path: root/Doc/library/typing.rst

:mod:`typing` --- Support for type hints
========================================

.. module:: typing
   :synopsis: Support for type hints (see PEP 484).

.. versionadded:: 3.5

**Source code:** :source:`Lib/typing.py`

--------------

This module supports type hints as specified by :pep:`484`.  The most
fundamental support consists of the type :class:`Any`, :class:`Union`,
:class:`Tuple`, :class:`Callable`, :class:`TypeVar`, and
:class:`Generic`.  For full specification please see :pep:`484`.  For
a simplified introduction to type hints see :pep:`483`.


The function below takes and returns a string and is annotated as follows::

   def greeting(name: str) -> str:
       return 'Hello ' + name

In the function ``greeting``, the argument ``name`` is expected to be of type
:class:`str` and the return type :class:`str`. Subtypes are accepted as
arguments.

Type aliases
------------

A type alias is defined by assigning the type to the alias. In this example,
``Vector`` and ``List[float]`` will be treated as interchangeable synonyms::

   from typing import List
   Vector = List[float]

   def scale(scalar: float, vector: Vector) -> Vector:
       return [scalar * num for num in vector]

   # typechecks; a list of floats qualifies as a Vector.
   new_vector = scale(2.0, [1.0, -4.2, 5.4])

Type aliases are useful for simplifying complex type signatures. For example::

   from typing import Dict, Tuple, List

   ConnectionOptions = Dict[str, str]
   Address = Tuple[str, int]
   Server = Tuple[Address, ConnectionOptions]

   def broadcast_message(message: str, servers: List[Server]) -> None:
       ...

   # The static type checker will treat the previous type signature as
   # being exactly equivalent to this one.
   def broadcast_message(
           message: str,
           servers: List[Tuple[Tuple[str, int], Dict[str, str]]]) -> None:
       ...

Note that ``None`` as a type hint is a special case and is replaced by
``type(None)``.

.. _distinct:

NewType
-------

Use the :func:`NewType` helper function to create distinct types::

   from typing import NewType

   UserId = NewType('UserId', int)
   some_id = UserId(524313)

The static type checker will treat the new type as if it were a subclass
of the original type. This is useful in helping catch logical errors::

   def get_user_name(user_id: UserId) -> str:
       ...

   # typechecks
   user_a = get_user_name(UserId(42351))

   # does not typecheck; an int is not a UserId
   user_b = get_user_name(-1)

You may still perform all ``int`` operations on a variable of type ``UserId``,
but the result will always be of type ``int``. This lets you pass in a
``UserId`` wherever an ``int`` might be expected, but will prevent you from
accidentally creating a ``UserId`` in an invalid way::

   # 'output' is of type 'int', not 'UserId'
   output = UserId(23413) + UserId(54341)

Note that these checks are enforced only by the static type checker. At runtime
the statement ``Derived = NewType('Derived', Base)`` will make ``Derived`` a
function that immediately returns whatever parameter you pass it. That means
the expression ``Derived(some_value)`` does not create a new class or introduce
any overhead beyond that of a regular function call.

More precisely, the expression ``some_value is Derived(some_value)`` is always
true at runtime.

This also means that it is not possible to create a subtype of ``Derived``
since it is an identity function at runtime, not an actual type. Similarly, it
is not possible to create another :func:`NewType` based on a ``Derived`` type::

   from typing import NewType

   UserId = NewType('UserId', int)

   # Fails at runtime and does not typecheck
   class AdminUserId(UserId): pass

   # Also does not typecheck
   ProUserId = NewType('ProUserId', UserId)

See :pep:`484` for more details.

.. note::

   Recall that the use of a type alias declares two types to be *equivalent* to
   one another. Doing ``Alias = Original`` will make the static type checker
   treat ``Alias`` as being *exactly equivalent* to ``Original`` in all cases.
   This is useful when you want to simplify complex type signatures.

   In contrast, ``NewType`` declares one type to be a *subtype* of another.
   Doing ``Derived = NewType('Derived', Original)`` will make the static type
   checker treat ``Derived`` as a *subclass* of ``Original``, which means a
   value of type ``Original`` cannot be used in places where a value of type
   ``Derived`` is expected. This is useful when you want to prevent logic
   errors with minimal runtime cost.

Callable
--------

Frameworks expecting callback functions of specific signatures might be
type hinted using ``Callable[[Arg1Type, Arg2Type], ReturnType]``.

