: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: ... NewType ------- Use the ``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 ``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]``. ``None`` as a type hint is a special case and is replaced by ``type(None)``. 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('{}: {}'.format(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=``. 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, type(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:: Iterable(Generic[T_co]) A generic version of the :class:`collections.abc.Iterable`. .. class:: Iterator(Iterable[T_co]) A generic version of the :class:`collections.abc.Iterator`. .. 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:: Reversible An ABC with one abstract method ``__reversed__`` returning an ``Iterator[T_co]``. .. class:: Container(Generic[T_co]) A generic version of :class:`collections.abc.Container`. .. 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 `. .. 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:: 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:: 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 ``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:: 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:: 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`.