Callables¶

Terminology¶

In this section, and throughout this specification, the term “parameter” refers to a named symbol associated with a function that receives the value of an argument (or multiple arguments) passed to the function. The term “argument” refers to a value passed to a function when it is called.

Python supports five kinds of parameters: positional-only, keyword-only, standard (positional or keyword), variadic positional (*args), and variadic keyword (**kwargs). Positional-only parameters can accept only positional arguments, and keyword-only parameters can accept only keyword arguments. Standard parameters can accept either positional or keyword arguments. Parameters of the form *args and **kwargs are variadic and accept zero or more positional or keyword arguments, respectively.

In the example below, a is a positional-only parameter, b is a standard (positional or keyword) parameter, c is a keyword-only parameter, args is a variadic parameter that accepts additional positional arguments, and kwargs is a variadic parameter that accepts additional keyword arguments:

def func(a: str, /, b, *args, c=0, **kwargs) -> None:
    ...

A function’s “signature” refers to its list of parameters (including the name, kind, optional declared type, and whether it has a default argument value) plus its return type. The signature of the function above is (a: str, /, b, *args, c=..., **kwargs) -> None. Note that the default argument value for parameter c is denoted as ... here because the presence of a default value is considered part of the signature, but the specific value is not.

The term “input signature” is used to refer to only the parameters of a function. In the example above, the input signature is (a: str, /, b, *args, c=..., **kwargs).

Positional-only parameters¶

Within a function signature, positional-only parameters are separated from non-positional-only parameters by a single forward slash (‘/’). This forward slash does not represent a parameter, but rather a delimiter. In this example, a is a positional-only parameter and b is a standard (positional or keyword) parameter:

def func(a: int, /, b: int) -> None:
    ...

func(1, 2)  # OK
func(1, b=2)  # OK
func(a=1, b=2)  # Error

Support for the / delimiter was introduced in Python 3.8 (PEP 570). For compatibility with earlier versions of Python, the type system also supports specifying positional-only parameters using a double leading underscore.

Default argument values¶

In certain cases, it may be desirable to omit the default argument value for a parameter. Examples include function definitions in stub files or methods within a protocol or abstract base class. In such cases, the default value may be given as an ellipsis. For example:

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

If a non-ellipsis default value is present and its type can be statically evaluated, a type checker should verify that this type is assignable to the declared parameter’s type:

def func(x: int = 0): ...  # OK
def func(x: int | None = None): ...  # OK
def func(x: int = 0.0): ...  # Error
def func(x: int = None): ...  # Error

Annotating `*args` and `**kwargs`¶

At runtime, the type of a variadic positional parameter (*args) is a tuple, and the type of a variadic keyword parameter (**kwargs) is a dict. However, when annotating these parameters, the type annotation refers to the type of items within the tuple or dict (unless Unpack is used).

Therefore, the definition:

def func(*args: str, **kwargs: int): ...

means that the function accepts an arbitrary number of positional arguments of type str and an arbitrary number of keyword arguments of type int. For example, all of the following represent function calls with valid arguments:

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

In the body of function func, the type of parameter args is tuple[str, ...], and the type of parameter kwargs is dict[str, int].

`Unpack` for keyword arguments¶

typing.Unpack has two use cases in the type system:

As introduced by PEP 646, a backward-compatible form for certain operations involving variadic generics. See the section on TypeVarTuple for details.
As introduced by PEP 692, a way to annotate the **kwargs of a function.

This second usage is described in this section. The following example:

from typing import TypedDict, Unpack

class Movie(TypedDict):
    name: str
    year: int

def foo(**kwargs: Unpack[Movie]) -> None: ...

means that the **kwargs comprise two keyword arguments specified by Movie (i.e. a name keyword of type str and a year keyword of type int). This indicates that the function should be called as follows:

kwargs: Movie = {"name": "Life of Brian", "year": 1979}

foo(**kwargs)                               # OK!
foo(name="The Meaning of Life", year=1983)  # OK!