For example::

   from typing import Callable

   def feeder(get_next_item: Callable[[], str]) -> None:
       # Body

   def async_query(on_success: Callable[[int], None],
                   on_error: Callable[[int, Exception], None]) -> None:
       # Body

It is possible to declare the return type of a callable without specifying
the call signature by substituting a literal ellipsis
for the list of arguments in the type hint: ``Callable[..., ReturnType]``.

Generics
--------

Since type information about objects kept in containers cannot be statically
inferred in a generic way, abstract base classes have been extended to support
subscription to denote expected types for container elements.

::

   from typing import Mapping, Sequence

   def notify_by_email(employees: Sequence[Employee],
                       overrides: Mapping[str, str]) -> None: ...

Generics can be parametrized by using a new factory available in typing
called :class:`TypeVar`.

::

   from typing import Sequence, TypeVar

   T = TypeVar('T')      # Declare type variable

   def first(l: Sequence[T]) -> T:   # Generic function
       return l[0]


User-defined generic types
--------------------------

A user-defined class can be defined as a generic class.

::

   from typing import TypeVar, Generic
   from logging import Logger

   T = TypeVar('T')

   class LoggedVar(Generic[T]):
       def __init__(self, value: T, name: str, logger: Logger) -> None:
           self.name = name
           self.logger = logger
           self.value = value

       def set(self, new: T) -> None:
           self.log('Set ' + repr(self.value))
           self.value = new

       def get(self) -> T:
           self.log('Get ' + repr(self.value))
           return self.value

       def log(self, message: str) -> None:
           self.logger.info('%s: %s', self.name, message)

``Generic[T]`` as a base class defines that the class ``LoggedVar`` takes a
single type parameter ``T`` . This also makes ``T`` valid as a type within the
class body.

The :class:`Generic` base class uses a metaclass that defines
:meth:`__getitem__` so that ``LoggedVar[t]`` is valid as a type::

   from typing import Iterable

   def zero_all_vars(vars: Iterable[LoggedVar[int]]) -> None:
       for var in vars:
           var.set(0)

A generic type can have any number of type variables, and type variables may
be constrained::

   from typing import TypeVar, Generic
   ...

   T = TypeVar('T')
   S = TypeVar('S', int, str)

   class StrangePair(Generic[T, S]):
       ...

Each type variable argument to :class:`Generic` must be distinct.
This is thus invalid::

   from typing import TypeVar, Generic
   ...

   T = TypeVar('T')

   class Pair(Generic[T, T]):   # INVALID
       ...

You can use multiple inheritance with :class:`Generic`::

   from typing import TypeVar, Generic, Sized

   T = TypeVar('T')

   class LinkedList(Sized, Generic[T]):
       ...

When inheriting from generic classes, some type variables could be fixed::

    from typing import TypeVar, Mapping

    T = TypeVar('T')

    class MyDict(Mapping[str, T]):
        ...

In this case ``MyDict`` has a single parameter, ``T``.

Subclassing a generic class without specifying type parameters assumes
:class:`Any` for each position. In the following example, ``MyIterable`` is
not generic but implicitly inherits from ``Iterable[Any]``::

   from typing import Iterable

   class MyIterable(Iterable): # Same as Iterable[Any]

The metaclass used by :class:`Generic` is a subclass of :class:`abc.ABCMeta`.
A generic class can be an ABC by including abstract methods or properties,
and generic classes can also have ABCs as base classes without a metaclass
conflict.  Generic metaclasses are not supported.


The :class:`Any` type
---------------------

A special kind of type is :class:`Any`. A static type checker will treat
every type as being compatible with :class:`Any` and :class:`Any` as being
compatible with every type.

This means that it is possible to perform any operation or method call on a
value of type on :class:`Any` and assign it to any variable::

   from typing import Any

   a = None    # type: Any
   a = []      # OK
   a = 2       # OK

   s = ''      # type: str
   s = a       # OK

   def foo(item: Any) -> int:
       # Typechecks; 'item' could be any type,
       # and that type might have a 'bar' method
       item.bar()
       ...

Notice that no typechecking is performed when assigning a value of type
:class:`Any` to a more precise type. For example, the static type checker did
not report an error when assigning ``a`` to ``s`` even though ``s`` was
declared to be of type :class:`str` and receives an :class:`int` value at
runtime!

