PEP: 484 Title: Type Hints Version: $Revision$ Last-Modified: $Date$
Author: Guido van Rossum <guido@python.org>, Jukka Lehtosalo
<jukka.lehtosalo@iki.fi>, Łukasz Langa <lukasz@python.org>
BDFL-Delegate: Mark Shannon Discussions-To: python-dev@python.org
Status: Final Type: Standards Track Topic: Typing Content-Type:
text/x-rst Created: 29-Sep-2014 Python-Version: 3.5 Post-History:
16-Jan-2015, 20-Mar-2015, 17-Apr-2015, 20-May-2015, 22-May-2015
Resolution:
https://mail.python.org/pipermail/python-dev/2015-May/140104.html

Abstract

PEP 3107 introduced syntax for function annotations, but the semantics
were deliberately left undefined. There has now been enough 3rd party
usage for static type analysis that the community would benefit from a
standard vocabulary and baseline tools within the standard library.

This PEP introduces a provisional module to provide these standard
definitions and tools, along with some conventions for situations where
annotations are not available.

Note that this PEP still explicitly does NOT prevent other uses of
annotations, nor does it require (or forbid) any particular processing
of annotations, even when they conform to this specification. It simply
enables better coordination, as PEP 333 did for web frameworks.

For example, here is a simple function whose argument and return type
are declared in the annotations:

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

While these annotations are available at runtime through the usual
__annotations__ attribute, no type checking happens at runtime. Instead,
the proposal assumes the existence of a separate off-line type checker
which users can run over their source code voluntarily. Essentially,
such a type checker acts as a very powerful linter. (While it would of
course be possible for individual users to employ a similar checker at
run time for Design By Contract enforcement or JIT optimization, those
tools are not yet as mature.)

The proposal is strongly inspired by mypy. For example, the type
"sequence of integers" can be written as Sequence[int]. The square
brackets mean that no new syntax needs to be added to the language. The
example here uses a custom type Sequence, imported from a pure-Python
module typing. The Sequence[int] notation works at runtime by
implementing __getitem__() in the metaclass (but its significance is
primarily to an offline type checker).

The type system supports unions, generic types, and a special type named
Any which is consistent with (i.e. assignable to and from) all types.
This latter feature is taken from the idea of gradual typing. Gradual
typing and the full type system are explained in PEP 483.

Other approaches from which we have borrowed or to which ours can be
compared and contrasted are described in PEP 482.

Rationale and Goals

PEP 3107 added support for arbitrary annotations on parts of a function
definition. Although no meaning was assigned to annotations then, there
has always been an implicit goal to use them for type hinting, which is
listed as the first possible use case in said PEP.

This PEP aims to provide a standard syntax for type annotations, opening
up Python code to easier static analysis and refactoring, potential
runtime type checking, and (perhaps, in some contexts) code generation
utilizing type information.

Of these goals, static analysis is the most important. This includes
support for off-line type checkers such as mypy, as well as providing a
standard notation that can be used by IDEs for code completion and
refactoring.

Non-goals

While the proposed typing module will contain some building blocks for
runtime type checking -- in particular the get_type_hints() function --
third party packages would have to be developed to implement specific
runtime type checking functionality, for example using decorators or
metaclasses. Using type hints for performance optimizations is left as
an exercise for the reader.

It should also be emphasized that Python will remain a dynamically typed
language, and the authors have no desire to ever make type hints
mandatory, even by convention.

The meaning of annotations

Any function without annotations should be treated as having the most
general type possible, or ignored, by any type checker. Functions with
the @no_type_check decorator should be treated as having no annotations.

It is recommended but not required that checked functions have
annotations for all arguments and the return type. For a checked
function, the default annotation for arguments and for the return type
is Any. An exception is the first argument of instance and class
methods. If it is not annotated, then it is assumed to have the type of
the containing class for instance methods, and a type object type
corresponding to the containing class object for class methods. For
example, in class A the first argument of an instance method has the
implicit type A. In a class method, the precise type of the first
argument cannot be represented using the available type notation.

(Note that the return type of __init__ ought to be annotated with
-> None. The reason for this is subtle. If __init__ assumed a return
annotation of -> None, would that mean that an argument-less,
un-annotated __init__ method should still be type-checked? Rather than
leaving this ambiguous or introducing an exception to the exception, we
simply say that __init__ ought to have a return annotation; the default
behavior is thus the same as for other methods.)

A type checker is expected to check the body of a checked function for
consistency with the given annotations. The annotations may also be used
to check correctness of calls appearing in other checked functions.

Type checkers are expected to attempt to infer as much information as
necessary. The minimum requirement is to handle the builtin decorators
@property, @staticmethod and @classmethod.

Type Definition Syntax

The syntax leverages PEP 3107-style annotations with a number of
extensions described in sections below. In its basic form, type hinting
is used by filling function annotation slots with classes:

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

This states that the expected type of the name argument is str.
Analogically, the expected return type is str.

Expressions whose type is a subtype of a specific argument type are also
accepted for that argument.

Acceptable type hints

Type hints may be built-in classes (including those defined in standard
library or third-party extension modules), abstract base classes, types
available in the types module, and user-defined classes (including those
defined in the standard library or third-party modules).

While annotations are normally the best format for type hints, there are
times when it is more appropriate to represent them by a special
comment, or in a separately distributed stub file. (See below for
examples.)

Annotations must be valid expressions that evaluate without raising
exceptions at the time the function is defined (but see below for
forward references).

Annotations should be kept simple or static analysis tools may not be
able to interpret the values. For example, dynamically computed types
are unlikely to be understood. (This is an intentionally somewhat vague
requirement, specific inclusions and exclusions may be added to future
versions of this PEP as warranted by the discussion.)

In addition to the above, the following special constructs defined below
may be used: None, Any, Union, Tuple, Callable, all ABCs and stand-ins
for concrete classes exported from typing (e.g. Sequence and Dict), type
variables, and type aliases.

All newly introduced names used to support features described in
following sections (such as Any and Union) are available in the typing
module.

Using None

When used in a type hint, the expression None is considered equivalent
to type(None).

Type aliases

Type aliases are defined by simple variable assignments:

    Url = str

    def retry(url: Url, retry_count: int) -> None: ...

Note that we recommend capitalizing alias names, since they represent
user-defined types, which (like user-defined classes) are typically
spelled that way.

Type aliases may be as complex as type hints in annotations --anything
that is acceptable as a type hint is acceptable in a type alias:

    from typing import TypeVar, Iterable, Tuple

    T = TypeVar('T', int, float, complex)
    Vector = Iterable[Tuple[T, T]]

    def inproduct(v: Vector[T]) -> T:
        return sum(x*y for x, y in v)
    def dilate(v: Vector[T], scale: T) -> Vector[T]:
        return ((x * scale, y * scale) for x, y in v)
    vec = []  # type: Vector[float]

This is equivalent to:

    from typing import TypeVar, Iterable, Tuple

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

    def inproduct(v: Iterable[Tuple[T, T]]) -> T:
        return sum(x*y for x, y in v)
    def dilate(v: Iterable[Tuple[T, T]], scale: T) -> Iterable[Tuple[T, T]]:
        return ((x * scale, y * scale) for x, y in v)
    vec = []  # type: Iterable[Tuple[float, float]]

Callable

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

    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 (three
dots) for the list of arguments:

    def partial(func: Callable[..., str], *args) -> Callable[..., str]:
        # Body

Note that there are no square brackets around the ellipsis. The
arguments of the callback are completely unconstrained in this case (and
keyword arguments are acceptable).

Since using callbacks with keyword arguments is not perceived as a
common use case, there is currently no support for specifying keyword
arguments with Callable. Similarly, there is no support for specifying
callback signatures with a variable number of arguments of a specific
type.

Because typing.Callable does double-duty as a replacement for
collections.abc.Callable, isinstance(x, typing.Callable) is implemented
by deferring to isinstance(x, collections.abc.Callable). However,
isinstance(x, typing.Callable[...]) is not supported.

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. Example:

    from typing import Mapping, Set

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

Generics can be parameterized by using a new factory available in typing
called TypeVar. Example:

    from typing import Sequence, TypeVar

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

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

In this case the contract is that the returned value is consistent with
the elements held by the collection.

A TypeVar() expression must always directly be assigned to a variable
(it should not be used as part of a larger expression). The argument to
TypeVar() must be a string equal to the variable name to which it is
assigned. Type variables must not be redefined.

