-                 traceback: Any = None) -> bool: ...
+    def __exit__(self, t: Optional[type] = None, value: Optional[BaseException] = None,
+                 traceback: Optional[Any] = None) -> bool: ...
     def close(self) -> None: ...
     def fileno(self) -> int: ...
     def flush(self) -> None: ...
@@ -210,8 +210,8 @@ class TextIOWrapper(TextIO):
         write_through: bool = ...
     ) -> None: ...
     # copied from IOBase
-    def __exit__(self, t: type = None, value: BaseException = None,
-                 traceback: Any = None) -> bool: ...
+    def __exit__(self, t: Optional[type] = None, value: Optional[BaseException] = None,
+                 traceback: Optional[Any] = None) -> bool: ...
     def close(self) -> None: ...
     def fileno(self) -> int: ...
     def flush(self) -> None: ...
diff --git a/stdlib/3/json/decoder.pyi b/stdlib/3/json/decoder.pyi
index 919039eac..1c3e9a05b 100644
--- a/stdlib/3/json/decoder.pyi
+++ b/stdlib/3/json/decoder.pyi
@@ -1,5 +1,5 @@
 import sys
-from typing import Any, Callable, Dict, List, Tuple
+from typing import Any, Callable, Dict, List, Optional, Tuple
 
 if sys.version_info >= (3, 5):
     class JSONDecodeError(ValueError):
@@ -18,11 +18,11 @@ class JSONDecoder:
     strict = ...  # type: bool
     object_pairs_hook = None  # type: Callable[[List[Tuple[str, Any]]], Any]
 
-    def __init__(self, object_hook: Callable[[Dict[str, Any]], Any]=None,
-            parse_float: Callable[[str], Any]=None,
-            parse_int: Callable[[str], Any]=None,
-            parse_constant: Callable[[str], Any]=None,
-            strict: bool=True,
-            object_pairs_hook: Callable[[List[Tuple[str, Any]]], Any]=None) -> None: ...
+    def __init__(self, object_hook: Optional[Callable[[Dict[str, Any]], Any]] = None,
+            parse_float: Optional[Callable[[str], Any]] = None,
+            parse_int: Optional[Callable[[str], Any]] = None,
+            parse_constant: Optional[Callable[[str], Any]] = None,
+            strict: bool = True,
+            object_pairs_hook: Optional[Callable[[List[Tuple[str, Any]]], Any]] = None) -> None: ...
     def decode(self, s: str) -> Any: ...
-    def raw_decode(self, s: str, idx: int=...) -> Tuple[Any, int]: ...
+    def raw_decode(self, s: str, idx: int = ...) -> Tuple[Any, int]: ...
diff --git a/stdlib/3/json/encoder.pyi b/stdlib/3/json/encoder.pyi
index f75304dba..c423c182a 100644
--- a/stdlib/3/json/encoder.pyi
+++ b/stdlib/3/json/encoder.pyi
@@ -1,4 +1,4 @@
-from typing import Any, Callable, Iterator, Tuple
+from typing import Any, Callable, Iterator, Optional, Tuple
 
 class JSONEncoder:
     item_separator = ...  # type: str
@@ -11,10 +11,10 @@ class JSONEncoder:
     sort_keys = ...  # type: bool
     indent = None  # type: int
 
-    def __init__(self, skipkeys: bool=..., ensure_ascii: bool=...,
-            check_circular: bool=..., allow_nan: bool=..., sort_keys: bool=...,
-            indent: int=None, separators: Tuple[str, str]=None, default: Callable=None) -> None: ...
+    def __init__(self, skipkeys: bool = ..., ensure_ascii: bool = ...,
+            check_circular: bool = ..., allow_nan: bool = ..., sort_keys: bool = ...,
+            indent: Optional[int] = None, separators: Optional[Tuple[str, str]] = None, default: Optional[Callable] = None) -> None: ...
 