When Unpack is used, type checkers treat kwargs inside the function body as a TypedDict:

def foo(**kwargs: Unpack[Movie]) -> None:
    assert_type(kwargs, Movie)  # OK!

Function calls with standard dictionaries¶

Passing a dictionary of type dict[str, object] as a **kwargs argument to a function that has **kwargs annotated with Unpack must generate a type checker error. On the other hand, the behavior for functions using standard, untyped dictionaries can depend on the type checker. For example:

def func(**kwargs: Unpack[Movie]) -> None: ...

movie: dict[str, object] = {"name": "Life of Brian", "year": 1979}
func(**movie)  # WRONG! Movie is of type dict[str, object]

typed_movie: Movie = {"name": "The Meaning of Life", "year": 1983}
func(**typed_movie)  # OK!

another_movie = {"name": "Life of Brian", "year": 1979}
func(**another_movie)  # Depends on the type checker.

Keyword collisions¶

A TypedDict that is used to type **kwargs could potentially contain keys that are already defined in the function’s signature. If the duplicate name is a standard parameter, an error should be reported by type checkers. If the duplicate name is a positional-only parameter, no errors should be generated. For example:

def foo(name, **kwargs: Unpack[Movie]) -> None: ...     # WRONG! "name" will
                                                        # always bind to the
                                                        # first parameter.

def foo(name, /, **kwargs: Unpack[Movie]) -> None: ...  # OK! "name" is a
                                                        # positional-only parameter,
                                                        # so **kwargs can contain
                                                        # a "name" keyword.

Required and non-required items¶

Items in a TypedDict may be either required or non-required. When using a TypedDict to type **kwargs all of the required and non-required keys should correspond to required and non-required function keyword parameters. Therefore, if a required key is not provided by the caller, then an error must be reported by type checkers.

Read-only items¶

TypedDict items may also be read-only. Marking one or more of the items of a TypedDict used to type **kwargs as read-only will have no effect on the type signature of the method. However, it will prevent the item from being modified in the body of the function:

class Args(TypedDict):
    key1: int
    key2: str

class ReadOnlyArgs(TypedDict):
    key1: ReadOnly[int]
    key2: ReadOnly[str]

class Function(Protocol):
    def __call__(self, **kwargs: Unpack[Args]) -> None: ...

def impl(**kwargs: Unpack[ReadOnlyArgs]) -> None:
    kwargs["key1"] = 3  # Type check error: key1 is readonly

fn: Function = impl  # Accepted by type checker: function signatures are identical

Extra items¶

If the TypedDict used for annotating **kwargs is defined to allow extra items, arbitrary additional keyword arguments of the right type may be passed to the function:

class MovieNoExtra(TypedDict):
    name: str

class MovieExtra(TypedDict, extra_items=int):
    name: str

def f(**kwargs: Unpack[MovieNoExtra]) -> None: ...
def g(**kwargs: Unpack[MovieExtra]) -> None: ...

# Should be equivalent to:
def f(*, name: str) -> None: ...
def g(*, name: str, **kwargs: int) -> None: ...

f(name="No Country for Old Men", year=2007) # Not OK. Unrecognized item
g(name="No Country for Old Men", year=2007) # OK

Assignment¶

Assignments of a function typed with **kwargs: Unpack[Movie] and another callable type should pass type checking only for the scenarios described below.

Source and destination contain `**kwargs`¶

Both destination and source functions have a **kwargs: Unpack[TypedDict] parameter and the destination function’s TypedDict is assignable to the source function’s TypedDict and the rest of the parameters are assignable:

class Animal(TypedDict):
    name: str

class Dog(Animal):
    breed: str

def accept_animal(**kwargs: Unpack[Animal]): ...
def accept_dog(**kwargs: Unpack[Dog]): ...

accept_dog = accept_animal  # OK! Expression of type Dog can be
                            # assigned to a variable of type Animal.

accept_animal = accept_dog  # WRONG! Expression of type Animal
                            # cannot be assigned to a variable of type Dog.