Furthermore, all functions without a return type or parameter types will
implicitly default to using :class:`Any`::

   def legacy_parser(text):
       ...
       return data

   # A static type checker will treat the above
   # as having the same signature as:
   def legacy_parser(text: Any) -> Any:
       ...
       return data

This behavior allows :class:`Any` to be used as an *escape hatch* when you
need to mix dynamically and statically typed code.

Contrast the behavior of :class:`Any` with the behavior of :class:`object`.
Similar to :class:`Any`, every type is a subtype of :class:`object`. However,
unlike :class:`Any`, the reverse is not true: :class:`object` is *not* a
subtype of every other type.

That means when the type of a value is :class:`object`, a type checker will
reject almost all operations on it, and assigning it to a variable (or using
it as a return value) of a more specialized type is a type error. For example::

   def hash_a(item: object) -> int:
       # Fails; an object does not have a 'magic' method.
       item.magic()
       ...

   def hash_b(item: Any) -> int:
       # Typechecks
       item.magic()
       ...

   # Typechecks, since ints and strs are subclasses of object
   hash_a(42)
   hash_a("foo")

   # Typechecks, since Any is compatible with all types
   hash_b(42)
   hash_b("foo")

Use :class:`object` to indicate that a value could be any type in a typesafe
manner. Use :class:`Any` to indicate that a value is dynamically typed.

Classes, functions, and decorators
----------------------------------

The module defines the following classes, functions and decorators:

.. class:: Any

   Special type indicating an unconstrained type.

   * Any object is an instance of :class:`Any`.
   * Any class is a subclass of :class:`Any`.
   * As a special case, :class:`Any` and :class:`object` are subclasses of
     each other.

.. class:: TypeVar

    Type variable.

    Usage::

      T = TypeVar('T')  # Can be anything
      A = TypeVar('A', str, bytes)  # Must be str or bytes

    Type variables exist primarily for the benefit of static type
    checkers.  They serve as the parameters for generic types as well
    as for generic function definitions.  See class Generic for more
    information on generic types.  Generic functions work as follows::

       def repeat(x: T, n: int) -> Sequence[T]:
           """Return a list containing n references to x."""
           return [x]*n

       def longest(x: A, y: A) -> A:
           """Return the longest of two strings."""
           return x if len(x) >= len(y) else y

    The latter example's signature is essentially the overloading
    of ``(str, str) -> str`` and ``(bytes, bytes) -> bytes``.  Also note
    that if the arguments are instances of some subclass of :class:`str`,
    the return type is still plain :class:`str`.

    At runtime, ``isinstance(x, T)`` will raise :exc:`TypeError`.  In general,
    :func:`isinstance` and :func:`issubclass` should not be used with types.

    Type variables may be marked covariant or contravariant by passing
    ``covariant=True`` or ``contravariant=True``.  See :pep:`484` for more
    details.  By default type variables are invariant.  Alternatively,
    a type variable may specify an upper bound using ``bound=<type>``.
    This means that an actual type substituted (explicitly or implicitly)
    for the type variable must be a subclass of the boundary type,
    see :pep:`484`.

.. class:: Union

   Union type; ``Union[X, Y]`` means either X or Y.

   To define a union, use e.g. ``Union[int, str]``.  Details:

   * The arguments must be types and there must be at least one.

   * Unions of unions are flattened, e.g.::

       Union[Union[int, str], float] == Union[int, str, float]

   * Unions of a single argument vanish, e.g.::

       Union[int] == int  # The constructor actually returns int

   * Redundant arguments are skipped, e.g.::

       Union[int, str, int] == Union[int, str]

   * When comparing unions, the argument order is ignored, e.g.::

       Union[int, str] == Union[str, int]

   * If :class:`Any` is present it is the sole survivor, e.g.::

       Union[int, Any] == Any

   * You cannot subclass or instantiate a union.

   * You cannot write ``Union[X][Y]``.

   * You can use ``Optional[X]`` as a shorthand for ``Union[X, None]``.

.. class:: Optional

   Optional type.

   ``Optional[X]`` is equivalent to ``Union[X, None]``.

   Note that this is not the same concept as an optional argument,
   which is one that has a default.  An optional argument with a
   default needn't use the ``Optional`` qualifier on its type
   annotation (although it is inferred if the default is ``None``).
   A mandatory argument may still have an ``Optional`` type if an
   explicit value of ``None`` is allowed.

