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PEP 613: Loosen class scope restriction of explicit TypeAlias #2154

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Jan 12, 2022
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PEP 613: Loosen class scope restriction of explicit TypeAlias #2154

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PEP 613: Loosen class scope restriction of explicit TypeAlias
nipunn1313 Nov 17, 2021
1f7ca33
Apply CAM-Gerlach's suggestions from code review
nipunn1313 Jan 12, 2022
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29 changes: 24 additions & 5 deletions pep-0613.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -95,6 +95,17 @@ across the codebase can be suppressed.
Scope Restrictions:
*******************

::

class Foo:
x = ClassName
y: TypeAlias = ClassName
z: Type[ClassName] = ClassName

Type aliases are valid within class scope, both implicitly (``x``) and
explicitly (``y``). If the line should be interpreted as a class
variable, it must be explicitly annotated (``z``).

::

x = ClassName
Expand All @@ -103,7 +114,7 @@ Scope Restrictions:

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.
inside of a function.
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.
Expand All @@ -116,10 +127,10 @@ but rather on every one of its subsequent use cases.
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.
With explicit aliases, the outer assignment is still a valid type variable.
Inside ``foo``, the inner assignment should be interpreted as ``x: Type[ClassName]``.
Inside ``bar``, the type checker should raise a clear error, communicating
to the author that type aliases cannot be defined inside a function.


Specification
Expand Down Expand Up @@ -200,6 +211,14 @@ appealing because it still sticks with the ``MyType = int`` assignment
syntax, and adds some information for the type checker purely as an annotation.


Version History
===============

* 2021-11-16

* Allow TypeAlias inside class scope


Copyright
=========

Expand Down