Source contains `**kwargs` and destination doesn’t¶

The destination callable doesn’t contain **kwargs, the source callable contains **kwargs: Unpack[TypedDict] and the destination function’s keyword arguments are assignable to the corresponding keys in source function’s TypedDict. Moreover, not required keys should correspond to optional function arguments, whereas required keys should correspond to required function arguments. Again, the rest of the parameters have to be assignable. Continuing the previous example:

class Example(TypedDict):
    animal: Animal
    string: str
    number: NotRequired[int]

def src(**kwargs: Unpack[Example]): ...
def dest(*, animal: Dog, string: str, number: int = ...): ...

dest = src  # OK!

It is worth pointing out that the destination function’s parameters that are to be assignable to the keys and values from the TypedDict must be keyword only:

def dest(dog: Dog, string: str, number: int = ...): ...

dog: Dog = {"name": "Daisy", "breed": "labrador"}

dest(dog, "some string")  # OK!

dest = src                # Type checker error!
dest(dog, "some string")  # The same call fails at
                          # runtime now because 'src' expects
                          # keyword arguments.

The reverse situation where the destination callable contains **kwargs: Unpack[TypedDict] and the source callable doesn’t contain **kwargs should be disallowed. This is because, we cannot be sure that additional keyword arguments are not being passed in when an instance of a subclass had been assigned to a variable with a base class type and then unpacked in the destination callable invocation:

def dest(**kwargs: Unpack[Animal]): ...
def src(name: str): ...

dog: Dog = {"name": "Daisy", "breed": "Labrador"}
animal: Animal = dog

dest = src      # WRONG!
dest(**animal)  # Fails at runtime.

A similar situation can happen even without inheritance as assignability between TypedDicts is structural.

Source contains untyped `**kwargs`¶

The destination callable contains **kwargs: Unpack[TypedDict] and the source callable contains untyped **kwargs:

def src(**kwargs): ...
def dest(**kwargs: Unpack[Movie]): ...

dest = src  # OK!

Source contains traditionally typed `**kwargs: T`¶

The destination callable contains **kwargs: Unpack[TypedDict], the source callable contains traditionally typed **kwargs: T and each of the destination function TypedDict’s fields is assignable to a variable of type T:

class Vehicle:
    ...

class Car(Vehicle):
    ...

class Motorcycle(Vehicle):
    ...

class Vehicles(TypedDict):
    car: Car
    moto: Motorcycle

def dest(**kwargs: Unpack[Vehicles]): ...
def src(**kwargs: Vehicle): ...

dest = src  # OK!

On the other hand, if the destination callable contains either untyped or traditionally typed **kwargs: T and the source callable is typed using **kwargs: Unpack[TypedDict] then an error should be generated, because traditionally typed **kwargs aren’t checked for keyword names.

To summarize, function parameters should behave contravariantly and function return types should behave covariantly.

Passing kwargs inside a function to another function¶

A previous point mentions the problem of possibly passing additional keyword arguments by assigning a subclass instance to a variable that has a base class type. Let’s consider the following example:

class Animal(TypedDict):
    name: str

class Dog(Animal):
    breed: str

def takes_name(name: str): ...

dog: Dog = {"name": "Daisy", "breed": "Labrador"}
animal: Animal = dog

def foo(**kwargs: Unpack[Animal]):
    print(kwargs["name"].capitalize())

def bar(**kwargs: Unpack[Animal]):
    takes_name(**kwargs)

def baz(animal: Animal):
    takes_name(**animal)

def spam(**kwargs: Unpack[Animal]):
    baz(kwargs)

foo(**animal)   # OK! foo only expects and uses keywords of 'Animal'.

bar(**animal)   # WRONG! This will fail at runtime because 'breed' keyword
                # will be passed to 'takes_name' as well.

spam(**animal)  # WRONG! Again, 'breed' keyword will be eventually passed
                # to 'takes_name'.

In the example above, the call to foo will not cause any issues at runtime. Even though foo expects kwargs of type Animal it doesn’t matter if it receives additional arguments because it only reads and uses what it needs completely ignoring any additional values.

The calls to bar and spam will fail because an unexpected keyword argument will be passed to the takes_name function.

