python-peps/pep-0585.rst

PEP: 585
Title: Type Hinting Generics In Standard Collections
Version: $Revision$
Last-Modified: $Date$
Author: Łukasz Langa <lukasz@python.org>
Discussions-To: Typing-Sig <typing-sig@python.org>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 03-Mar-2019
Python-Version: 3.9

Abstract
========

Static typing as defined by PEPs 484, 526, 544, 560, and 563 was built
incrementally on top of the existing Python runtime and constrained by
existing syntax and runtime behavior.  This led to the existence of
a duplicated collection hierarchy in the ``typing`` module due to
generics (for example ``typing.List`` and the built-in ``list``).

This PEP proposes to enable support for the generics syntax in all
standard collections available by the ``typing`` module.


Rationale and Goals
===================

This change removes the necessity for a parallel type hierarchy in the
``typing`` module, making it easier for users to annotate their programs
and easier for teachers to teach Python.


Terminology
===========

Generic (n.) - a type that can be parametrized, typically a container.
Also known as a *parametric type* or a *generic type*.  For example:
``dict``.

Parametrized generic - a specific instance of a generic with the
expected types for container elements provided.  For example:
``dict[str, int]``.


Backwards compatibility
=======================

The newly described functionality requires Python 3.9.  For use cases
restricted to type annotations, Python files with the "annotations"
future-import (available since Python 3.7) can use generics in
combination with standard library collections.

Tooling, including type checkers and linters, will have to be adapted to
recognize such generics usage as valid.


Implementation
==============

Starting with Python 3.7, when ``from __future__ import annotations`` is
used, function and variable annotations can specify generics directly on
builtin types.  Example::

    from __future__ import annotations

    def find(haystack: dict[str, list[int]]) -> int:
        ...

Certain features of typing like type aliases or casting require putting
types in runtime context, outside of annotations.  While these are
relatively less common than type annotations, it's important to allow
using the same type syntax in all contexts. This is why starting with
Python 3.9, the following collections gain `__class_getitem__()` support
for generics:

* ``tuple``  # typing.Tuple
* ``list``  # typing.List
* ``dict``  # typing.Dict
* ``set``  # typing.Set
* ``frozenset``  # typing.FrozenSet
* ``type``  # typing.Type
* ``collections.deque``
* ``collections.defaultdict``
* ``collections.OrderedDict``
* ``collections.Counter``
* ``collections.ChainMap``
* ``collections.abc.Awaitable``
* ``collections.abc.Coroutine``
* ``collections.abc.AsyncIterable``
* ``collections.abc.AsyncIterator``
* ``collections.abc.AsyncGenerator``
* ``collections.abc.Iterable``
* ``collections.abc.Iterator``
* ``collections.abc.Generator``
* ``collections.abc.Reversible``
* ``collections.abc.Container``
* ``collections.abc.Collection``
* ``collections.abc.Callable``
* ``collections.abc.Set``  # typing.AbstractSet
* ``collections.abc.MutableSet``
* ``collections.abc.Mapping``
* ``collections.abc.MutableMapping``
* ``collections.abc.Sequence``
* ``collections.abc.MutableSequence``
* ``collections.abc.ByteString``
* ``collections.abc.MappingView``
* ``collections.abc.KeysView``
* ``collections.abc.ItemsView``
* ``collections.abc.ValuesView``
* ``contextlib.AbstractContextManager``  # typing.ContextManager
* ``contextlib.AbstractAsyncContextManager``  # typing.AsyncContextManager

Importing those from ``typing`` is deprecated.  Type checkers may warn
about such deprecated usage when the target version of the checked
program is signalled to be Python 3.9 or newer.


Parameters to generics are available at runtime
-----------------------------------------------

Preserving the generic type at runtime enables introspection of the type
which can be used for API generation or runtime type checking.  Such
usage is already present in the wild.

Just like with the ``typing`` module today, the parametrized generic
types listed in the previous section all preserve their type parameters
at runtime::

    >>> list[str]
    list[str]
    >>> tuple[int, ...]
    tuple[int, ...]
    >>> ChainMap[str, list[str]]
    collections.ChainMap[str, list[str]]

This is implemented using a thin proxy type that forwards all method
calls and attribute accesses to the bare origin type with the following
exceptions:

* the ``__repr__`` shows the parametrized type;
* the ``__origin__`` attribute points at the non-parametrized
  generic class;
* the ``__parameters__`` attribute is a tuple (possibly of length
  1) of generic types passed to the original ``__class_getitem__``;
* the ``__class_getitem__`` raises an exception to disallow mistakes
  like ``dict[str][str]``.