TypeVar supports constraining parametric types to a fixed set of
possible types (note: those types cannot be parameterized by type
variables). For example, we can define a type variable that ranges over
just str and bytes. By default, a type variable ranges over all possible
types. Example of constraining a type variable:

    from typing import TypeVar, Text

    AnyStr = TypeVar('AnyStr', Text, bytes)

    def concat(x: AnyStr, y: AnyStr) -> AnyStr:
        return x + y

The function concat can be called with either two str arguments or two
bytes arguments, but not with a mix of str and bytes arguments.

There should be at least two constraints, if any; specifying a single
constraint is disallowed.

Subtypes of types constrained by a type variable should be treated as
their respective explicitly listed base types in the context of the type
variable. Consider this example:

    class MyStr(str): ...

    x = concat(MyStr('apple'), MyStr('pie'))

The call is valid but the type variable AnyStr will be set to str and
not MyStr. In effect, the inferred type of the return value assigned to
x will also be str.

Additionally, Any is a valid value for every type variable. Consider the
following:

    def count_truthy(elements: List[Any]) -> int:
        return sum(1 for elem in elements if elem)

This is equivalent to omitting the generic notation and just saying
elements: List.

User-defined generic types

You can include a Generic base class to define a user-defined class as
generic. Example:

    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. This is valid:

    from typing import TypeVar, Generic
    ...

    T = TypeVar('T')
    S = TypeVar('S')

    class Pair(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
        ...

The Generic[T] base class is redundant in simple cases where you
subclass some other generic class and specify type variables for its
parameters:

    from typing import TypeVar, Iterator

    T = TypeVar('T')

    class MyIter(Iterator[T]):
        ...

That class definition is equivalent to:

    class MyIter(Iterator[T], Generic[T]):
        ...

You can use multiple inheritance with Generic:

    from typing import TypeVar, Generic, Sized, Iterable, Container, Tuple

    T = TypeVar('T')

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

    K = TypeVar('K')
    V = TypeVar('V')

    class MyMapping(Iterable[Tuple[K, V]],
                    Container[Tuple[K, V]],
                    Generic[K, V]):
        ...

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]
        ...

Generic metaclasses are not supported.

Scoping rules for type variables

Type variables follow normal name resolution rules. However, there are
some special cases in the static typechecking context:

-   A type variable used in a generic function could be inferred to
    represent different types in the same code block. Example:

        from typing import TypeVar, Generic

        T = TypeVar('T')

        def fun_1(x: T) -> T: ...  # T here
        def fun_2(x: T) -> T: ...  # and here could be different

        fun_1(1)                   # This is OK, T is inferred to be int
        fun_2('a')                 # This is also OK, now T is str

-   A type variable used in a method of a generic class that coincides
    with one of the variables that parameterize this class is always
    bound to that variable. Example:

        from typing import TypeVar, Generic

        T = TypeVar('T')

        class MyClass(Generic[T]):
            def meth_1(self, x: T) -> T: ...  # T here
            def meth_2(self, x: T) -> T: ...  # and here are always the same

        a = MyClass()  # type: MyClass[int]
        a.meth_1(1)    # OK
        a.meth_2('a')  # This is an error!

-   A type variable used in a method that does not match any of the
    variables that parameterize the class makes this method a generic
    function in that variable:

        T = TypeVar('T')
        S = TypeVar('S')
        class Foo(Generic[T]):
            def method(self, x: T, y: S) -> S:
                ...

        x = Foo()               # type: Foo[int]
        y = x.method(0, "abc")  # inferred type of y is str

-   Unbound type variables should not appear in the bodies of generic
    functions, or in the class bodies apart from method definitions:

        T = TypeVar('T')
        S = TypeVar('S')

        def a_fun(x: T) -> None:
            # this is OK
            y = []  # type: List[T]
            # but below is an error!
            y = []  # type: List[S]

        class Bar(Generic[T]):
            # this is also an error
            an_attr = []  # type: List[S]

            def do_something(x: S) -> S:  # this is OK though
                ...

-   A generic class definition that appears inside a generic function
    should not use type variables that parameterize the generic
    function:

        from typing import List

        def a_fun(x: T) -> None:

            # This is OK
            a_list = []  # type: List[T]
            ...

            # This is however illegal
            class MyGeneric(Generic[T]):
                ...

-   A generic class nested in another generic class cannot use same type
    variables. The scope of the type variables of the outer class
    doesn't cover the inner one:

        T = TypeVar('T')
        S = TypeVar('S')

        class Outer(Generic[T]):
            class Bad(Iterable[T]):       # Error
                ...
            class AlsoBad:
                x = None  # type: List[T] # Also an error

            class Inner(Iterable[S]):     # OK
                ...
            attr = None  # type: Inner[T] # Also OK

Instantiating generic classes and type erasure

User-defined generic classes can be instantiated. Suppose we write a
Node class inheriting from Generic[T]:

    from typing import TypeVar, Generic

    T = TypeVar('T')

    class Node(Generic[T]):
        ...

To create Node instances you call Node() just as for a regular class. At
runtime the type (class) of the instance will be Node. But what type
does it have to the type checker? The answer depends on how much
information is available in the call. If the constructor (__init__ or
__new__) uses T in its signature, and a corresponding argument value is
passed, the type of the corresponding argument(s) is substituted.
Otherwise, Any is assumed. Example:

    from typing import TypeVar, Generic

    T = TypeVar('T')

    class Node(Generic[T]):
        x = None  # type: T # Instance attribute (see below)
        def __init__(self, label: T = None) -> None:
            ...

    x = Node('')  # Inferred type is Node[str]
    y = Node(0)   # Inferred type is Node[int]
    z = Node()    # Inferred type is Node[Any]

In case the inferred type uses [Any] but the intended type is more
specific, you can use a type comment (see below) to force the type of
the variable, e.g.:

    # (continued from previous example)
    a = Node()  # type: Node[int]
    b = Node()  # type: Node[str]

Alternatively, you can instantiate a specific concrete type, e.g.:

    # (continued from previous example)
    p = Node[int]()
    q = Node[str]()
    r = Node[int]('')  # Error
    s = Node[str](0)   # Error

Note that the runtime type (class) of p and q is still just Node --
Node[int] and Node[str] are distinguishable class objects, but the
runtime class of the objects created by instantiating them doesn't
record the distinction. This behavior is called "type erasure"; it is
common practice in languages with generics (e.g. Java, TypeScript).

Using generic classes (parameterized or not) to access attributes will
result in type check failure. Outside the class definition body, a class
attribute cannot be assigned, and can only be looked up by accessing it
through a class instance that does not have an instance attribute with
the same name:

    # (continued from previous example)
    Node[int].x = 1  # Error
    Node[int].x      # Error
    Node.x = 1       # Error
    Node.x           # Error
    type(p).x        # Error
    p.x              # Ok (evaluates to None)
    Node[int]().x    # Ok (evaluates to None)
    p.x = 1          # Ok, but assigning to instance attribute

Generic versions of abstract collections like Mapping or Sequence and
generic versions of built-in classes -- List, Dict, Set, and FrozenSet
-- cannot be instantiated. However, concrete user-defined subclasses
thereof and generic versions of concrete collections can be
instantiated:

    data = DefaultDict[int, bytes]()

Note that one should not confuse static types and runtime classes. The
type is still erased in this case and the above expression is just a
shorthand for:

    data = collections.defaultdict()  # type: DefaultDict[int, bytes]

It is not recommended to use the subscripted class (e.g. Node[int])
directly in an expression -- using a type alias (e.g.
IntNode = Node[int]) instead is preferred. (First, creating the
subscripted class, e.g. Node[int], has a runtime cost. Second, using a
type alias is more readable.)

Arbitrary generic types as base classes

Generic[T] is only valid as a base class -- it's not a proper type.
However, user-defined generic types such as LinkedList[T] from the above
example and built-in generic types and ABCs such as List[T] and
Iterable[T] are valid both as types and as base classes. For example, we
can define a subclass of Dict that specializes type arguments:

    from typing import Dict, List, Optional

    class Node:
        ...

    class SymbolTable(Dict[str, List[Node]]):
        def push(self, name: str, node: Node) -> None:
            self.setdefault(name, []).append(node)

        def pop(self, name: str) -> Node:
            return self[name].pop()

        def lookup(self, name: str) -> Optional[Node]:
            nodes = self.get(name)
            if nodes:
                return nodes[-1]
            return None

SymbolTable is a subclass of dict and a subtype of
Dict[str, List[Node]].