Therefore, kwargs hinted with an unpacked TypedDict can only be passed to another function if the function to which unpacked kwargs are being passed to has **kwargs in its signature as well, because then additional keywords would not cause errors at runtime during function invocation. Otherwise, the type checker should generate an error.

In cases similar to the bar function above the problem could be worked around by explicitly dereferencing desired fields and using them as arguments to perform the function call:

def bar(**kwargs: Unpack[Animal]):
    name = kwargs["name"]
    takes_name(name)

Using `Unpack` with types other than `TypedDict`¶

TypedDict is the only permitted heterogeneous type for typing **kwargs. Therefore, in the context of typing **kwargs, using Unpack with types other than TypedDict should not be allowed and type checkers should generate errors in such cases.

Callable¶

The Callable special form can be used to specify the signature of a function within a type expression. The syntax is Callable[[Param1Type, Param2Type], ReturnType]. For example:

from collections.abc import Callable

def func(cb: Callable[[int], str]) -> None:
    ...

x: Callable[[], str]

Parameters specified using Callable are assumed to be positional-only. The Callable form provides no way to specify keyword-only parameters, variadic parameters, or default argument values. For these use cases, see the section on Callback protocols.

Meaning of `...` in `Callable`¶

The Callable special form supports the use of ... in place of the list of parameter types. This is a gradual form indicating that the type is consistent with any input signature:

cb1: Callable[..., str]
cb1 = lambda x: str(x)  # OK
cb1 = lambda : ""  # OK

cb2: Callable[[], str] = cb1  # OK

A ... can also be used with Concatenate. In this case, the parameters prior to the ... are required to be present in the input signature and be assignable, but any additional parameters are permitted:

cb3: Callable[Concatenate[int, ...], str]
cb3 = lambda x: str(x)  # OK
cb3 = lambda a, b, c: str(a)  # OK
cb3 = lambda : ""  # Error
cb3 = lambda *, a: str(a)  # Error

If the input signature in a function definition includes both a *args and **kwargs parameter and both are typed as Any (explicitly or implicitly because it has no annotation), a type checker should treat this as the equivalent of .... Any other parameters in the signature are unaffected and are retained as part of the signature:

class Proto1(Protocol):
    def __call__(self, *args: Any, **kwargs: Any) -> None: ...

class Proto2(Protocol):
    def __call__(self, a: int, /, *args, **kwargs) -> None: ...

class Proto3(Protocol):
    def __call__(self, a: int, *args: Any, **kwargs: Any) -> None: ...

class Proto4[**P](Protocol):
    def __call__(self, a: int, *args: P.args, **kwargs: P.kwargs) -> None: ...

def func(p1: Proto1, p2: Proto2, p3: Proto3):
    assert_type(p1, Callable[..., None])  # OK
    assert_type(p2, Callable[Concatenate[int, ...], None])  # OK
    assert_type(p3, Callable[..., None])  # Error
    assert_type(p3, Proto4[...])  # OK

class A:
    def method(self, a: int, /, *args: Any, k: str, **kwargs: Any) -> None:
        pass

class B(A):
    # This override is OK because it is assignable to the parent's method.
    def method(self, a: float, /, b: int, *, k: str, m: str) -> None:
        pass

The ... syntax can also be used to provide a specialized value for a ParamSpec in a generic class or type alias. For example:

type Callback[**P] = Callable[P, str]

def func(cb: Callable[[], str]) -> None:
    f: Callback[...] = cb  # OK

If ... is used with signature concatenation, the ... portion continues to be consistent with any input parameters:

type CallbackWithInt[**P] = Callable[Concatenate[int, P], str]
type CallbackWithStr[**P] = Callable[Concatenate[str, P], str]

def func(cb: Callable[[int, str], str]) -> None:
    f1: Callable[Concatenate[int, ...], str] = cb # OK
    f2: Callable[Concatenate[str, ...], str] = cb # Error
    f3: CallbackWithInt[...] = cb  # OK
    f4: CallbackWithStr[...] = cb  # Error

