
"typing" --- Support for type hints
***********************************

New in version 3.5.

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

======================================================================

This module supports type hints as specified by **PEP 484**.  The most
fundamental support consists of the type "Any", "Union", "Tuple",
"Callable", "TypeVar", and "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 "str" and the return type "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 "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 "Generic" base class uses a metaclass that defines "__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 "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 "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
"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 "Generic" is a subclass of "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 "Any" type
==============

A special kind of type is "Any". A static type checker will treat
every type as being compatible with "Any" and "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 "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 "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 "str" and receives an "int" value at
runtime!

Furthermore, all functions without a return type or parameter types
will implicitly default to using "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 "Any" to be used as an *escape hatch* when you
need to mix dynamically and statically typed code.

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

That means when the type of a value is "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 "object" to indicate that a value could be any type in a typesafe
manner. Use "Any" to indicate that a value is dynamically typed.


Classes, functions, and decorators
==================================

The module defines the following classes, functions and decorators:

class typing.Any

   Special type indicating an unconstrained type.

   * Any object is an instance of "Any".

   * Any class is a subclass of "Any".

   * As a special case, "Any" and "object" are subclasses of each
     other.

class typing.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 "str", the
   return type is still plain "str".

   At runtime, "isinstance(x, T)" will raise "TypeError".  In general,
   "isinstance()" and "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 typing.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 "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 typing.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 typing.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 typing.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 "Callable"
   is equivalent to "Callable[..., Any]".

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

   A generic version of the "collections.abc.Iterable".

class typing.Iterator(Iterable[T_co])

   A generic version of the "collections.abc.Iterator".

class typing.SupportsInt

   An ABC with one abstract method "__int__".

class typing.SupportsFloat

   An ABC with one abstract method "__float__".

class typing.SupportsAbs

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

class typing.SupportsRound

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

class typing.Reversible

   An ABC with one abstract method "__reversed__" returning an
   "Iterator[T_co]".

class typing.Container(Generic[T_co])

   A generic version of "collections.abc.Container".

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

   A generic version of "collections.abc.Set".

class typing.MutableSet(AbstractSet[T])

   A generic version of "collections.abc.MutableSet".

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

   A generic version of "collections.abc.Mapping".

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

   A generic version of "collections.abc.MutableMapping".

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

   A generic version of "collections.abc.Sequence".

class typing.MutableSequence(Sequence[T])

   A generic version of "collections.abc.MutableSequence".

class typing.ByteString(Sequence[int])

   A generic version of "collections.abc.ByteString".

   This type represents the types "bytes", "bytearray", and
   "memoryview".

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

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

   Generic version of "list". Useful for annotating return types. To
   annotate arguments it is preferred to use abstract collection types
   such as "Mapping", "Sequence", or "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 typing.Set(set, MutableSet[T])

   A generic version of "builtins.set".

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

   A generic version of "collections.abc.MappingView".

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

   A generic version of "collections.abc.KeysView".

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

   A generic version of "collections.abc.ItemsView".

class typing.ValuesView(MappingView[VT_co])

   A generic version of "collections.abc.ValuesView".

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

   A generic version of "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 typing.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 "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 typing.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 "open()".

class typing.re

   Wrapper namespace for regular expression matching types.

   This defines the type aliases "Pattern" and "Match" which
   correspond to the return types from "re.compile()" and
   "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]".

typing.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.)

typing.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).

typing.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.

@typing.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.

@typing.no_type_check_decorator(decorator)

   Decorator to give another decorator the "no_type_check()" effect.

   This wraps the decorator with something that wraps the decorated
   function in "no_type_check()".