If a generic base class has a type variable as a type argument, this
makes the defined class generic. For example, we can define a generic
LinkedList class that is iterable and a container:

    from typing import TypeVar, Iterable, Container

    T = TypeVar('T')

    class LinkedList(Iterable[T], Container[T]):
        ...

Now LinkedList[int] is a valid type. Note that we can use T multiple
times in the base class list, as long as we don't use the same type
variable T multiple times within Generic[...].

Also consider the following example:

    from typing import TypeVar, Mapping

    T = TypeVar('T')

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

In this case MyDict has a single parameter, T.

Abstract generic types

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.

Type variables with an upper bound

A type variable may specify an upper bound using bound=<type> (note:
<type> itself cannot be parameterized by type variables). This means
that an actual type substituted (explicitly or implicitly) for the type
variable must be a subtype of the boundary type. Example:

    from typing import TypeVar, Sized

    ST = TypeVar('ST', bound=Sized)

    def longer(x: ST, y: ST) -> ST:
        if len(x) > len(y):
            return x
        else:
            return y

    longer([1], [1, 2])  # ok, return type List[int]
    longer({1}, {1, 2})  # ok, return type Set[int]
    longer([1], {1, 2})  # ok, return type Collection[int]

An upper bound cannot be combined with type constraints (as in used
AnyStr, see the example earlier); type constraints cause the inferred
type to be _exactly one of the constraint types, while an upper bound
just requires that the actual type is a subtype of the boundary type.

Covariance and contravariance

Consider a class Employee with a subclass Manager. Now suppose we have a
function with an argument annotated with List[Employee]. Should we be
allowed to call this function with a variable of type List[Manager] as
its argument? Many people would answer "yes, of course" without even
considering the consequences. But unless we know more about the
function, a type checker should reject such a call: the function might
append an Employee instance to the list, which would violate the
variable's type in the caller.

It turns out such an argument acts contravariantly, whereas the
intuitive answer (which is correct in case the function doesn't mutate
its argument!) requires the argument to act covariantly. A longer
introduction to these concepts can be found on Wikipedia and in PEP 483;
here we just show how to control a type checker's behavior.

By default generic types are considered invariant in all type variables,
which means that values for variables annotated with types like
List[Employee] must exactly match the type annotation -- no subclasses
or superclasses of the type parameter (in this example Employee) are
allowed.

To facilitate the declaration of container types where covariant or
contravariant type checking is acceptable, type variables accept keyword
arguments covariant=True or contravariant=True. At most one of these may
be passed. Generic types defined with such variables are considered
covariant or contravariant in the corresponding variable. By convention,
it is recommended to use names ending in _co for type variables defined
with covariant=True and names ending in _contra for that defined with
contravariant=True.

A typical example involves defining an immutable (or read-only)
container class:

    from typing import TypeVar, Generic, Iterable, Iterator

    T_co = TypeVar('T_co', covariant=True)

    class ImmutableList(Generic[T_co]):
        def __init__(self, items: Iterable[T_co]) -> None: ...
        def __iter__(self) -> Iterator[T_co]: ...
        ...

    class Employee: ...

    class Manager(Employee): ...

    def dump_employees(emps: ImmutableList[Employee]) -> None:
        for emp in emps:
            ...

    mgrs = ImmutableList([Manager()])  # type: ImmutableList[Manager]
    dump_employees(mgrs)  # OK

The read-only collection classes in typing are all declared covariant in
their type variable (e.g. Mapping and Sequence). The mutable collection
classes (e.g. MutableMapping and MutableSequence) are declared
invariant. The one example of a contravariant type is the Generator
type, which is contravariant in the send() argument type (see below).

Note: Covariance or contravariance is not a property of a type variable,
but a property of a generic class defined using this variable. Variance
is only applicable to generic types; generic functions do not have this
property. The latter should be defined using only type variables without
covariant or contravariant keyword arguments. For example, the following
example is fine:

    from typing import TypeVar

    class Employee: ...

    class Manager(Employee): ...

    E = TypeVar('E', bound=Employee)

    def dump_employee(e: E) -> None: ...

    dump_employee(Manager())  # OK

while the following is prohibited:

    B_co = TypeVar('B_co', covariant=True)

    def bad_func(x: B_co) -> B_co:  # Flagged as error by a type checker
        ...

The numeric tower

PEP 3141 defines Python's numeric tower, and the stdlib module numbers
implements the corresponding ABCs (Number, Complex, Real, Rational and
Integral). There are some issues with these ABCs, but the built-in
concrete numeric classes complex, float and int are ubiquitous
(especially the latter two :-).

Rather than requiring that users write import numbers and then use
numbers.Float etc., this PEP proposes a straightforward shortcut that is
almost as effective: when an argument is annotated as having type float,
an argument of type int is acceptable; similar, for an argument
annotated as having type complex, arguments of type float or int are
acceptable. This does not handle classes implementing the corresponding
ABCs or the fractions.Fraction class, but we believe those use cases are
exceedingly rare.

Forward references

When a type hint contains names that have not been defined yet, that
definition may be expressed as a string literal, to be resolved later.

A situation where this occurs commonly is the definition of a container
class, where the class being defined occurs in the signature of some of
the methods. For example, the following code (the start of a simple
binary tree implementation) does not work:

    class Tree:
        def __init__(self, left: Tree, right: Tree):
            self.left = left
            self.right = right

To address this, we write:

    class Tree:
        def __init__(self, left: 'Tree', right: 'Tree'):
            self.left = left
            self.right = right

The string literal should contain a valid Python expression (i.e.,
compile(lit, '', 'eval') should be a valid code object) and it should
evaluate without errors once the module has been fully loaded. The local
and global namespace in which it is evaluated should be the same
namespaces in which default arguments to the same function would be
evaluated.

Moreover, the expression should be parseable as a valid type hint, i.e.,
it is constrained by the rules from the section Acceptable type hints
above.

It is allowable to use string literals as part of a type hint, for
example:

    class Tree:
        ...
        def leaves(self) -> List['Tree']:
            ...

A common use for forward references is when e.g. Django models are
needed in the signatures. Typically, each model is in a separate file,
and has methods taking arguments whose type involves other models.
Because of the way circular imports work in Python, it is often not
possible to import all the needed models directly:

    # File models/a.py
    from models.b import B
    class A(Model):
        def foo(self, b: B): ...

    # File models/b.py
    from models.a import A
    class B(Model):
        def bar(self, a: A): ...

    # File main.py
    from models.a import A
    from models.b import B

Assuming main is imported first, this will fail with an ImportError at
the line from models.a import A in models/b.py, which is being imported
from models/a.py before a has defined class A. The solution is to switch
to module-only imports and reference the models by their _module._class
name:

    # File models/a.py
    from models import b
    class A(Model):
        def foo(self, b: 'b.B'): ...

    # File models/b.py
    from models import a
    class B(Model):
        def bar(self, a: 'a.A'): ...

    # File main.py
    from models.a import A
    from models.b import B

Union types

Since accepting a small, limited set of expected types for a single
argument is common, there is a new special factory called Union.
Example:

    from typing import Union

    def handle_employees(e: Union[Employee, Sequence[Employee]]) -> None:
        if isinstance(e, Employee):
            e = [e]
        ...

A type factored by Union[T1, T2, ...] is a supertype of all types T1,
T2, etc., so that a value that is a member of one of these types is
acceptable for an argument annotated by Union[T1, T2, ...].

One common case of union types are optional types. By default, None is
an invalid value for any type, unless a default value of None has been
provided in the function definition. Examples:

    def handle_employee(e: Union[Employee, None]) -> None: ...

As a shorthand for Union[T1, None] you can write Optional[T1]; for
example, the above is equivalent to:

    from typing import Optional

    def handle_employee(e: Optional[Employee]) -> None: ...

A past version of this PEP allowed type checkers to assume an optional
type when the default value is None, as in this code:

    def handle_employee(e: Employee = None): ...

This would have been treated as equivalent to:

    def handle_employee(e: Optional[Employee] = None) -> None: ...

This is no longer the recommended behavior. Type checkers should move
towards requiring the optional type to be made explicit.