Callback protocols¶

Protocols can be used to define flexible callback types that are impossible to express using the Callable special form as specified above. This includes keyword parameters, variadic parameters, default argument values, and overloads. They can be defined as protocols with a __call__ member:

from typing import Protocol

class Combiner(Protocol):
    def __call__(self, *args: bytes, max_len: int | None = None) -> list[bytes]: ...

def good_cb(*args: bytes, max_len: int | None = None) -> list[bytes]:
    ...
def bad_cb(*args: bytes, max_items: int | None) -> list[bytes]:
    ...

comb: Combiner = good_cb  # OK
comb = bad_cb  # Error! Argument 2 is not assignable because of
               # different parameter name and kind in the callback

Callback protocols and Callable[...] types can generally be used interchangeably.

Assignability rules for callables¶

A callable type B is assignable to a callable type A if the return type of B is assignable to the return type of A and the input signature of B accepts all possible combinations of arguments that the input signature of A accepts. All of the specific assignability rules described below derive from this general rule.

Parameter types¶

Callable types are covariant with respect to their return types but contravariant with respect to their parameter types. This means a callable B is assignable to callable A if the types of the parameters of A are assignable to the parameters of B. For example, (x: float) -> int is assignable to (x: int) -> float:

def func(cb: Callable[[float], int]):
    f1: Callable[[int], float] = cb  # OK

Parameter kinds¶

Callable B is assignable to callable A only if all keyword-only parameters in A are present in B as either keyword-only parameters or standard (positional or keyword) parameters. For example, (a: int) -> None is assignable to (*, a: int) -> None, but the converse is not true. The order of keyword-only parameters is ignored for purposes of assignability:

class KwOnly(Protocol):
    def __call__(self, *, b: int, a: int) -> None: ...

class Standard(Protocol):
    def __call__(self, a: int, b: int) -> None: ...

def func(standard: Standard, kw_only: KwOnly):
    f1: KwOnly = standard  # OK
    f2: Standard = kw_only  # Error

Likewise, callable B is assignable to callable A only if all positional-only parameters in A are present in B as either positional-only parameters or standard (positional or keyword) parameters. The names of positional-only parameters are ignored for purposes of assignability:

class PosOnly(Protocol):
    def __call__(self, not_a: int, /) -> None: ...

class Standard(Protocol):
    def __call__(self, a: int) -> None: ...

def func(standard: Standard, pos_only: PosOnly):
    f1: PosOnly = standard  # OK
    f2: Standard = pos_only  # Error

`*args` parameters¶

If a callable A has a signature with a *args parameter, callable B must also have a *args parameter to be assignable to A, and the type of A’s *args parameter must be assignable to B’s *args parameter:

class NoArgs(Protocol):
    def __call__(self) -> None: ...

class IntArgs(Protocol):
    def __call__(self, *args: int) -> None: ...

class FloatArgs(Protocol):
    def __call__(self, *args: float) -> None: ...

def func(no_args: NoArgs, int_args: IntArgs, float_args: FloatArgs):
    f1: NoArgs = int_args  # OK
    f2: NoArgs = float_args  # OK

    f3: IntArgs = no_args  # Error: missing *args parameter
    f4: IntArgs = float_args  # OK

    f5: FloatArgs = no_args  # Error: missing *args parameter
    f6: FloatArgs = int_args  # Error: float is not assignable to int

If a callable A has a signature with one or more positional-only parameters, a callable B is assignable to A only if B has an *args parameter whose type is assignable from the types of any otherwise-unmatched positional-only parameters in A:

class PosOnly(Protocol):
    def __call__(self, a: int, b: str, /) -> None: ...

class IntArgs(Protocol):
    def __call__(self, *args: int) -> None: ...

class IntStrArgs(Protocol):
    def __call__(self, *args: int | str) -> None: ...

class StrArgs(Protocol):
    def __call__(self, a: int, /, *args: str) -> None: ...

class Standard(Protocol):
    def __call__(self, a: int, b: str) -> None: ...

