python-peps/peps/pep-0692.rst

PEP: 692
Title: Using TypedDict for more precise \*\*kwargs typing
Author: Franek Magiera <framagie@gmail.com>
Sponsor: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Discussions-To: https://discuss.python.org/t/pep-692-using-typeddict-for-more-precise-kwargs-typing/17314
Status: Final
Type: Standards Track
Topic: Typing
Created: 29-May-2022
Python-Version: 3.12
Post-History: `29-May-2022 <https://mail.python.org/archives/list/typing-sig@python.org/thread/U42MJE6QZYWPVIFHJIGIT7OE52ZGIQV3/>`__,
              `12-Jul-2022 <https://mail.python.org/archives/list/python-dev@python.org/thread/PLCNW2XR4OOKAKHEZQM7R2AYVYUXPZGW/>`__,
              `12-Jul-2022 <https://discuss.python.org/t/pep-692-using-typeddict-for-more-precise-kwargs-typing/17314>`__,
Resolution: https://discuss.python.org/t/pep-692-using-typeddict-for-more-precise-kwargs-typing/17314/81

.. canonical-typing-spec:: :ref:`typing:unpack-kwargs`

Abstract
========

Currently ``**kwargs`` can be type hinted as long as all of the keyword
arguments specified by them are of the same type. However, that behaviour can
be very limiting. Therefore, in this PEP we propose a new way to enable more
precise ``**kwargs`` typing. The new approach revolves around using
``TypedDict`` to type ``**kwargs`` that comprise keyword arguments of different
types.

Motivation
==========

Currently annotating ``**kwargs`` with a type ``T`` means that the ``kwargs``
type is in fact ``dict[str, T]``. For example::

    def foo(**kwargs: str) -> None: ...

means that all keyword arguments in ``foo`` are strings (i.e., ``kwargs`` is
of type ``dict[str, str]``). This behaviour limits the ability to type
annotate ``**kwargs`` only to the cases where all of them are of the same type.
However, it is often the case that keyword arguments conveyed by ``**kwargs``
have different types that are dependent on the keyword's name. In those cases
type annotating ``**kwargs`` is not possible. This is especially a problem for
already existing codebases where the need of refactoring the code in order to
introduce proper type annotations may be considered not worth the effort. This
in turn prevents the project from getting all of the benefits that type hinting
can provide.

Moreover, ``**kwargs`` can be used to reduce the amount of code needed in
cases when there is a top-level function that is a part of a public API and it
calls a bunch of helper functions, all of which expect the same keyword
arguments. Unfortunately, if those helper functions were to use ``**kwargs``,
there is no way to properly type hint them if the keyword arguments they expect
are of different types. In addition, even if the keyword arguments are of the
same type, there is no way to check whether the function is being called with
keyword names that it actually expects.

As described in the `Intended Usage`_ section,
using ``**kwargs`` is not always the best tool for the job. Despite that, it is
still a widely used pattern. As a consequence, there has been a lot of
discussion around supporting more precise ``**kwargs`` typing and it became a
feature that would be valuable for a large part of the Python community. This
is best illustrated by the `mypy GitHub issue 4441 <mypyIssue4441_>`__ which
contains a lot of real world cases that could benefit from this propsal.

One more use case worth mentioning for which ``**kwargs`` are also convenient,
is when a function should accommodate optional keyword-only arguments that
don't have default values. A need for a pattern like that can arise when values
that are usually used as defaults to indicate no user input, such as ``None``,
can be passed in by a user and should result in a valid, non-default behavior.
For example, this issue `came up <httpxIssue1384_>`__ in the popular ``httpx`` library.