Support for singleton types in unions

A singleton instance is frequently used to mark some special condition,
in particular in situations where None is also a valid value for a
variable. Example:

    _empty = object()

    def func(x=_empty):
        if x is _empty:  # default argument value
            return 0
        elif x is None:  # argument was provided and it's None
            return 1
        else:
            return x * 2

To allow precise typing in such situations, the user should use the
Union type in conjunction with the enum.Enum class provided by the
standard library, so that type errors can be caught statically:

    from typing import Union
    from enum import Enum

    class Empty(Enum):
        token = 0
    _empty = Empty.token

    def func(x: Union[int, None, Empty] = _empty) -> int:

        boom = x * 42  # This fails type check

        if x is _empty:
            return 0
        elif x is None:
            return 1
        else:  # At this point typechecker knows that x can only have type int
            return x * 2

Since the subclasses of Enum cannot be further subclassed, the type of
variable x can be statically inferred in all branches of the above
example. The same approach is applicable if more than one singleton
object is needed: one can use an enumeration that has more than one
value:

    class Reason(Enum):
        timeout = 1
        error = 2

    def process(response: Union[str, Reason] = '') -> str:
        if response is Reason.timeout:
            return 'TIMEOUT'
        elif response is Reason.error:
            return 'ERROR'
        else:
            # response can be only str, all other possible values exhausted
            return 'PROCESSED: ' + response

The Any type

A special kind of type is Any. Every type is consistent with Any. It can
be considered a type that has all values and all methods. Note that Any
and builtin type object are completely different.

When the type of a value is object, the 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. On the other
hand, when a value has type Any, the type checker will allow all
operations on it, and a value of type Any can be assigned to a variable
(or used as a return value) of a more constrained type.

A function parameter without an annotation is assumed to be annotated
with Any. If a generic type is used without specifying type parameters,
they are assumed to be Any:

    from typing import Mapping

    def use_map(m: Mapping) -> None:  # Same as Mapping[Any, Any]
        ...

This rule also applies to Tuple, in annotation context it is equivalent
to Tuple[Any, ...] and, in turn, to tuple. As well, a bare Callable in
an annotation is equivalent to Callable[..., Any] and, in turn, to
collections.abc.Callable:

    from typing import Tuple, List, Callable

    def check_args(args: Tuple) -> bool:
        ...

    check_args(())           # OK
    check_args((42, 'abc'))  # Also OK
    check_args(3.14)         # Flagged as error by a type checker

    # A list of arbitrary callables is accepted by this function
    def apply_callbacks(cbs: List[Callable]) -> None:
        ...

The NoReturn type

The typing module provides a special type NoReturn to annotate functions
that never return normally. For example, a function that unconditionally
raises an exception:

    from typing import NoReturn

    def stop() -> NoReturn:
        raise RuntimeError('no way')

The NoReturn annotation is used for functions such as sys.exit. Static
type checkers will ensure that functions annotated as returning NoReturn
truly never return, either implicitly or explicitly:

    import sys
    from typing import NoReturn

      def f(x: int) -> NoReturn:  # Error, f(0) implicitly returns None
          if x != 0:
              sys.exit(1)

The checkers will also recognize that the code after calls to such
functions is unreachable and will behave accordingly:

    # continue from first example
    def g(x: int) -> int:
        if x > 0:
            return x
        stop()
        return 'whatever works'  # Error might be not reported by some checkers
                                 # that ignore errors in unreachable blocks

The NoReturn type is only valid as a return annotation of functions, and
considered an error if it appears in other positions:

    from typing import List, NoReturn

    # All of the following are errors
    def bad1(x: NoReturn) -> int:
        ...
    bad2 = None  # type: NoReturn
    def bad3() -> List[NoReturn]:
        ...

The type of class objects

Sometimes you want to talk about class objects, in particular class
objects that inherit from a given class. This can be spelled as Type[C]
where C is a class. To clarify: while C (when used as an annotation)
refers to instances of class C, Type[C] refers to subclasses of C. (This
is a similar distinction as between object and type.)

For example, suppose we have the following classes:

    class User: ...  # Abstract base for User classes
    class BasicUser(User): ...
    class ProUser(User): ...
    class TeamUser(User): ...

And suppose we have a function that creates an instance of one of these
classes if you pass it a class object:

    def new_user(user_class):
        user = user_class()
        # (Here we could write the user object to a database)
        return user

Without Type[] the best we could do to annotate new_user() would be:

    def new_user(user_class: type) -> User:
        ...

However using Type[] and a type variable with an upper bound we can do
much better:

    U = TypeVar('U', bound=User)
    def new_user(user_class: Type[U]) -> U:
        ...

Now when we call new_user() with a specific subclass of User a type
checker will infer the correct type of the result:

    joe = new_user(BasicUser)  # Inferred type is BasicUser

The value corresponding to Type[C] must be an actual class object that's
a subtype of C, not a special form. In other words, in the above example
calling e.g. new_user(Union[BasicUser, ProUser]) is rejected by the type
checker (in addition to failing at runtime because you can't instantiate
a union).

Note that it is legal to use a union of classes as the parameter for
Type[], as in:

    def new_non_team_user(user_class: Type[Union[BasicUser, ProUser]]):
        user = new_user(user_class)
        ...

However the actual argument passed in at runtime must still be a
concrete class object, e.g. in the above example:

    new_non_team_user(ProUser)  # OK
    new_non_team_user(TeamUser)  # Disallowed by type checker

Type[Any] is also supported (see below for its meaning).

Type[T] where T is a type variable is allowed when annotating the first
argument of a class method (see the relevant section).

Any other special constructs like Tuple or Callable are not allowed as
an argument to Type.

There are some concerns with this feature: for example when new_user()
calls user_class() this implies that all subclasses of User must support
this in their constructor signature. However this is not unique to
Type[]: class methods have similar concerns. A type checker ought to
flag violations of such assumptions, but by default constructor calls
that match the constructor signature in the indicated base class (User
in the example above) should be allowed. A program containing a complex
or extensible class hierarchy might also handle this by using a factory
class method. A future revision of this PEP may introduce better ways of
dealing with these concerns.

When Type is parameterized it requires exactly one parameter. Plain Type
without brackets is equivalent to Type[Any] and this in turn is
equivalent to type (the root of Python's metaclass hierarchy). This
equivalence also motivates the name, Type, as opposed to alternatives
like Class or SubType, which were proposed while this feature was under
discussion; this is similar to the relationship between e.g. List and
list.

Regarding the behavior of Type[Any] (or Type or type), accessing
attributes of a variable with this type only provides attributes and
methods defined by type (for example, __repr__() and __mro__). Such a
variable can be called with arbitrary arguments, and the return type is
Any.

Type is covariant in its parameter, because Type[Derived] is a subtype
of Type[Base]:

    def new_pro_user(pro_user_class: Type[ProUser]):
        user = new_user(pro_user_class)  # OK
        ...

Annotating instance and class methods

In most cases the first argument of class and instance methods does not
need to be annotated, and it is assumed to have the type of the
containing class for instance methods, and a type object type
corresponding to the containing class object for class methods. In
addition, the first argument in an instance method can be annotated with
a type variable. In this case the return type may use the same type
variable, thus making that method a generic function. For example:

    T = TypeVar('T', bound='Copyable')
    class Copyable:
        def copy(self: T) -> T:
            # return a copy of self

    class C(Copyable): ...
    c = C()
    c2 = c.copy()  # type here should be C

The same applies to class methods using Type[] in an annotation of the
first argument:

    T = TypeVar('T', bound='C')
    class C:
        @classmethod
        def factory(cls: Type[T]) -> T:
            # make a new instance of cls

    class D(C): ...
    d = D.factory()  # type here should be D

Note that some type checkers may apply restrictions on this use, such as
requiring an appropriate upper bound for the type variable used (see
examples).

Version and platform checking

Type checkers are expected to understand simple version and platform
checks, e.g.:

    import sys

    if sys.version_info[0] >= 3:
        # Python 3 specific definitions
    else:
        # Python 2 specific definitions

    if sys.platform == 'win32':
        # Windows specific definitions
    else:
        # Posix specific definitions

Don't expect a checker to understand obfuscations like
"".join(reversed(sys.platform)) == "xunil".

Runtime or type checking?

Sometimes there's code that must be seen by a type checker (or other
static analysis tools) but should not be executed. For such situations
the typing module defines a constant, TYPE_CHECKING, that is considered
True during type checking (or other static analysis) but False at
runtime. Example:

    import typing

    if typing.TYPE_CHECKING:
        import expensive_mod

    def a_func(arg: 'expensive_mod.SomeClass') -> None:
        a_var = arg  # type: expensive_mod.SomeClass
        ...

(Note that the type annotation must be enclosed in quotes, making it a
"forward reference", to hide the expensive_mod reference from the
interpreter runtime. In the # type comment no quotes are needed.)