     def default(self, o: Any) -> Any: ...
     def encode(self, o: Any) -> str: ...
-    def iterencode(self, o: Any, _one_shot: bool=False) -> Iterator[str]: ...
+    def iterencode(self, o: Any, _one_shot: bool = False) -> Iterator[str]: ...
diff --git a/stdlib/3/multiprocessing/__init__.pyi b/stdlib/3/multiprocessing/__init__.pyi
index 40aed12f4..da9333533 100644
--- a/stdlib/3/multiprocessing/__init__.pyi
+++ b/stdlib/3/multiprocessing/__init__.pyi
@@ -25,7 +25,7 @@ class Pool():
                  initializer: Optional[Callable[..., None]] = ...,
                  initargs: Iterable[Any] = ...,
                  maxtasksperchild: Optional[int] = ...,
-                 context: Any = None) -> None: ...
+                 context: Optional[Any] = None) -> None: ...
     def apply(self,
               func: Callable[..., Any],
               args: Iterable[Any] = ...,
@@ -34,8 +34,8 @@ class Pool():
                 func: Callable[..., Any],
                 args: Iterable[Any] = ...,
                 kwds: Dict[str, Any] = ...,
-                callback: Callable[..., None] = None,
-                error_callback: Callable[[BaseException], None] = None) -> AsyncResult: ...
+                callback: Optional[Callable[..., None]] = None,
+                error_callback: Optional[Callable[[BaseException], None]] = None) -> AsyncResult: ...
     def map(self,
             func: Callable[..., Any],
             iterable: Iterable[Any] = ...,
@@ -43,8 +43,8 @@ class Pool():
     def map_async(self, func: Callable[..., Any],
                   iterable: Iterable[Any] = ...,
                   chunksize: Optional[int] = ...,
-                  callback: Callable[..., None] = None,
-                  error_callback: Callable[[BaseException], None] = None) -> AsyncResult: ...
+                  callback: Optional[Callable[..., None]] = None,
+                  error_callback: Optional[Callable[[BaseException], None]] = None) -> AsyncResult: ...
     def imap(self,
              func: Callable[..., Any],
              iterable: Iterable[Any] = ...,
@@ -61,8 +61,8 @@ class Pool():
                       func: Callable[..., Any],
                       iterable: Iterable[Iterable[Any]] = ...,
                       chunksize: Optional[int] = ...,
-                      callback: Callable[..., None] = None,
-                      error_callback: Callable[[BaseException], None] = None) -> AsyncResult: ...
+                      callback: Optional[Callable[..., None]] = None,
+                      error_callback: Optional[Callable[[BaseException], None]] = None) -> AsyncResult: ...
     def close(self) -> None: ...
     def terminate(self) -> None: ...
     def join(self) -> None: ...
diff --git a/stdlib/3/multiprocessing/pool.pyi b/stdlib/3/multiprocessing/pool.pyi
index af9f9ebae..c3b789393 100644
--- a/stdlib/3/multiprocessing/pool.pyi
+++ b/stdlib/3/multiprocessing/pool.pyi
@@ -22,17 +22,17 @@ class ThreadPool():
                 func: Callable[..., Any],
                 args: Iterable[Any] = ...,
                 kwds: Dict[str, Any] = ...,
-                callback: Callable[..., None] = None,
-                error_callback: Callable[[BaseException], None] = None) -> AsyncResult: ...
+                callback: Optional[Callable[..., None]] = None,
+                error_callback: Optional[Callable[[BaseException], None]] = None) -> AsyncResult: ...
     def map(self,
             func: Callable[..., Any],
             iterable: Iterable[Any] = ...,
             chunksize: Optional[int] = None) -> List[Any]: ...
     def map_async(self, func: Callable[..., Any],
                   iterable: Iterable[Any] = ...,
-                  chunksize: Optional[int] = None,
-                  callback: Callable[..., None] = None,
-                  error_callback: Callable[[BaseException], None] = None) -> AsyncResult: ...
+                  chunksize: Optional[Optional[int]] = None,
+                  callback: Optional[Callable[..., None]] = None,
+                  error_callback: Optional[Callable[[BaseException], None]] = None) -> AsyncResult: ...
     def imap(self,
              func: Callable[..., Any],
              iterable: Iterable[Any] = ...,
@@ -49,8 +49,8 @@ class ThreadPool():
                       func: Callable[..., Any],
                       iterable: Iterable[Iterable[Any]] = ...,
                       chunksize: Optional[int] = None,
-                      callback: Callable[..., None] = None,
-                      error_callback: Callable[[BaseException], None] = None) -> AsyncResult: ...
+                      callback: Optional[Callable[..., None]] = None,
+                      error_callback: Optional[Callable[[BaseException], None]] = None) -> AsyncResult: ...
     def close(self) -> None: ...
     def terminate(self) -> None: ...
     def join(self) -> None: ...
diff --git a/stdlib/3/unittest/mock.pyi b/stdlib/3/unittest/mock.pyi
index d884fb95c..38b4669d2 100644
--- a/stdlib/3/unittest/mock.pyi
+++ b/stdlib/3/unittest/mock.pyi
@@ -1,7 +1,7 @@
 # Stubs for unittest.mock
 
 import sys
-from typing import Any
+from typing import Any, Optional
 
 if sys.version_info >= (3, 3):
     FILTER_DIR = ...  # type: Any
@@ -33,7 +33,7 @@ if sys.version_info >= (3, 3):
 