Rationale
=========

:pep:`589` introduced the ``TypedDict`` type constructor that supports dictionary
types consisting of string keys and values of potentially different types. A
function's keyword arguments represented by a formal parameter that begins with
double asterisk, such as ``**kwargs``, are received as a dictionary.
Additionally, such functions are often called using unpacked dictionaries to
provide keyword arguments. This makes ``TypedDict`` a perfect candidate to be
used for more precise ``**kwargs`` typing. In addition, with ``TypedDict``
keyword names can be taken into account during static type analysis. However,
specifying ``**kwargs`` type with a ``TypedDict`` means, as mentioned earlier,
that each keyword argument specified by ``**kwargs`` is a ``TypedDict`` itself.
For instance::

    class Movie(TypedDict):
        name: str
        year: int
    
    def foo(**kwargs: Movie) -> None: ...

means that each keyword argument in ``foo`` is itself a ``Movie`` dictionary
that has a ``name`` key with a string type value and a ``year`` key with an
integer type value. Therefore, in order to support specifying ``kwargs`` type
as a ``TypedDict`` without breaking current behaviour, a new construct has to
be introduced. 

To support this use case, we propose reusing ``Unpack`` which
was initially introduced in :pep:`646`. There are several reasons for doing so:

* Its name is quite suitable and intuitive for the ``**kwargs`` typing use case
  as our intention is to "unpack" the keywords arguments from the supplied
  ``TypedDict``.
* The current way of typing ``*args`` would be extended to ``**kwargs``
  and those are supposed to behave similarly.
* There would be no need to introduce any new special forms.
* The use of ``Unpack`` for the purposes described in this PEP does not
  interfere with the use cases described in :pep:`646`.

Specification
=============

With ``Unpack`` we introduce a new way of annotating ``**kwargs``.
Continuing the previous example::

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

would mean 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!


Using the new annotation will not have any runtime effect - it should only be
taken into account by type checkers. Any mention of errors in the following
sections relates to type checker errors.

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 behaviour for functions using
standard, untyped dictionaries can depend on the type checker. For example::

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

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

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

    another_movie = {"name": "Life of Brian", "year": 1979}
    foo(**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 keys
------------------------------

By default all keys in a ``TypedDict`` are required. This behaviour can be
overridden by setting the dictionary's ``total`` parameter as ``False``.
Moreover, :pep:`655` introduced new type qualifiers - ``typing.Required`` and
``typing.NotRequired`` - that enable specifying whether a particular key is
required or not::

    class Movie(TypedDict):
        title: str
        year: NotRequired[int]

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 supported by the
caller, then an error must be reported by type checkers.

Assignment
----------

Assignments of a function typed with ``**kwargs: Unpack[Movie]`` and
another callable type should pass type checking only if they are compatible.
This can happen 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
compatible::

    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.

.. _PEP 692 assignment dest no kwargs:

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 compatible.
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 compatible with 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.

Similar situation can happen even without inheritance as compatibility
between ``TypedDict``\s is based on structural subtyping.

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 <PEP 692 assignment dest no kwargs>`_
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``
----------------------------------------------------

As described in the Rationale_ section,
``TypedDict`` is the most natural candidate 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.

Changes to ``Unpack``
---------------------

Currently using ``Unpack`` in the context of
typing is interchangeable with using the asterisk syntax::

    >>> Unpack[Movie]
    *<class '__main__.Movie'>

Therefore, in order to be compatible with the new use case, ``Unpack``'s
``repr`` should be changed to simply ``Unpack[T]``.

Intended Usage
==============
The intended use cases for this proposal are described in the
Motivation_ section. In summary, more precise ``**kwargs`` typing
can bring benefits to already existing codebases that decided to use
``**kwargs`` initially, but now are mature enough to use a stricter contract
via type hints. Using ``**kwargs`` can also help in reducing code duplication
and the amount of copy-pasting needed when there is a bunch of functions that
require the same set of keyword arguments. Finally, ``**kwargs`` are useful for
cases when a function needs to facilitate optional keyword arguments that don't
have obvious default values.

However, it has to be pointed out that in some cases there are better tools
for the job than using ``TypedDict`` to type ``**kwargs`` as proposed in this
PEP. For example, when writing new code if all the keyword arguments are
required or have default values then writing everything explicitly is better
than using ``**kwargs`` and a ``TypedDict``::

    def foo(name: str, year: int): ...     # Preferred way.
    def foo(**kwargs: Unpack[Movie]): ...