This approach may also be useful to handle import cycles.

Arbitrary argument lists and default argument values

Arbitrary argument lists can as well be type annotated, so that the
definition:

    def foo(*args: str, **kwds: int): ...

is acceptable and it means that, e.g., all of the following represent
function calls with valid types of arguments:

    foo('a', 'b', 'c')
    foo(x=1, y=2)
    foo('', z=0)

In the body of function foo, the type of variable args is deduced as
Tuple[str, ...] and the type of variable kwds is Dict[str, int].

In stubs it may be useful to declare an argument as having a default
without specifying the actual default value. For example:

    def foo(x: AnyStr, y: AnyStr = ...) -> AnyStr: ...

What should the default value look like? Any of the options "", b"" or
None fails to satisfy the type constraint.

In such cases the default value may be specified as a literal ellipsis,
i.e. the above example is literally what you would write.

Positional-only arguments

Some functions are designed to take their arguments only positionally,
and expect their callers never to use the argument's name to provide
that argument by keyword. All arguments with names beginning with __ are
assumed to be positional-only, except if their names also end with __:

    def quux(__x: int, __y__: int = 0) -> None: ...

    quux(3, __y__=1)  # This call is fine.

    quux(__x=3)  # This call is an error.

Annotating generator functions and coroutines

The return type of generator functions can be annotated by the generic
type Generator[yield_type, send_type, return_type] provided by typing.py
module:

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

Coroutines introduced in PEP 492 are annotated with the same syntax as
ordinary functions. However, the return type annotation corresponds to
the type of await expression, not to the coroutine type:

    async def spam(ignored: int) -> str:
        return 'spam'

    async def foo() -> None:
        bar = await spam(42)  # type: str

The typing.py module provides a generic version of ABC
collections.abc.Coroutine to specify awaitables that also support send()
and throw() methods. The variance and order of type variables correspond
to those of Generator, namely Coroutine[T_co, T_contra, V_co], for
example:

    from typing import List, Coroutine
    c = None  # type: Coroutine[List[str], str, int]
    ...
    x = c.send('hi')  # type: List[str]
    async def bar() -> None:
        x = await c  # type: int

The module also provides generic ABCs Awaitable, AsyncIterable, and
AsyncIterator for situations where more precise types cannot be
specified:

    def op() -> typing.Awaitable[str]:
        if cond:
            return spam(42)
        else:
            return asyncio.Future(...)

Compatibility with other uses of function annotations

A number of existing or potential use cases for function annotations
exist, which are incompatible with type hinting. These may confuse a
static type checker. However, since type hinting annotations have no
runtime behavior (other than evaluation of the annotation expression and
storing annotations in the __annotations__ attribute of the function
object), this does not make the program incorrect -- it just may cause a
type checker to emit spurious warnings or errors.

To mark portions of the program that should not be covered by type
hinting, you can use one or more of the following:

-   a # type: ignore comment;
-   a @no_type_check decorator on a class or function;
-   a custom class or function decorator marked with
    @no_type_check_decorator.

For more details see later sections.

In order for maximal compatibility with offline type checking it may
eventually be a good idea to change interfaces that rely on annotations
to switch to a different mechanism, for example a decorator. In Python
3.5 there is no pressure to do this, however. See also the longer
discussion under Rejected alternatives below.

Type comments

No first-class syntax support for explicitly marking variables as being
of a specific type is added by this PEP. To help with type inference in
complex cases, a comment of the following format may be used:

    x = []                # type: List[Employee]
    x, y, z = [], [], []  # type: List[int], List[int], List[str]
    x, y, z = [], [], []  # type: (List[int], List[int], List[str])
    a, b, *c = range(5)   # type: float, float, List[float]
    x = [1, 2]            # type: List[int]

Type comments should be put on the last line of the statement that
contains the variable definition. They can also be placed on with
statements and for statements, right after the colon.

Examples of type comments on with and for statements:

    with frobnicate() as foo:  # type: int
        # Here foo is an int
        ...

    for x, y in points:  # type: float, float
        # Here x and y are floats
        ...

In stubs it may be useful to declare the existence of a variable without
giving it an initial value. This can be done using PEP 526 variable
annotation syntax:

    from typing import IO

    stream: IO[str]

The above syntax is acceptable in stubs for all versions of Python.
However, in non-stub code for versions of Python 3.5 and earlier there
is a special case:

    from typing import IO

    stream = None  # type: IO[str]

Type checkers should not complain about this (despite the value None not
matching the given type), nor should they change the inferred type to
Optional[...] (despite the rule that does this for annotated arguments
with a default value of None). The assumption here is that other code
will ensure that the variable is given a value of the proper type, and
all uses can assume that the variable has the given type.

The # type: ignore comment should be put on the line that the error
refers to:

    import http.client
    errors = {
        'not_found': http.client.NOT_FOUND  # type: ignore
    }

A # type: ignore comment on a line by itself at the top of a file,
before any docstrings, imports, or other executable code, silences all
errors in the file. Blank lines and other comments, such as shebang
lines and coding cookies, may precede the # type: ignore comment.

In some cases, linting tools or other comments may be needed on the same
line as a type comment. In these cases, the type comment should be
before other comments and linting markers:

  # type: ignore # <comment or other marker>

If type hinting proves useful in general, a syntax for typing variables
may be provided in a future Python version. (UPDATE: This syntax was
added in Python 3.6 through PEP 526.)

Casts

Occasionally the type checker may need a different kind of hint: the
programmer may know that an expression is of a more constrained type
than a type checker may be able to infer. For example:

    from typing import List, cast

    def find_first_str(a: List[object]) -> str:
        index = next(i for i, x in enumerate(a) if isinstance(x, str))
        # We only get here if there's at least one string in a
        return cast(str, a[index])

Some type checkers may not be able to infer that the type of a[index] is
str and only infer object or Any, but we know that (if the code gets to
that point) it must be a string. The cast(t, x) call tells the type
checker that we are confident that the type of x is t. At runtime a cast
always returns the expression unchanged -- it does not check the type,
and it does not convert or coerce the value.

Casts differ from type comments (see the previous section). When using a
type comment, the type checker should still verify that the inferred
type is consistent with the stated type. When using a cast, the type
checker should blindly believe the programmer. Also, casts can be used
in expressions, while type comments only apply to assignments.

NewType helper function

There are also situations where a programmer might want to avoid logical
errors by creating simple classes. For example:

    class UserId(int):
        pass

    def get_by_user_id(user_id: UserId):
        ...

However, this approach introduces a runtime overhead. To avoid this,
typing.py provides a helper function NewType that creates simple unique
types with almost zero runtime overhead. For a static type checker
Derived = NewType('Derived', Base) is roughly equivalent to a
definition:

    class Derived(Base):
        def __init__(self, _x: Base) -> None:
            ...

While at runtime, NewType('Derived', Base) returns a dummy function that
simply returns its argument. Type checkers require explicit casts from
int where UserId is expected, while implicitly casting from UserId where
int is expected. Examples:

    UserId = NewType('UserId', int)

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

    UserId('user')          # Fails type check

    name_by_id(42)          # Fails type check
    name_by_id(UserId(42))  # OK

    num = UserId(5) + 1     # type: int

NewType accepts exactly two arguments: a name for the new unique type,
and a base class. The latter should be a proper class (i.e., not a type
construct like Union, etc.), or another unique type created by calling
NewType. The function returned by NewType accepts only one argument;
this is equivalent to supporting only one constructor accepting an
instance of the base class (see above). Example:

    class PacketId:
        def __init__(self, major: int, minor: int) -> None:
            self._major = major
            self._minor = minor

    TcpPacketId = NewType('TcpPacketId', PacketId)

    packet = PacketId(100, 100)
    tcp_packet = TcpPacketId(packet)  # OK

    tcp_packet = TcpPacketId(127, 0)  # Fails in type checker and at runtime

Both isinstance and issubclass, as well as subclassing will fail for
NewType('Derived', Base) since function objects don't support these
operations.

Stub Files

Stub files are files containing type hints that are only for use by the
type checker, not at runtime. There are several use cases for stub
files:

-   Extension modules
-   Third-party modules whose authors have not yet added type hints
-   Standard library modules for which type hints have not yet been
    written
-   Modules that must be compatible with Python 2 and 3
-   Modules that use annotations for other purposes

Stub files have the same syntax as regular Python modules. There is one
feature of the typing module that is different in stub files: the
@overload decorator described below.