     class NonCallableMock(Any):
         def __new__(cls, *args: Any, **kw: Any) -> Any: ...
-        def __init__(self, spec: Any = None, wraps: Any = None, name: Any = None, spec_set: Any = None, parent: Any = None, _spec_state: Any = None, _new_name: Any ='', _new_parent: Any = None, _spec_as_instance: Any = False, _eat_self: Any = None, unsafe: Any = False, **kwargs: Any) -> None: ...
+        def __init__(self, spec: Optional[Any] = None, wraps: Optional[Any] = None, name: Optional[Any] = None, spec_set: Optional[Any] = None, parent: Optional[Any] = None, _spec_state: Optional[Any] = None, _new_name: Any ='', _new_parent: Optional[Any] = None, _spec_as_instance: Any = False, _eat_self: Optional[Any] = None, unsafe: Any = False, **kwargs: Any) -> None: ...
         def attach_mock(self, mock: Any, attribute: Any) -> Any: ...
         def mock_add_spec(self, spec: Any, spec_set: Any = False) -> Any: ...
         return_value = ...  # type: Any
@@ -45,7 +45,7 @@ if sys.version_info >= (3, 3):
         mock_calls = ...  # type: Any
         side_effect = ...  # type: Any
         method_calls = ...  # type: Any
-        def reset_mock(self, visited: bool = None) -> None: ...
+        def reset_mock(self, visited: Optional[bool] = None) -> None: ...
         def configure_mock(self, **kwargs: Any) -> None: ...
         def __getattr__(self, name: Any) -> Any: ...
         def __dir__(self) -> Any: ...
@@ -59,7 +59,7 @@ if sys.version_info >= (3, 3):
 
     class CallableMixin(Base):
         side_effect = ...  # type: Any
-        def __init__(self, spec: Any = None, side_effect: Any = None, return_value: Any = ..., wraps: Any = None, name: Any = None, spec_set: Any = None, parent: Any = None, _spec_state: Any = None, _new_name: Any = '', _new_parent: Any = None, **kwargs: Any) -> None: ...
+        def __init__(self, spec: Optional[Any] = None, side_effect: Optional[Any] = None, return_value: Any = ..., wraps: Optional[Any] = None, name: Optional[Any] = None, spec_set: Optional[Any] = None, parent: Optional[Any] = None, _spec_state: Optional[Any] = None, _new_name: Any = '', _new_parent: Optional[Any] = None, **kwargs: Any) -> None: ...
         def __call__(_mock_self, *args: Any, **kwargs: Any) -> Any: ...
 
     class Mock(CallableMixin, NonCallableMock):
@@ -93,8 +93,8 @@ if sys.version_info >= (3, 3):
         def stop(self) -> Any: ...
 
     class _patcher:
-        def __call__(self, target: Any, new: Any = None, spec: Any = None, create: Any = False, spec_set: Any = None, autospec: Any = None, new_callable: Any = None, **kwargs: Any) -> Any: ...
-        def object(self, target: Any, attribute: str, new: Any = None, spec: Any = None, create: Any = False, spec_set: Any = None, autospec: Any = None, new_callable: Any = None, **kwargs: Any) -> _patch: ...
+        def __call__(self, target: Any, new: Optional[Any] = None, spec: Optional[Any] = None, create: Any = False, spec_set: Optional[Any] = None, autospec: Optional[Any] = None, new_callable: Optional[Any] = None, **kwargs: Any) -> Any: ...
+        def object(self, target: Any, attribute: str, new: Optional[Any] = None, spec: Optional[Any] = None, create: Any = False, spec_set: Optional[Any] = None, autospec: Optional[Any] = None, new_callable: Optional[Any] = None, **kwargs: Any) -> _patch: ...
 
     patch = ...  # type: _patcher
 
@@ -127,7 +127,7 @@ if sys.version_info >= (3, 3):
         def __init__(self, name: Any, parent: Any) -> None: ...
         def __call__(self, *args: Any, **kwargs: Any) -> Any: ...
         def create_mock(self) -> Any: ...
-        def __get__(self, obj: Any, _type: Any = None) -> Any: ...
+        def __get__(self, obj: Any, _type: Optional[Any] = None) -> Any: ...
 