.. class:: Tuple

  Tuple type; ``Tuple[X, Y]`` is the type of a tuple of two items
  with the first item of type X and the second of type Y.

  Example: ``Tuple[T1, T2]`` is a tuple of two elements corresponding
  to type variables T1 and T2.  ``Tuple[int, float, str]`` is a tuple
  of an int, a float and a string.

  To specify a variable-length tuple of homogeneous type,
  use literal ellipsis, e.g. ``Tuple[int, ...]``.

.. class:: Callable

   Callable type; ``Callable[[int], str]`` is a function of (int) -> str.

   The subscription syntax must always be used with exactly two
   values: the argument list and the return type.  The argument list
   must be a list of types; the return type must be a single type.

   There is no syntax to indicate optional or keyword arguments,
   such function types are rarely used as callback types.
   ``Callable[..., ReturnType]`` could be used to type hint a callable
   taking any number of arguments and returning ``ReturnType``.
   A plain :class:`Callable` is equivalent to ``Callable[..., Any]``.

.. class:: Generic

   Abstract base class for generic types.

   A generic type is typically declared by inheriting from an
   instantiation of this class with one or more type variables.
   For example, a generic mapping type might be defined as::

      class Mapping(Generic[KT, VT]):
          def __getitem__(self, key: KT) -> VT:
              ...
              # Etc.

   This class can then be used as follows::

      X = TypeVar('X')
      Y = TypeVar('Y')

      def lookup_name(mapping: Mapping[X, Y], key: X, default: Y) -> Y:
          try:
              return mapping[key]
          except KeyError:
              return default

.. class:: Type

   A variable annotated with ``C`` may accept a value of type ``C``. In
   contrast, a variable annotated with ``Type[C]`` may accept values that are
   classes themselves -- specifically, it will accept the *class object* of
   ``C``. For example::

      a = 3         # Has type 'int'
      b = int       # Has type 'Type[int]'
      c = type(a)   # Also has type 'Type[int]'

   Note that ``Type[C]`` is covariant::

      class User: ...
      class BasicUser(User): ...
      class ProUser(User): ...
      class TeamUser(User): ...

      # Accepts User, BasicUser, ProUser, TeamUser, ...
      def make_new_user(user_class: Type[User]) -> User:
          # ...
          return user_class()

   The fact that ``Type[C]`` is covariant implies that all subclasses of
   ``C`` should implement the same constructor signature and class method
   signatures as ``C``. The type checker should flag violations of this,
   but should also allow constructor calls in subclasses that match the
   constructor calls in the indicated base class. How the type checker is
   required to handle this particular case may change in future revisions of
   :pep:`484`.

   The only legal parameters for :class:`Type` are classes, unions of classes, and
   :class:`Any`. For example::

      def new_non_team_user(user_class: Type[Union[BaseUser, ProUser]]): ...

   ``Type[Any]`` is equivalent to ``Type`` which in turn is equivalent
   to ``type``, which is the root of Python's metaclass hierarchy.

.. class:: Iterable(Generic[T_co])

    A generic version of :class:`collections.abc.Iterable`.

.. class:: Iterator(Iterable[T_co])

    A generic version of :class:`collections.abc.Iterator`.

.. class:: Reversible(Iterable[T_co])

    A generic version of :class:`collections.abc.Reversible`.

.. class:: SupportsInt

    An ABC with one abstract method ``__int__``.

.. class:: SupportsFloat

    An ABC with one abstract method ``__float__``.

.. class:: SupportsAbs

    An ABC with one abstract method ``__abs__`` that is covariant
    in its return type.

.. class:: SupportsRound

    An ABC with one abstract method ``__round__``
    that is covariant in its return type.

.. class:: Container(Generic[T_co])

    A generic version of :class:`collections.abc.Container`.

.. class:: Hashable

   An alias to :class:`collections.abc.Hashable`

.. class:: Sized

   An alias to :class:`collections.abc.Sized`

.. class:: AbstractSet(Sized, Iterable[T_co], Container[T_co])

    A generic version of :class:`collections.abc.Set`.

.. class:: MutableSet(AbstractSet[T])

    A generic version of :class:`collections.abc.MutableSet`.

.. class:: Mapping(Sized, Iterable[KT], Container[KT], Generic[VT_co])

    A generic version of :class:`collections.abc.Mapping`.

.. class:: MutableMapping(Mapping[KT, VT])

    A generic version of :class:`collections.abc.MutableMapping`.