The type checker should only check function signatures in stub files; It
is recommended that function bodies in stub files just be a single
ellipsis (...).

The type checker should have a configurable search path for stub files.
If a stub file is found the type checker should not read the
corresponding "real" module.

While stub files are syntactically valid Python modules, they use the
.pyi extension to make it possible to maintain stub files in the same
directory as the corresponding real module. This also reinforces the
notion that no runtime behavior should be expected of stub files.

Additional notes on stub files:

-   Modules and variables imported into the stub are not considered
    exported from the stub unless the import uses the import ... as ...
    form or the equivalent from ... import ... as ... form. (UPDATE: To
    clarify, the intention here is that only names imported using the
    form X as X will be exported, i.e. the name before and after as must
    be the same.)

-   However, as an exception to the previous bullet, all objects
    imported into a stub using from ... import * are considered
    exported. (This makes it easier to re-export all objects from a
    given module that may vary by Python version.)

-   Just like in normal Python files, submodules automatically become
    exported attributes of their parent module when imported. For
    example, if the spam package has the following directory structure:

        spam/
            __init__.pyi
            ham.pyi

    where __init__.pyi contains a line such as from . import ham or
    from .ham import Ham, then ham is an exported attribute of spam.

-   Stub files may be incomplete. To make type checkers aware of this,
    the file can contain the following code:

        def __getattr__(name) -> Any: ...

    Any identifier not defined in the stub is therefore assumed to be of
    type Any.

Function/method overloading

The @overload decorator allows describing functions and methods that
support multiple different combinations of argument types. This pattern
is used frequently in builtin modules and types. For example, the
__getitem__() method of the bytes type can be described as follows:

    from typing import overload

    class bytes:
        ...
        @overload
        def __getitem__(self, i: int) -> int: ...
        @overload
        def __getitem__(self, s: slice) -> bytes: ...

This description is more precise than would be possible using unions
(which cannot express the relationship between the argument and return
types):

    from typing import Union

    class bytes:
        ...
        def __getitem__(self, a: Union[int, slice]) -> Union[int, bytes]: ...

Another example where @overload comes in handy is the type of the
builtin map() function, which takes a different number of arguments
depending on the type of the callable:

    from typing import Callable, Iterable, Iterator, Tuple, TypeVar, overload

    T1 = TypeVar('T1')
    T2 = TypeVar('T2')
    S = TypeVar('S')

    @overload
    def map(func: Callable[[T1], S], iter1: Iterable[T1]) -> Iterator[S]: ...
    @overload
    def map(func: Callable[[T1, T2], S],
            iter1: Iterable[T1], iter2: Iterable[T2]) -> Iterator[S]: ...
    # ... and we could add more items to support more than two iterables

Note that we could also easily add items to support map(None, ...):

    @overload
    def map(func: None, iter1: Iterable[T1]) -> Iterable[T1]: ...
    @overload
    def map(func: None,
            iter1: Iterable[T1],
            iter2: Iterable[T2]) -> Iterable[Tuple[T1, T2]]: ...

Uses of the @overload decorator as shown above are suitable for stub
files. In regular modules, 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.
Here's an example of a non-stub overload that can't easily be expressed
using a union or a type variable:

    @overload
    def utf8(value: None) -> None:
        pass
    @overload
    def utf8(value: bytes) -> bytes:
        pass
    @overload
    def utf8(value: unicode) -> bytes:
        pass
    def utf8(value):
        <actual implementation>

NOTE: While it would be possible to provide a multiple dispatch
implementation using this syntax, its implementation would require using
sys._getframe(), which is frowned upon. Also, designing and implementing
an efficient multiple dispatch mechanism is hard, which is why previous
attempts were abandoned in favor of functools.singledispatch(). (See PEP
443, especially its section "Alternative approaches".) In the future we
may come up with a satisfactory multiple dispatch design, but we don't
want such a design to be constrained by the overloading syntax defined
for type hints in stub files. It is also possible that both features
will develop independent from each other (since overloading in the type
checker has different use cases and requirements than multiple dispatch
at runtime -- e.g. the latter is unlikely to support generic types).

A constrained TypeVar type can often be used instead of using the
@overload decorator. For example, the definitions of concat1 and concat2
in this stub file are equivalent:

    from typing import TypeVar, Text

    AnyStr = TypeVar('AnyStr', Text, bytes)

    def concat1(x: AnyStr, y: AnyStr) -> AnyStr: ...

    @overload
    def concat2(x: str, y: str) -> str: ...
    @overload
    def concat2(x: bytes, y: bytes) -> bytes: ...

Some functions, such as map or bytes.__getitem__ above, can't be
represented precisely using type variables. However, unlike @overload,
type variables can also be used outside stub files. We recommend that
@overload is only used in cases where a type variable is not sufficient,
due to its special stub-only status.

Another important difference between type variables such as AnyStr and
using @overload is that the prior can also be used to define constraints
for generic class type parameters. For example, the type parameter of
the generic class typing.IO is constrained (only IO[str], IO[bytes] and
IO[Any] are valid):

    class IO(Generic[AnyStr]): ...

Storing and distributing stub files

The easiest form of stub file storage and distribution is to put them
alongside Python modules in the same directory. This makes them easy to
find by both programmers and the tools. However, since package
maintainers are free not to add type hinting to their packages,
third-party stubs installable by pip from PyPI are also supported. In
this case we have to consider three issues: naming, versioning,
installation path.

This PEP does not provide a recommendation on a naming scheme that
should be used for third-party stub file packages. Discoverability will
hopefully be based on package popularity, like with Django packages for
example.

Third-party stubs have to be versioned using the lowest version of the
source package that is compatible. Example: FooPackage has versions 1.0,
1.1, 1.2, 1.3, 2.0, 2.1, 2.2. There are API changes in versions 1.1, 2.0
and 2.2. The stub file package maintainer is free to release stubs for
all versions but at least 1.0, 1.1, 2.0 and 2.2 are needed to enable the
end user type check all versions. This is because the user knows that
the closest lower or equal version of stubs is compatible. In the
provided example, for FooPackage 1.3 the user would choose stubs version
1.1.

Note that if the user decides to use the "latest" available source
package, using the "latest" stub files should generally also work if
they're updated often.

Third-party stub packages can use any location for stub storage. Type
checkers should search for them using PYTHONPATH. A default fallback
directory that is always checked is shared/typehints/pythonX.Y/ (for
some PythonX.Y as determined by the type checker, not just the installed
version). Since there can only be one package installed for a given
Python version per environment, no additional versioning is performed
under that directory (just like bare directory installs by pip in
site-packages). Stub file package authors might use the following
snippet in setup.py:

    ...
    data_files=[
        (
            'shared/typehints/python{}.{}'.format(*sys.version_info[:2]),
            pathlib.Path(SRC_PATH).glob('**/*.pyi'),
        ),
    ],
    ...

(UPDATE: As of June 2018 the recommended way to distribute type hints
for third-party packages has changed -- in addition to typeshed (see the
next section) there is now a standard for distributing type hints, PEP
561. It supports separately installable packages containing stubs, stub
files included in the same distribution as the executable code of a
package, and inline type hints, the latter two options enabled by
including a file named py.typed in the package.)

The Typeshed Repo

There is a shared repository where useful stubs are being collected.
Policies regarding the stubs collected here will be decided separately
and reported in the repo's documentation. Note that stubs for a given
package will not be included here if the package owners have
specifically requested that they be omitted.

Exceptions

No syntax for listing explicitly raised exceptions is proposed.
Currently the only known use case for this feature is documentational,
in which case the recommendation is to put this information in a
docstring.

The typing Module

To open the usage of static type checking to Python 3.5 as well as older
versions, a uniform namespace is required. For this purpose, a new
module in the standard library is introduced called typing.

It defines the fundamental building blocks for constructing types (e.g.
Any), types representing generic variants of builtin collections (e.g.
List), types representing generic collection ABCs (e.g. Sequence), and a
small collection of convenience definitions.

Note that special type constructs, such as Any, Union, and type
variables defined using TypeVar are only supported in the type
annotation context, and Generic may only be used as a base class. All of
these (except for unparameterized generics) will raise TypeError if
appear in isinstance or issubclass.

