PEP: 613
Title: Explicit Type Aliases
Author: Shannon Zhu <szhu@fb.com>
Sponsor: Guido van Rossum <guido@python.org>
Discussions-To: https://mail.python.org/archives/list/typing-sig@python.org/thread/MWRJOBEEEMFVXE7CAKO7B4P46IPM4AN3/
Status: Accepted
Type: Standards Track
Content-Type: text/x-rst
Created: 21-Jan-2020
Post-History: 21-Jan-2020


Abstract
========

Type aliases are user-specified types which may be as complex as any type hint,
and are specified with a simple variable assignment on a module top level.

This PEP formalizes a way to explicitly declare an assignment as a type alias.

Motivation
==========

Type aliases are declared as top level variable assignments.
In `PEP 484 <https://www.python.org/dev/peps/pep-0484/#type-aliases/>`_,
the distinction between a valid type alias and a global variable was implicitly
determined: if a top level assignment is unannotated, and the assigned value is
a valid type, then the name being assigned to is a valid type alias. Otherwise,
that name is simply a global value that cannot be used as a type hint.

These implicit type alias declaration rules create confusion when type aliases
involve forward references, invalid types, or violate other restrictions
enforced on type alias declaration. Because the distinction between an
unannotated value and a type alias is implicit, ambiguous or incorrect type
alias declarations implicitly default to a valid value assignment. This creates
expressions that are impossible to express as type aliases and punts error
diagnosis of malformed type aliases downstream.

The following examples each include an illustration of some of the suboptimal
or confusing behaviors resulting from existing implicit alias declarations.
We also introduce explicit aliases of the format ``TypeName: TypeAlias = Expression``
here for the sake of comparison, but the syntax is discussed in further detail
in later sections.

Forward References:
*******************

::

  MyType = "ClassName"
  def foo() -> MyType: ...

This code snippet should not error so long as ``ClassName`` is defined
later on. However, a type checker is forced to assume that MyType is a value
assignment rather than a type alias, and therefore may throw spurious errors
that (1) ``MyType`` is an unannotated global string, and (2) ``MyType``
cannot be used as a return annotation because it is not a valid type.

::

  MyType: TypeAlias = “ClassName”
  def foo() -> MyType: ...

Explicit aliases remove ambiguity so neither of the above errors will be
thrown. Additionally, if something is wrong with ``ClassName``
(i.e., it’s not actually defined later), the type checker can throw an error.


Error Messaging:
****************

::

  MyType1 = InvalidType
  MyType2 = MyGeneric(int)  # i.e., intention was MyGeneric[int]

A type checker should warn on this code snippet that ``InvalidType`` is not
a valid type, and therefore cannot be used to annotate an expression or to
construct a type alias. Instead, type checkers are forced to throw spurious
errors that (1) ``MyType`` is a global expression missing an annotation,
and (2) ``MyType`` is not a valid type in all usages of ``MyType``
across the codebase.

::

  MyType1: TypeAlias = InvalidType
  MyType2: TypeAlias = MyGeneric(int)

With explicit aliases, the type checker has enough information to error on the
actual definition of the bad type alias, and explain why: that ``MyGeneric(int)``
and ``InvalidType`` are not valid types. When the value expression is no longer
evaluated as a global value, unactionable type errors on all usages of ``MyType``
across the codebase can be suppressed.

Scope Restrictions:
*******************

::

  x = ClassName
  def foo() -> None:
    x = ClassName

The outer ``x`` is a valid type alias, but type checkers must error if the
inner ``x`` is ever used as a type because type aliases cannot be defined
inside a nested scope.
This is confusing because the alias declaration rule is not explicit, and because
a type error will not be thrown on the location of the inner type alias declaration
but rather on every one of its subsequent use cases.

::

  x: TypeAlias = ClassName
  def foo() -> None:
    x = ClassName
  def bar() -> None:
    x: TypeAlias = ClassName

With explicit aliases, the outer assignment is still a valid type variable,
and the inner assignment can either be a valid local variable or a clear error,
communicating to the author that type aliases cannot be defined inside a nested
scope.


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

The explicit alias declaration syntax clearly differentiates between the three
possible kinds of assignments: typed global expressions, untyped global
expressions, and type aliases. This avoids the existence of assignments that
break type checking when an annotation is added, and avoids classifying the
nature of the assignment based on the type of the value.

Implicit syntax (pre-existing):

::

  x = 1  # untyped global expression
  x: int = 1  # typed global expression

  x = int  # type alias
  x: Type[int] = int  # typed global expression


Explicit syntax:

::

  x = 1  # untyped global expression
  x: int = 1  # typed global expression

  x = int  # untyped global expression (see note below)
  x: Type[int] = int  # typed global expression

  x: TypeAlias = int  # type alias
  x: TypeAlias = “MyClass”  # type alias


Note: The examples above illustrate implicit and explicit alias declarations in
isolation. For the sake of backwards compatibility, type checkers should support
both simultaneously, meaning an untyped global expression ``x = int`` will
still be considered a valid type alias.


Backwards Compatibility
=======================

Explicit aliases provide an alternative way to declare type aliases, but all
pre-existing code and old alias declarations will work as before.


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

The `Pyre <https://pyre-check.org/>`_ type checker supports explicit type
alias declarations.


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

Some alternative syntaxes were considered for explicit aliases:

::

  MyType: TypeAlias[int]

This looks a lot like an uninitialized variable.

::

  MyType = TypeAlias[int]

Along with the option above, this format potentially adds confusion around
what the runtime value of ``MyType`` is.


In comparison, the chosen syntax option ``MyType: TypeAlias = int`` is
appealing because it still sticks with the ``MyType = int`` assignment
syntax, and adds some information for the type checker purely as an annotation.


Copyright
=========

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


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