This design means that it is possible to create instances of
parametrized collections, like::

    >>> l = list[str]()
    []
    >>> isinstance([1, 2, 3], list[str])
    True
    >>> list is list[str]
    False
    >>> list == list[str]
    True

Objects created with bare types and parametrized types are exactly the
same.  The generic parameters are not preserved in instances created
with parametrized types, in other words generic types erase type
parameters during object creation.

One important consequence of this is that the interpreter does **not**
attempt to type check operations on the collection created with
a parametrized type.  This provides symmetry between::

    l: list[str] = []

and::

    l = list[str]()


Forward compatibility
---------------------

Future standard collections must implement the same behavior.


Rejected alternatives
=====================

Do nothing
----------

Keeping the status quo forces Python programmers to perform book-keeping
of imports from the ``typing`` module for standard collections, making
all but the simplest annotations cumbersome to maintain.  The existence
of parallel types is confusing to newcomers (why is there both ``list``
and ``List``?).

The above problems also don't exist in user-built generic classes which
share runtime functionality and the ability to use them as generic type
annotations.  Making standard collections harder to use in type hinting
from user classes hindered typing adoption and usability.

Generics erasure
----------------

It would be easier to implement ``__class_getitem__`` on the listed
standard collections in a way that doesn't preserve the generic type,
in other words::

    >>> list[str]
    <class 'list'>
    >>> tuple[int, ...]
    <class 'tuple'>
    >>> collections.ChainMap[str, list[str]]
    <class 'collections.ChainMap'>

This is problematic as it breaks backwards compatibility: current
equivalents of those types in the ``typing`` module **do** preserve
the generic type::

    >>> from typing import List, Tuple, ChainMap
    >>> List[str]
    typing.List[str]
    >>> Tuple[int, ...]
    typing.Tuple[int, ...]
    >>> ChainMap[str, List[str]]
    typing.ChainMap[str, typing.List[str]]

As mentioned in the "Implementation" section, preserving the generic
type at runtime enables runtime introspection of the type which can be
used for API generation or runtime type checking.  Such usage is already
present in the wild.

Additionally, implementing subscripts as identity functions would make
Python less friendly to beginners.  Let's demonstrate this with an
example.  If a user is passing a list type instead of a list object to
a function, and that function is using indexing, the code would no
longer raise an error.

Today::

    >>> l = list
    >>> l[-1]
    TypeError: 'type' object is not subscriptable

With ``__class_getitem__`` as an identity function::

    >>> l = list
    >>> l[-1]
    list

The indexing being successful here would likely end up raising an
exception at a distance and with a confusing error message to the user.

Disallowing instantiation of parametrized types
-----------------------------------------------

Given that the proxy type which preserves ``__origin__`` and
``__parameters__`` is mostly useful for static analysis or runtime
introspection purposes, we might have disallowed instantiation of
parametrized types.

In fact, this is what the ``typing`` module does today for the parallels
of builtin collections only.  Instantiation of other parametrized types
is allowed.

The original reason for this decision was to discourage spurious
parametrization which made object creation up to two orders of magnitude
slower compared to the special syntax available for builtin types.

This rationale is not strong enough to allow the exceptional treatment
of builtins.  All other parametrized types can still be instantiated,
including parallels of collections in the standard library.  Moreover,
Python allows for instantiation of lists using ``list()`` and some
builtin collections don't provide special syntax for instantiation.

Making ``isinstance(obj, list[str])`` perform a runtime type check
------------------------------------------------------------------

This functionality requires iterating over the collection which is
a destructive operation in some of them.  This functionality would have
been useful, however implementing the type checker within Python that
would deal with complex types, nested type checking, type variables,
string forward references, and so on is out of scope for this PEP.  This
can be revised in the future.


Note on the initial draft
=========================

An early version of this PEP discussed matters beyond generics in
standard collections.  Those unrelated topics were removed for clarity.


Copyright
=========

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