Fundamental building blocks:

-   Any, used as def get(key: str) -> Any: ...
-   Union, used as Union[Type1, Type2, Type3]
-   Callable, used as Callable[[Arg1Type, Arg2Type], ReturnType]
-   Tuple, used by listing the element types, for example
    Tuple[int, int, str]. The empty tuple can be typed as Tuple[()].
    Arbitrary-length homogeneous tuples can be expressed using one type
    and ellipsis, for example Tuple[int, ...]. (The ... here are part of
    the syntax, a literal ellipsis.)
-   TypeVar, used as X = TypeVar('X', Type1, Type2, Type3) or simply
    Y = TypeVar('Y') (see above for more details)
-   Generic, used to create user-defined generic classes
-   Type, used to annotate class objects

Generic variants of builtin collections:

-   Dict, used as Dict[key_type, value_type]
-   DefaultDict, used as DefaultDict[key_type, value_type], a generic
    variant of collections.defaultdict
-   List, used as List[element_type]
-   Set, used as Set[element_type]. See remark for AbstractSet below.
-   FrozenSet, used as FrozenSet[element_type]

Note: Dict, DefaultDict, List, Set and FrozenSet are mainly useful for
annotating return values. For arguments, prefer the abstract collection
types defined below, e.g. Mapping, Sequence or AbstractSet.

Generic variants of container ABCs (and a few non-containers):

-   Awaitable
-   AsyncIterable
-   AsyncIterator
-   ByteString
-   Callable (see above, listed here for completeness)
-   Collection
-   Container
-   ContextManager
-   Coroutine
-   Generator, used as Generator[yield_type, send_type, return_type].
    This represents the return value of generator functions. It is a
    subtype of Iterable and it has additional type variables for the
    type accepted by the send() method (it is contravariant in this
    variable -- a generator that accepts sending it Employee instance is
    valid in a context where a generator is required that accepts
    sending it Manager instances) and the return type of the generator.
-   Hashable (not generic, but present for completeness)
-   ItemsView
-   Iterable
-   Iterator
-   KeysView
-   Mapping
-   MappingView
-   MutableMapping
-   MutableSequence
-   MutableSet
-   Sequence
-   Set, renamed to AbstractSet. This name change was required because
    Set in the typing module means set() with generics.
-   Sized (not generic, but present for completeness)
-   ValuesView

A few one-off types are defined that test for single special methods
(similar to Hashable or Sized):

-   Reversible, to test for __reversed__
-   SupportsAbs, to test for __abs__
-   SupportsComplex, to test for __complex__
-   SupportsFloat, to test for __float__
-   SupportsInt, to test for __int__
-   SupportsRound, to test for __round__
-   SupportsBytes, to test for __bytes__

Convenience definitions:

-   Optional, defined by Optional[t] == Union[t, None]
-   Text, a simple alias for str in Python 3, for unicode in Python 2
-   AnyStr, defined as TypeVar('AnyStr', Text, bytes)
-   NamedTuple, used as
    NamedTuple(type_name, [(field_name, field_type), ...]) and
    equivalent to collections.namedtuple(type_name, [field_name, ...]).
    This is useful to declare the types of the fields of a named tuple
    type.
-   NewType, used to create unique types with little runtime overhead
    UserId = NewType('UserId', int)
-   cast(), described earlier
-   @no_type_check, a decorator to disable type checking per class or
    function (see below)
-   @no_type_check_decorator, a decorator to create your own decorators
    with the same meaning as @no_type_check (see below)
-   @type_check_only, a decorator only available during type checking
    for use in stub files (see above); marks a class or function as
    unavailable during runtime
-   @overload, described earlier
-   get_type_hints(), a utility function to retrieve the type hints from
    a function or method. Given a function or method object, it returns
    a dict with the same format as __annotations__, but evaluating
    forward references (which are given as string literals) as
    expressions in the context of the original function or method
    definition.
-   TYPE_CHECKING, False at runtime but True to type checkers

I/O related types:

-   IO (generic over AnyStr)
-   BinaryIO (a simple subtype of IO[bytes])
-   TextIO (a simple subtype of IO[str])

Types related to regular expressions and the re module:

-   Match and Pattern, types of re.match() and re.compile() results
    (generic over AnyStr)

Suggested syntax for Python 2.7 and straddling code

Some tools may want to support type annotations in code that must be
compatible with Python 2.7. For this purpose this PEP has a suggested
(but not mandatory) extension where function annotations are placed in a
# type: comment. Such a comment must be placed immediately following the
function header (before the docstring). An example: the following Python
3 code:

    def embezzle(self, account: str, funds: int = 1000000, *fake_receipts: str) -> None:
        """Embezzle funds from account using fake receipts."""
        <code goes here>

is equivalent to the following:

    def embezzle(self, account, funds=1000000, *fake_receipts):
        # type: (str, int, *str) -> None
        """Embezzle funds from account using fake receipts."""
        <code goes here>

Note that for methods, no type is needed for self.

For an argument-less method it would look like this:

    def load_cache(self):
        # type: () -> bool
        <code>

Sometimes you want to specify the return type for a function or method
without (yet) specifying the argument types. To support this explicitly,
the argument list may be replaced with an ellipsis. Example:

    def send_email(address, sender, cc, bcc, subject, body):
        # type: (...) -> bool
        """Send an email message.  Return True if successful."""
        <code>

Sometimes you have a long list of parameters and specifying their types
in a single # type: comment would be awkward. To this end you may list
the arguments one per line and add a # type: comment per line after an
argument's associated comma, if any. To specify the return type use the
ellipsis syntax. Specifying the return type is not mandatory and not
every argument needs to be given a type. A line with a # type: comment
should contain exactly one argument. The type comment for the last
argument (if any) should precede the close parenthesis. Example:

    def send_email(address,     # type: Union[str, List[str]]
                   sender,      # type: str
                   cc,          # type: Optional[List[str]]
                   bcc,         # type: Optional[List[str]]
                   subject='',
                   body=None    # type: List[str]
                   ):
        # type: (...) -> bool
        """Send an email message.  Return True if successful."""
        <code>

Notes:

-   Tools that support this syntax should support it regardless of the
    Python version being checked. This is necessary in order to support
    code that straddles Python 2 and Python 3.

-   It is not allowed for an argument or return value to have both a
    type annotation and a type comment.

-   When using the short form (e.g. # type: (str, int) -> None) every
    argument must be accounted for, except the first argument of
    instance and class methods (those are usually omitted, but it's
    allowed to include them).

-   The return type is mandatory for the short form. If in Python 3 you
    would omit some argument or the return type, the Python 2 notation
    should use Any.

-   When using the short form, for *args and **kwds, put 1 or 2 stars in
    front of the corresponding type annotation. (As with Python 3
    annotations, the annotation here denotes the type of the individual
    argument values, not of the tuple/dict that you receive as the
    special argument value args or kwds.)

-   Like other type comments, any names used in the annotations must be
    imported or defined by the module containing the annotation.

-   When using the short form, the entire annotation must be one line.

-   The short form may also occur on the same line as the close
    parenthesis, e.g.:

        def add(a, b):  # type: (int, int) -> int
            return a + b

-   Misplaced type comments will be flagged as errors by a type checker.
    If necessary, such comments could be commented twice. For example:

        def f():
            '''Docstring'''
            # type: () -> None  # Error!

        def g():
            '''Docstring'''
            # # type: () -> None  # This is OK

When checking Python 2.7 code, type checkers should treat the int and
long types as equivalent. For parameters typed as Text, arguments of
type str as well as unicode should be acceptable.

Rejected Alternatives

During discussion of earlier drafts of this PEP, various objections were
raised and alternatives were proposed. We discuss some of these here and
explain why we reject them.

Several main objections were raised.

Which brackets for generic type parameters?

Most people are familiar with the use of angular brackets (e.g.
List<int>) in languages like C++, Java, C# and Swift to express the
parameterization of generic types. The problem with these is that they
are really hard to parse, especially for a simple-minded parser like
Python. In most languages the ambiguities are usually dealt with by only
allowing angular brackets in specific syntactic positions, where general
expressions aren't allowed. (And also by using very powerful parsing
techniques that can backtrack over an arbitrary section of code.)

But in Python, we'd like type expressions to be (syntactically) the same
as other expressions, so that we can use e.g. variable assignment to
create type aliases. Consider this simple type expression:

    List<int>

From the Python parser's perspective, the expression begins with the
same four tokens (NAME, LESS, NAME, GREATER) as a chained comparison:

    a < b > c  # I.e., (a < b) and (b > c)

We can even make up an example that could be parsed both ways:

    a < b > [ c ]

Assuming we had angular brackets in the language, this could be
interpreted as either of the following two:

    (a<b>)[c]      # I.e., (a<b>).__getitem__(c)
    a < b > ([c])  # I.e., (a < b) and (b > [c])

It would surely be possible to come up with a rule to disambiguate such
cases, but to most users the rules would feel arbitrary and complex. It
would also require us to dramatically change the CPython parser (and
every other parser for Python). It should be noted that Python's current
parser is intentionally "dumb" -- a simple grammar is easier for users
to reason about.

