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annotations: expand documentation on "simple" assignment targets (#120535)

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This behavior is rather surprising and it was not clearly specified.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
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Jelle Zijlstra 2024-06-15 08:18:16 -07:00 • committed by GitHub
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2 changed files with 12 additions and 5 deletions
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10
Doc/library/ast.rst
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@ -891,9 +891,13 @@ Statements
An assignment with a type annotation. ``target`` is a single node and can An assignment with a type annotation. ``target`` is a single node and can
be a :class:`Name`, a :class:`Attribute` or a :class:`Subscript`. be a :class:`Name`, a :class:`Attribute` or a :class:`Subscript`.
``annotation`` is the annotation, such as a :class:`Constant` or :class:`Name` ``annotation`` is the annotation, such as a :class:`Constant` or :class:`Name`
node. ``value`` is a single optional node. ``simple`` is a boolean integer node. ``value`` is a single optional node.
set to True for a :class:`Name` node in ``target`` that do not appear in
between parenthesis and are hence pure names and not expressions. ``simple`` is always either 0 (indicating a "complex" target) or 1
(indicating a "simple" target). A "simple" target consists solely of a
:class:`Name` node that does not appear between parentheses; all other
targets are considered complex. Only simple targets appear in
the :attr:`__annotations__` dictionary of modules and classes.
.. doctest:: .. doctest::

7
Doc/reference/simple_stmts.rst
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@ -333,7 +333,9 @@ statement, of a variable or attribute annotation and an optional assignment stat
The difference from normal :ref:`assignment` is that only a single target is allowed. The difference from normal :ref:`assignment` is that only a single target is allowed.
For simple names as assignment targets, if in class or module scope, The assignment target is considered "simple" if it consists of a single
name that is not enclosed in parentheses.
For simple assignment targets, if in class or module scope,
the annotations are evaluated and stored in a special class or module the annotations are evaluated and stored in a special class or module
attribute :attr:`__annotations__` attribute :attr:`__annotations__`
that is a dictionary mapping from variable names (mangled if private) to that is a dictionary mapping from variable names (mangled if private) to
@ -341,7 +343,8 @@ evaluated annotations. This attribute is writable and is automatically
created at the start of class or module body execution, if annotations created at the start of class or module body execution, if annotations
are found statically. are found statically.
For expressions as assignment targets, the annotations are evaluated if If the assignment target is not simple (an attribute, subscript node, or
parenthesized name), the annotation is evaluated if
in class or module scope, but not stored. in class or module scope, but not stored.
If a name is annotated in a function scope, then this name is local for If a name is annotated in a function scope, then this name is local for