def func(int_args: IntArgs, int_str_args: IntStrArgs, str_args: StrArgs):
    f1: PosOnly = int_args  # Error: str is not assignable to int
    f2: PosOnly = int_str_args  # OK
    f3: PosOnly = str_args  # OK
    f4: IntStrArgs = str_args  # Error: int | str is not assignable to str
    f5: IntStrArgs = int_args  # Error: int | str is not assignable to int
    f6: StrArgs = int_str_args  # OK
    f7: StrArgs = int_args  # Error: str is not assignable to int
    f8: IntArgs = int_str_args  # OK
    f9: IntArgs = str_args  # Error: int is not assignable to str
    f10: Standard = int_str_args  # Error: keyword parameters a and b missing
    f11: Standard = str_args  # Error: keyword parameter b missing

`**kwargs` parameters¶

If a callable A has a signature with a **kwargs parameter (without an unpacked TypedDict type annotation), callable B must also have a **kwargs parameter to be assignable to A, and the type of A’s **kwargs parameter must be assignable to B’s **kwargs parameter:

class NoKwargs(Protocol):
    def __call__(self) -> None: ...

class IntKwargs(Protocol):
    def __call__(self, **kwargs: int) -> None: ...

class FloatKwargs(Protocol):
    def __call__(self, **kwargs: float) -> None: ...

def func(no_kwargs: NoKwargs, int_kwargs: IntKwargs, float_kwargs: FloatKwargs):
    f1: NoKwargs = int_kwargs  # OK
    f2: NoKwargs = float_kwargs  # OK

    f3: IntKwargs = no_kwargs  # Error: missing **kwargs parameter
    f4: IntKwargs = float_kwargs  # OK

    f5: FloatKwargs = no_kwargs  # Error: missing **kwargs parameter
    f6: FloatKwargs = int_kwargs  # Error: float is not assignable to int

If a callable A has a signature with one or more keyword-only parameters, a callable B is assignable to A if B has a **kwargs parameter whose type is assignable from the types of any otherwise-unmatched keyword-only parameters in A:

class KwOnly(Protocol):
    def __call__(self, *, a: int, b: str) -> None: ...

class IntKwargs(Protocol):
    def __call__(self, **kwargs: int) -> None: ...

class IntStrKwargs(Protocol):
    def __call__(self, **kwargs: int | str) -> None: ...

class StrKwargs(Protocol):
    def __call__(self, *, a: int, **kwargs: str) -> None: ...

class Standard(Protocol):
    def __call__(self, a: int, b: str) -> None: ...

def func(int_kwargs: IntKwargs, int_str_kwargs: IntStrKwargs, str_kwargs: StrKwargs):
    f1: KwOnly = int_kwargs  # Error: str is not assignable to int
    f2: KwOnly = int_str_kwargs  # OK
    f3: KwOnly = str_kwargs  # OK
    f4: IntStrKwargs = str_kwargs  # Error: int | str is not assignable to str
    f5: IntStrKwargs = int_kwargs  # Error: int | str is not assignable to int
    f6: StrKwargs = int_str_kwargs  # OK
    f7: StrKwargs = int_kwargs  # Error: str is not assignable to int
    f8: IntKwargs = int_str_kwargs  # OK
    f9: IntKwargs = str_kwargs  # Error: int is not assignable to str
    f10: Standard = int_str_kwargs  # Error: Does not accept positional arguments
    f11: Standard = str_kwargs  # Error: Does not accept positional arguments

Assignability rules for callable signatures that contain a **kwargs with an unpacked TypedDict are described in the section above.

Signatures with ParamSpecs¶

A signature that includes *args: P.args, **kwargs: P.kwargs is equivalent to a Callable parameterized by P:

class ProtocolWithP[**P](Protocol):
    def __call__(self, *args: P.args, **kwargs: P.kwargs) -> None: ...

type TypeAliasWithP[**P] = Callable[P, None]

def func[**P](proto: ProtocolWithP[P], ta: TypeAliasWithP[P]):
    # These two types are equivalent
    f1: TypeAliasWithP[P] = proto  # OK
    f2: ProtocolWithP[P] = ta  # OK