For all these reasons, square brackets (e.g. List[int]) are (and have
long been) the preferred syntax for generic type parameters. They can be
implemented by defining the __getitem__() method on the metaclass, and
no new syntax is required at all. This option works in all recent
versions of Python (starting with Python 2.2). Python is not alone in
this syntactic choice -- generic classes in Scala also use square
brackets.

What about existing uses of annotations?

One line of argument points out that PEP 3107 explicitly supports the
use of arbitrary expressions in function annotations. The new proposal
is then considered incompatible with the specification of PEP 3107.

Our response to this is that, first of all, the current proposal does
not introduce any direct incompatibilities, so programs using
annotations in Python 3.4 will still work correctly and without
prejudice in Python 3.5.

We do hope that type hints will eventually become the sole use for
annotations, but this will require additional discussion and a
deprecation period after the initial roll-out of the typing module with
Python 3.5. The current PEP will have provisional status (see PEP 411)
until Python 3.6 is released. The fastest conceivable scheme would
introduce silent deprecation of non-type-hint annotations in 3.6, full
deprecation in 3.7, and declare type hints as the only allowed use of
annotations in Python 3.8. This should give authors of packages that use
annotations plenty of time to devise another approach, even if type
hints become an overnight success.

(UPDATE: As of fall 2017, the timeline for the end of provisional status
for this PEP and for the typing.py module has changed, and so has the
deprecation schedule for other uses of annotations. For the updated
schedule see PEP 563.)

Another possible outcome would be that type hints will eventually become
the default meaning for annotations, but that there will always remain
an option to disable them. For this purpose the current proposal defines
a decorator @no_type_check which disables the default interpretation of
annotations as type hints in a given class or function. It also defines
a meta-decorator @no_type_check_decorator which can be used to decorate
a decorator (!), causing annotations in any function or class decorated
with the latter to be ignored by the type checker.

There are also # type: ignore comments, and static checkers should
support configuration options to disable type checking in selected
packages.

Despite all these options, proposals have been circulated to allow type
hints and other forms of annotations to coexist for individual
arguments. One proposal suggests that if an annotation for a given
argument is a dictionary literal, each key represents a different form
of annotation, and the key 'type' would be use for type hints. The
problem with this idea and its variants is that the notation becomes
very "noisy" and hard to read. Also, in most cases where existing
libraries use annotations, there would be little need to combine them
with type hints. So the simpler approach of selectively disabling type
hints appears sufficient.

The problem of forward declarations

The current proposal is admittedly sub-optimal when type hints must
contain forward references. Python requires all names to be defined by
the time they are used. Apart from circular imports this is rarely a
problem: "use" here means "look up at runtime", and with most "forward"
references there is no problem in ensuring that a name is defined before
the function using it is called.

The problem with type hints is that annotations (per PEP 3107, and
similar to default values) are evaluated at the time a function is
defined, and thus any names used in an annotation must be already
defined when the function is being defined. A common scenario is a class
definition whose methods need to reference the class itself in their
annotations. (More general, it can also occur with mutually recursive
classes.) This is natural for container types, for example:

    class Node:
        """Binary tree node."""

        def __init__(self, left: Node, right: Node):
            self.left = left
            self.right = right

As written this will not work, because of the peculiarity in Python that
class names become defined once the entire body of the class has been
executed. Our solution, which isn't particularly elegant, but gets the
job done, is to allow using string literals in annotations. Most of the
time you won't have to use this though -- most uses of type hints are
expected to reference builtin types or types defined in other modules.

A counterproposal would change the semantics of type hints so they
aren't evaluated at runtime at all (after all, type checking happens
off-line, so why would type hints need to be evaluated at runtime at
all). This of course would run afoul of backwards compatibility, since
the Python interpreter doesn't actually know whether a particular
annotation is meant to be a type hint or something else.

A compromise is possible where a __future__ import could enable turning
all annotations in a given module into string literals, as follows:

    from __future__ import annotations

    class ImSet:
        def add(self, a: ImSet) -> List[ImSet]: ...

    assert ImSet.add.__annotations__ == {'a': 'ImSet', 'return': 'List[ImSet]'}

Such a __future__ import statement may be proposed in a separate PEP.

(UPDATE: That __future__ import statement and its consequences are
discussed in PEP 563.)

The double colon

A few creative souls have tried to invent solutions for this problem.
For example, it was proposed to use a double colon (::) for type hints,
solving two problems at once: disambiguating between type hints and
other annotations, and changing the semantics to preclude runtime
evaluation. There are several things wrong with this idea, however.

-   It's ugly. The single colon in Python has many uses, and all of them
    look familiar because they resemble the use of the colon in English
    text. This is a general rule of thumb by which Python abides for
    most forms of punctuation; the exceptions are typically well known
    from other programming languages. But this use of :: is unheard of
    in English, and in other languages (e.g. C++) it is used as a
    scoping operator, which is a very different beast. In contrast, the
    single colon for type hints reads naturally -- and no wonder, since
    it was carefully designed for this purpose (the idea long predates
    PEP 3107). It is also used in the same fashion in other languages
    from Pascal to Swift.
-   What would you do for return type annotations?
-   It's actually a feature that type hints are evaluated at runtime.
    -   Making type hints available at runtime allows runtime type
        checkers to be built on top of type hints.
    -   It catches mistakes even when the type checker is not run. Since
        it is a separate program, users may choose not to run it (or
        even install it), but might still want to use type hints as a
        concise form of documentation. Broken type hints are no use even
        for documentation.
-   Because it's new syntax, using the double colon for type hints would
    limit them to code that works with Python 3.5 only. By using
    existing syntax, the current proposal can easily work for older
    versions of Python 3. (And in fact mypy supports Python 3.2 and
    newer.)
-   If type hints become successful we may well decide to add new syntax
    in the future to declare the type for variables, for example
    var age: int = 42. If we were to use a double colon for argument
    type hints, for consistency we'd have to use the same convention for
    future syntax, perpetuating the ugliness.

Other forms of new syntax

A few other forms of alternative syntax have been proposed, e.g. the
introduction of a where keyword, and Cobra-inspired requires clauses.
But these all share a problem with the double colon: they won't work for
earlier versions of Python 3. The same would apply to a new __future__
import.

Other backwards compatible conventions

The ideas put forward include:

-   A decorator, e.g. @typehints(name=str, returns=str). This could
    work, but it's pretty verbose (an extra line, and the argument names
    must be repeated), and a far cry in elegance from the PEP 3107
    notation.
-   Stub files. We do want stub files, but they are primarily useful for
    adding type hints to existing code that doesn't lend itself to
    adding type hints, e.g. 3rd party packages, code that needs to
    support both Python 2 and Python 3, and especially extension
    modules. For most situations, having the annotations in line with
    the function definitions makes them much more useful.
-   Docstrings. There is an existing convention for docstrings, based on
    the Sphinx notation (:type arg1: description). This is pretty
    verbose (an extra line per parameter), and not very elegant. We
    could also make up something new, but the annotation syntax is hard
    to beat (because it was designed for this very purpose).

It's also been proposed to simply wait another release. But what problem
would that solve? It would just be procrastination.

PEP Development Process

A live draft for this PEP lives on GitHub. There is also an issue
tracker, where much of the technical discussion takes place.

The draft on GitHub is updated regularly in small increments. The
official PEPS repo is (usually) only updated when a new draft is posted
to python-dev.

Acknowledgements

This document could not be completed without valuable input,
encouragement and advice from Jim Baker, Jeremy Siek, Michael Matson
Vitousek, Andrey Vlasovskikh, Radomir Dopieralski, Peter Ludemann, and
the BDFL-Delegate, Mark Shannon.

Influences include existing languages, libraries and frameworks
mentioned in PEP 482. Many thanks to their creators, in alphabetical
order: Stefan Behnel, William Edwards, Greg Ewing, Larry Hastings,
Anders Hejlsberg, Alok Menghrajani, Travis E. Oliphant, Joe Pamer,
Raoul-Gabriel Urma, and Julien Verlaguet.

Copyright

This document has been placed in the public domain.