     class _ANY:
         def __eq__(self, other: Any) -> bool: ...
@@ -136,11 +136,11 @@ if sys.version_info >= (3, 3):
     ANY = ...  # type: Any
 
     class _Call(tuple):
-        def __new__(cls, value: Any = ..., name: Any = None, parent: Any = None, two: bool = False, from_kall: bool = True) -> Any: ...
+        def __new__(cls, value: Any = ..., name: Optional[Any] = None, parent: Optional[Any] = None, two: bool = False, from_kall: bool = True) -> Any: ...
         name = ...  # type: Any
         parent = ...  # type: Any
         from_kall = ...  # type: Any
-        def __init__(self, value: Any = ..., name: Any = None, parent: Any = None, two: bool = False, from_kall: bool = True) -> None: ...
+        def __init__(self, value: Any = ..., name: Optional[Any] = None, parent: Optional[Any] = None, two: bool = False, from_kall: bool = True) -> None: ...
         def __eq__(self, other: Any) -> bool: ...
         __ne__ = ...  # type: Any
         def __call__(self, *args: Any, **kwargs: Any) -> Any: ...
@@ -151,7 +151,7 @@ if sys.version_info >= (3, 3):
 
     call = ...  # type: Any
 
-    def create_autospec(spec: Any, spec_set: Any = False, instance: Any = False, _parent: Any = None, _name: Any = None, **kwargs: Any) -> Any: ...
+    def create_autospec(spec: Any, spec_set: Any = False, instance: Any = False, _parent: Optional[Any] = None, _name: Optional[Any] = None, **kwargs: Any) -> Any: ...
 
     class _SpecState:
         spec = ...  # type: Any
@@ -160,9 +160,9 @@ if sys.version_info >= (3, 3):
         parent = ...  # type: Any
         instance = ...  # type: Any
         name = ...  # type: Any
-        def __init__(self, spec: Any, spec_set: Any = False, parent: Any = None, name: Any = None, ids: Any = None, instance: Any = False) -> None: ...
+        def __init__(self, spec: Any, spec_set: Any = False, parent: Optional[Any] = None, name: Optional[Any] = None, ids: Optional[Any] = None, instance: Any = False) -> None: ...
 
-    def mock_open(mock: Any = None, read_data: Any = '') -> Any: ...
+    def mock_open(mock: Optional[Any] = None, read_data: Any = '') -> Any: ...
 
     class PropertyMock(Mock):
         def __get__(self, obj: Any, obj_type: Any) -> Any: ...
diff --git a/third_party/3/six/__init__.pyi b/third_party/3/six/__init__.pyi
index 333bd27ce..80f6766b1 100644
--- a/third_party/3/six/__init__.pyi
+++ b/third_party/3/six/__init__.pyi
@@ -84,12 +84,12 @@ def byte2int(bs: binary_type) -> int: ...
 def indexbytes(buf: binary_type, i: int) -> int: ...
 def iterbytes(buf: binary_type) -> typing.Iterator[int]: ...
 
-def assertCountEqual(self: unittest.TestCase, first: Iterable[_T], second: Iterable[_T], msg: str = None) -> None: ...
+def assertCountEqual(self: unittest.TestCase, first: Iterable[_T], second: Iterable[_T], msg: Optional[str] = None) -> None: ...
 @overload
-def assertRaisesRegex(self: unittest.TestCase, msg: str = None) -> Any: ...
+def assertRaisesRegex(self: unittest.TestCase, msg: Optional[str] = None) -> Any: ...
 @overload
 def assertRaisesRegex(self: unittest.TestCase, callable_obj: Callable[..., Any], *args: Any, **kwargs: Any) -> Any: ...
-def assertRegex(self: unittest.TestCase, text: AnyStr, expected_regex: Union[AnyStr, Pattern[AnyStr]], msg: str = None) -> None: ...
+def assertRegex(self: unittest.TestCase, text: AnyStr, expected_regex: Union[AnyStr, Pattern[AnyStr]], msg: Optional[str] = None) -> None: ...
 
 exec_ = exec
 
diff --git a/third_party/3/werkzeug/wrappers.pyi b/third_party/3/werkzeug/wrappers.pyi
index 59b5b9359..031e37766 100644
--- a/third_party/3/werkzeug/wrappers.pyi
+++ b/third_party/3/werkzeug/wrappers.pyi
@@ -76,7 +76,14 @@ class BaseResponse:
     status = ...  # type: str
     direct_passthrough = ...  # type: bool
     response = ...  # type: Iterable[bytes]
-    def __init__(self, response: Union[Iterable[bytes], bytes]=None, status: Union[str, int]=None, headers: Union[Headers, Mapping[str, str], Sequence[Tuple[str, str]]]=None, mimetype: str=None, content_type: str=None, direct_passthrough: bool=False) -> None: ...
+    def __init__(self, response: Optional[Union[Iterable[bytes], bytes]]=None,
+                 status: Optional[Union[str, int]]=None,
+                 headers: Optional[Union[Headers,
+                                         Mapping[str, str],
+                                         Sequence[Tuple[str, str]]]]=None,
+                 mimetype: Optional[str] = None,
+                 content_type: Optional[str] = None,
+                 direct_passthrough: bool=False) -> None: ...
     def call_on_close(self, func): ...
     @classmethod
     def force_type(cls, response, environ=None): ...