Default argument values¶

If a callable C has a parameter x with a default argument value and A is the same as C except that x has no default argument, then C is assignable to A. C is also assignable to A if A is the same as C with parameter x removed:

class DefaultArg(Protocol):
    def __call__(self, x: int = 0) -> None: ...

class NoDefaultArg(Protocol):
    def __call__(self, x: int) -> None: ...

class NoX(Protocol):
    def __call__(self) -> None: ...

def func(default_arg: DefaultArg):
    f1: NoDefaultArg = default_arg  # OK
    f2: NoX = default_arg  # OK

Overloads¶

If a callable B is overloaded with two or more signatures, it is assignable to callable A if at least one of the overloaded signatures in B is assignable to A:

class Overloaded(Protocol):
    @overload
    def __call__(self, x: int) -> int: ...
    @overload
    def __call__(self, x: str) -> str: ...

class IntArg(Protocol):
    def __call__(self, x: int) -> int: ...

class StrArg(Protocol):
    def __call__(self, x: str) -> str: ...

class FloatArg(Protocol):
    def __call__(self, x: float) -> float: ...

def func(overloaded: Overloaded):
    f1: IntArg = overloaded  # OK
    f2: StrArg = overloaded  # OK
    f3: FloatArg = overloaded  # Error

If a callable A is overloaded with two or more signatures, callable B is assignable to A if B is assignable to all of the signatures in A:

class Overloaded(Protocol):
    @overload
    def __call__(self, x: int, y: str) -> float: ...
    @overload
    def __call__(self, x: str) -> complex: ...

class StrArg(Protocol):
    def __call__(self, x: str) -> complex: ...

class IntStrArg(Protocol):
    def __call__(self, x: int | str, y: str = "") -> int: ...

def func(int_str_arg: IntStrArg, str_arg: StrArg):
    f1: Overloaded = int_str_arg  # OK
    f2: Overloaded = str_arg  # Error

Callables¶

Terminology¶

Positional-only parameters¶

Default argument values¶

Annotating `*args` and `**kwargs`¶

`Unpack` for keyword arguments¶

Function calls with standard dictionaries¶

Keyword collisions¶

Required and non-required items¶

Read-only items¶

Extra items¶

Assignment¶

Source and destination contain `**kwargs`¶

Source contains `**kwargs` and destination doesn’t¶

Source contains untyped `**kwargs`¶

Source contains traditionally typed `**kwargs: T`¶

Passing kwargs inside a function to another function¶

Using `Unpack` with types other than `TypedDict`¶

Callable¶

Meaning of `...` in `Callable`¶

Callback protocols¶

Assignability rules for callables¶

Parameter types¶

Parameter kinds¶

`*args` parameters¶

`**kwargs` parameters¶

Signatures with ParamSpecs¶

Default argument values¶

Overloads¶

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Callables¶

Terminology¶

Positional-only parameters¶

Default argument values¶

Annotating *args and **kwargs¶

Unpack for keyword arguments¶

Function calls with standard dictionaries¶

Keyword collisions¶

Required and non-required items¶

Read-only items¶

Extra items¶

Assignment¶

Source and destination contain **kwargs¶

Source contains **kwargs and destination doesn’t¶

Source contains untyped **kwargs¶

Source contains traditionally typed **kwargs: T¶

Passing kwargs inside a function to another function¶

Using Unpack with types other than TypedDict¶

Callable¶

Meaning of ... in Callable¶

Callback protocols¶

Assignability rules for callables¶

Parameter types¶

Parameter kinds¶

*args parameters¶

**kwargs parameters¶

Signatures with ParamSpecs¶

Default argument values¶

Overloads¶

Annotating `*args` and `**kwargs`¶

`Unpack` for keyword arguments¶

Source and destination contain `**kwargs`¶

Source contains `**kwargs` and destination doesn’t¶

Source contains untyped `**kwargs`¶

Source contains traditionally typed `**kwargs: T`¶

Using `Unpack` with types other than `TypedDict`¶

Meaning of `...` in `Callable`¶

`*args` parameters¶

`**kwargs` parameters¶