.. class:: Sequence(Sized, Iterable[T_co], Container[T_co])

    A generic version of :class:`collections.abc.Sequence`.

.. class:: MutableSequence(Sequence[T])

   A generic version of :class:`collections.abc.MutableSequence`.

.. class:: ByteString(Sequence[int])

   A generic version of :class:`collections.abc.ByteString`.

   This type represents the types :class:`bytes`, :class:`bytearray`,
   and :class:`memoryview`.

   As a shorthand for this type, :class:`bytes` can be used to
   annotate arguments of any of the types mentioned above.

.. class:: List(list, MutableSequence[T])

   Generic version of :class:`list`.
   Useful for annotating return types. To annotate arguments it is preferred
   to use abstract collection types such as :class:`Mapping`, :class:`Sequence`,
   or :class:`AbstractSet`.

   This type may be used as follows::

      T = TypeVar('T', int, float)

      def vec2(x: T, y: T) -> List[T]:
          return [x, y]

      def keep_positives(vector: Sequence[T]) -> List[T]:
          return [item for item in vector if item > 0]

.. class:: Set(set, MutableSet[T])

   A generic version of :class:`builtins.set <set>`.

.. class:: MappingView(Sized, Iterable[T_co])

   A generic version of :class:`collections.abc.MappingView`.

.. class:: KeysView(MappingView[KT_co], AbstractSet[KT_co])

   A generic version of :class:`collections.abc.KeysView`.

.. class:: ItemsView(MappingView, Generic[KT_co, VT_co])

   A generic version of :class:`collections.abc.ItemsView`.

.. class:: ValuesView(MappingView[VT_co])

   A generic version of :class:`collections.abc.ValuesView`.

.. class:: Awaitable(Generic[T_co])

   A generic version of :class:`collections.abc.Awaitable`.

.. class:: AsyncIterable(Generic[T_co])

   A generic version of :class:`collections.abc.AsyncIterable`.

.. class:: AsyncIterator(AsyncIterable[T_co])

  A generic version of :class:`collections.abc.AsyncIterator`.

.. class:: Dict(dict, MutableMapping[KT, VT])

   A generic version of :class:`dict`.
   The usage of this type is as follows::

      def get_position_in_index(word_list: Dict[str, int], word: str) -> int:
          return word_list[word]

.. class:: DefaultDict(collections.defaultdict, MutableMapping[KT, VT])

   A generic version of :class:`collections.defaultdict`

.. class:: Generator(Iterator[T_co], Generic[T_co, T_contra, V_co])

   A generator can be annotated by the generic type
   ``Generator[YieldType, SendType, ReturnType]``. For example::

      def echo_round() -> Generator[int, float, str]:
          sent = yield 0
          while sent >= 0:
              sent = yield round(sent)
          return 'Done'

   Note that unlike many other generics in the typing module, the ``SendType``
   of :class:`Generator` behaves contravariantly, not covariantly or
   invariantly.

   If your generator will only yield values, set the ``SendType`` and
   ``ReturnType`` to ``None``::

      def infinite_stream(start: int) -> Generator[int, None, None]:
          while True:
              yield start
              start += 1

   Alternatively, annotate your generator as having a return type of
   either ``Iterable[YieldType]`` or ``Iterator[YieldType]``::

      def infinite_stream(start: int) -> Iterator[int]:
          while True:
              yield start
              start += 1

.. class:: AnyStr

   ``AnyStr`` is a type variable defined as
   ``AnyStr = TypeVar('AnyStr', str, bytes)``.

   It is meant to be used for functions that may accept any kind of string
   without allowing different kinds of strings to mix. For example::

      def concat(a: AnyStr, b: AnyStr) -> AnyStr:
          return a + b

      concat(u"foo", u"bar")  # Ok, output has type 'unicode'
      concat(b"foo", b"bar")  # Ok, output has type 'bytes'
      concat(u"foo", b"bar")  # Error, cannot mix unicode and bytes

.. class:: Text

   ``Text`` is an alias for ``str``. It is provided to supply a forward
   compatible path for Python 2 code: in Python 2, ``Text`` is an alias for
   ``unicode``.

   Use ``Text`` to indicate that a value must contain a unicode string in
   a manner that is compatible with both Python 2 and Python 3::

       def add_unicode_checkmark(text: Text) -> Text:
           return text + u' \u2713'

.. class:: io

   Wrapper namespace for I/O stream types.