Similarly, when type hinting third party libraries via stubs it is again better
to state the function signature explicitly - this is the only way to type such
a function if it has default arguments. Another issue that may arise in this
case when trying to type hint the function with a ``TypedDict`` is that some
standard function parameters may be treated as keyword only::

    def foo(name, year): ...              # Function in a third party library.

    def foo(Unpack[Movie]): ...           # Function signature in a stub file.

    foo("Life of Brian", 1979)            # This would be now failing type
                                          # checking but is fine.

    foo(name="Life of Brian", year=1979)  # This would be the only way to call
                                          # the function now that passes type
                                          # checking.

Therefore, in this case it is again preferred to type hint such function
explicitly as::

    def foo(name: str, year: int): ...

Also, for the benefit of IDEs and documentation pages, functions that are part
of the public API should prefer explicit keyword parameters whenever possible.

How to Teach This
=================

This PEP could be linked in the ``typing`` module's documentation. Moreover, a
new section on using ``Unpack`` could be added to the aforementioned docs.
Similar sections could be also added to the
`mypy documentation <https://mypy.readthedocs.io/>`_ and the
`typing RTD documentation <https://typing.readthedocs.io/>`_.

Reference Implementation
========================

The `mypy type checker <https://github.com/python/mypy>`_ already 
`supports <https://github.com/python/mypy/pull/13471>`_ more precise
``**kwargs`` typing using ``Unpack``.

`Pyright type checker <https://github.com/microsoft/pyright>`_ also
`provides provisional support <pyrightProvisionalImplementation_>`__
for `this feature <pyrightIssue3002_>`__.

Rejected Ideas
==============

``TypedDict`` unions
--------------------

It is possible to create unions of typed dictionaries. However, supporting
typing ``**kwargs`` with a union of typed dicts would greatly increase the
complexity of the implementation of this PEP and there seems to be no
compelling use case to justify the support for this. Therefore, using unions of
typed dictionaries to type ``**kwargs`` as described in the context of this PEP
can result in an error::

    class Book(TypedDict):
        genre: str
        pages: int
    
    TypedDictUnion = Movie | Book

    def foo(**kwargs: Unpack[TypedDictUnion]) -> None: ...  # WRONG! Unsupported use
                                                            # of a union of
                                                            # TypedDicts to type
                                                            # **kwargs

Instead, a function that expects a union of ``TypedDict``\s can be
overloaded::

    @overload
    def foo(**kwargs: Unpack[Movie]): ...

    @overload
    def foo(**kwargs: Unpack[Book]): ...

Changing the meaning of ``**kwargs`` annotations
------------------------------------------------

One way to achieve the purpose of this PEP would be to change the
meaning of ``**kwargs`` annotations, so that the annotations would
apply to the entire ``**kwargs`` dict, not to individual elements.
For consistency, we would have to make an analogous change to ``*args``
annotations.

This idea was discussed in a meeting of the typing community, and the
consensus was that the change would not be worth the cost. There is no
clear migration path, the current meaning of ``*args`` and ``**kwargs``
annotations is well-established in the ecosystem, and type checkers
would have to introduce new errors for code that is currently legal.

Introducing a new syntax
------------------------

In the previous versions of this PEP, using a double asterisk syntax was
proposed to support more precise ``**kwargs`` typing. Using this syntax,
functions could be annotated as follows::

    def foo(**kwargs: **Movie): ...

Which would have the same meaning as::

    def foo(**kwargs: Unpack[Movie]): ...

This greatly increased the scope of the PEP, as it would require a grammar
change and adding a new dunder for the ``Unpack`` special form. At the same
the justification for introducing a new syntax was not strong enough and
became a blocker for the whole PEP. Therefore, we decided to abandon the idea
of introducing a new syntax as a part of this PEP and may propose it again in a
separate one.

References
==========

.. _httpxIssue1384: https://github.com/encode/httpx/issues/1384
.. _mypyIssue4441: https://github.com/python/mypy/issues/4441
.. _pyrightIssue3002: https://github.com/microsoft/pyright/issues/3002
.. _pyrightProvisionalImplementation: https://github.com/microsoft/pyright/commit/5bee749eb171979e3f526cd8e5bf66b00593378a

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.