   This defines the generic type ``IO[AnyStr]`` and aliases ``TextIO``
   and ``BinaryIO`` for respectively ``IO[str]`` and ``IO[bytes]``.
   These representing the types of I/O streams such as returned by
   :func:`open`.

.. class:: re

   Wrapper namespace for regular expression matching types.

   This defines the type aliases ``Pattern`` and ``Match`` which
   correspond to the return types from :func:`re.compile` and
   :func:`re.match`.  These types (and the corresponding functions)
   are generic in ``AnyStr`` and can be made specific by writing
   ``Pattern[str]``, ``Pattern[bytes]``, ``Match[str]``, or
   ``Match[bytes]``.

.. function:: NamedTuple(typename, fields)

   Typed version of namedtuple.

   Usage::

       Employee = typing.NamedTuple('Employee', [('name', str), ('id', int)])

   This is equivalent to::

       Employee = collections.namedtuple('Employee', ['name', 'id'])

   The resulting class has one extra attribute: _field_types,
   giving a dict mapping field names to types.  (The field names
   are in the _fields attribute, which is part of the namedtuple
   API.)

.. function:: NewType(typ)

   A helper function to indicate a distinct types to a typechecker,
   see :ref:`distinct`. At runtime it returns a function that returns
   its argument. Usage::

      UserId = NewType('UserId', int)
      first_user = UserId(1)

.. function:: cast(typ, val)

   Cast a value to a type.

   This returns the value unchanged.  To the type checker this
   signals that the return value has the designated type, but at
   runtime we intentionally don't check anything (we want this
   to be as fast as possible).

.. function:: get_type_hints(obj)

   Return type hints for a function or method object.

   This is often the same as ``obj.__annotations__``, but it handles
   forward references encoded as string literals, and if necessary
   adds ``Optional[t]`` if a default value equal to None is set.

.. decorator:: overload

   The ``@overload`` decorator allows describing functions and methods
   that support multiple different combinations of argument types. A series
   of ``@overload``-decorated definitions must be followed by exactly one
   non-``@overload``-decorated definition (for the same function/method).
   The ``@overload``-decorated definitions are for the benefit of the
   type checker only, since they will be overwritten by the
   non-``@overload``-decorated definition, while the latter is used at
   runtime but should be ignored by a type checker.  At runtime, calling
   a ``@overload``-decorated function directly will raise
   ``NotImplementedError``. An example of overload that gives a more
   precise type than can be expressed using a union or a type variable::

      @overload
      def process(response: None) -> None:
          ...
      @overload
      def process(response: int) -> Tuple[int, str]:
          ...
      @overload
      def process(response: bytes) -> str:
          ...
      def process(response):
          <actual implementation>

   See :pep:`484` for details and comparison with other typing semantics.

.. decorator:: no_type_check(arg)

   Decorator to indicate that annotations are not type hints.

   The argument must be a class or function; if it is a class, it
   applies recursively to all methods defined in that class (but not
   to methods defined in its superclasses or subclasses).

   This mutates the function(s) in place.

.. decorator:: no_type_check_decorator(decorator)

   Decorator to give another decorator the :func:`no_type_check` effect.

   This wraps the decorator with something that wraps the decorated
   function in :func:`no_type_check`.

.. data:: ClassVar

   Special type construct to mark class variables.

   As introduced in :pep:`526`, a variable annotation wrapped in ClassVar
   indicates that a given attribute is intended to be used as a class variable
   and should not be set on instances of that class. Usage::

      class Starship:
          stats: ClassVar[Dict[str, int]] = {} # class variable
          damage: int = 10                     # instance variable

   ClassVar accepts only types and cannot be further subscribed.

   ClassVar is not a class itself, and should not
   be used with isinstance() or issubclass(). Note that ClassVar
   does not change Python runtime behavior, it can be used by
   3rd party type checkers, so that the following code will
   flagged as an error by those::

      enterprise_d = Starship(3000)
      enterprise_d.stats = {} # Error, setting class variable on instance
      Starship.stats = {}     # This is OK

   .. versionadded:: 3.6

.. data:: TYPE_CHECKING

   A special constant that is assumed to be ``True`` by 3rd party static
   type checkers. It is ``False`` at runtime. Usage::

      if TYPE_CHECKING:
          import expensive_mod

      def fun():
          local_var: expensive_mod.some_type = other_fun()